CA1255946A - Blocker device for eliminating specular reflectance from a diffuse reflection spectrum - Google Patents
Blocker device for eliminating specular reflectance from a diffuse reflection spectrumInfo
- Publication number
- CA1255946A CA1255946A CA000501488A CA501488A CA1255946A CA 1255946 A CA1255946 A CA 1255946A CA 000501488 A CA000501488 A CA 000501488A CA 501488 A CA501488 A CA 501488A CA 1255946 A CA1255946 A CA 1255946A
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- Canada
- Prior art keywords
- sample
- energy
- reflected
- blocking
- blocker
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000001228 spectrum Methods 0.000 title claims description 39
- 238000000985 reflectance spectrum Methods 0.000 claims abstract description 30
- 230000000903 blocking effect Effects 0.000 claims abstract description 24
- 238000000034 method Methods 0.000 claims abstract description 19
- 238000010521 absorption reaction Methods 0.000 claims description 5
- 239000000203 mixture Substances 0.000 claims description 4
- 230000035515 penetration Effects 0.000 claims description 2
- 230000005855 radiation Effects 0.000 claims description 2
- 230000008030 elimination Effects 0.000 claims 2
- 238000003379 elimination reaction Methods 0.000 claims 2
- 239000000126 substance Substances 0.000 claims 1
- 150000001875 compounds Chemical class 0.000 abstract description 11
- 150000002484 inorganic compounds Chemical class 0.000 abstract description 4
- 229910010272 inorganic material Inorganic materials 0.000 abstract description 4
- 238000004566 IR spectroscopy Methods 0.000 abstract description 2
- 239000000523 sample Substances 0.000 description 84
- 230000003595 spectral effect Effects 0.000 description 17
- 239000000843 powder Substances 0.000 description 12
- 230000000694 effects Effects 0.000 description 8
- 239000000463 material Substances 0.000 description 8
- 239000013078 crystal Substances 0.000 description 7
- 239000008187 granular material Substances 0.000 description 7
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 6
- 239000011159 matrix material Substances 0.000 description 6
- 238000004611 spectroscopical analysis Methods 0.000 description 5
- 238000000411 transmission spectrum Methods 0.000 description 5
- 230000003287 optical effect Effects 0.000 description 4
- 238000007788 roughening Methods 0.000 description 4
- 238000002474 experimental method Methods 0.000 description 3
- 238000002329 infrared spectrum Methods 0.000 description 3
- 230000000149 penetrating effect Effects 0.000 description 3
- 229910021532 Calcite Inorganic materials 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 238000001055 reflectance spectroscopy Methods 0.000 description 2
- 239000004576 sand Substances 0.000 description 2
- 239000000377 silicon dioxide Substances 0.000 description 2
- 235000013619 trace mineral Nutrition 0.000 description 2
- 239000011573 trace mineral Substances 0.000 description 2
- 241000282836 Camelus dromedarius Species 0.000 description 1
- 241000950314 Figura Species 0.000 description 1
- 238000005033 Fourier transform infrared spectroscopy Methods 0.000 description 1
- 101100328463 Mus musculus Cmya5 gene Proteins 0.000 description 1
- LEHOTFFKMJEONL-UHFFFAOYSA-N Uric Acid Chemical compound N1C(=O)NC(=O)C2=C1NC(=O)N2 LEHOTFFKMJEONL-UHFFFAOYSA-N 0.000 description 1
- 230000002745 absorbent Effects 0.000 description 1
- 239000002250 absorbent Substances 0.000 description 1
- 238000004847 absorption spectroscopy Methods 0.000 description 1
- 239000002390 adhesive tape Substances 0.000 description 1
- 230000001680 brushing effect Effects 0.000 description 1
- JHIVVAPYMSGYDF-UHFFFAOYSA-N cyclohexanone Chemical compound O=C1CCCCC1 JHIVVAPYMSGYDF-UHFFFAOYSA-N 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000007865 diluting Methods 0.000 description 1
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 1
- 239000010931 gold Substances 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 238000009499 grossing Methods 0.000 description 1
- 230000000873 masking effect Effects 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 238000003908 quality control method Methods 0.000 description 1
- 238000004445 quantitative analysis Methods 0.000 description 1
- 230000005639 quantum mechanical wave Effects 0.000 description 1
- 239000013074 reference sample Substances 0.000 description 1
- 238000002235 transmission spectroscopy Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials
- G01N21/474—Details of optical heads therefor, e.g. using optical fibres
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N2021/3595—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using FTIR
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials
- G01N2021/4764—Special kinds of physical applications
- G01N2021/4769—Fluid samples, e.g. slurries, granulates; Compressible powdery of fibrous samples
Abstract
ABSTRACT OF THE DISCLOSURE
Diffuse reflectance spectra may be obtained that are free from the distortions caused by specular reflections by using an apparatus and method for physically blocking out specularly reflected energy. The apparatus consists of ablocker that is positioned substantially in contact with the surface of the sample at the region wherein an input beam of energy is focused on the sample.
Specularly reflected energy that would otherwise be reflected to a detector is physically blocked by the blocker. That part of the input energy beam that penetrates into the sample is diffusely reflected. Part of the diffusely reflected energy passes under the blocker where it is collected and focused on the detector. The blocker is found to have particular application in the field of infrared spectroscopy of inorganic compounds, particularly compounds having a powdered structure.
Diffuse reflectance spectra may be obtained that are free from the distortions caused by specular reflections by using an apparatus and method for physically blocking out specularly reflected energy. The apparatus consists of ablocker that is positioned substantially in contact with the surface of the sample at the region wherein an input beam of energy is focused on the sample.
Specularly reflected energy that would otherwise be reflected to a detector is physically blocked by the blocker. That part of the input energy beam that penetrates into the sample is diffusely reflected. Part of the diffusely reflected energy passes under the blocker where it is collected and focused on the detector. The blocker is found to have particular application in the field of infrared spectroscopy of inorganic compounds, particularly compounds having a powdered structure.
Description
s~ 6 BLOCKER D~VICE FOR ELll!nINATING SPECUL,AR REFLEGTANCE
._ FROM A DIFFU~E REFLECTIC~N SPECTRUM
Field of the Invention This invention deals with a method and apparatus for obtaining diffuse reflectance spectra, wherein the specular component of the diffuse spectra is substantially eliminated. Distortions in the diffuse refl~ctance spectr~ ca~lsed5 by the specular component are thus eliminRted.
Back~rround of the Invention _.. . . _ It is kr1own thnt partlcul~r compounds possess uniqu~ spectral signatures. One method to obtain a spectrum indicative of a particular compound is transmission spectroscopy. A transmission spectrum can be 10 obtained by transmitting an energy beam of known intensity and frequency through an at least partially transmissive sample and recorcling the intensity of the energy transmitted through the sample at various incident wRvelengths.
This method works quite well for a wide range of compounas having known transmission spectra. More recent technology for obtaining infrared spectra 15 uses interferometers and computers in what is commonly called Fourier Transform Infrared (FTIE~) spectroscopy~ This technology has proved to have significant advantages over prior art methods of obtaining infrared spectra. A
transmission spectrum cannot, generally, be obtained for a comp~und composed of powder grains or small size granules, particularly when the powder is 20 substantially opaque to the frequencies of the incident energy at common granule thickness. One solution in such an instance is to embed the powder or granules in a matrix not having spectral features in the frequency range of the incident energy beam. This method only works so long as a suitable matrix compound can be found and the powder or granules are not so opaque as to ~bsorb all input ~...
~2~ 6 energy when the matrix contains a sufficient density of the sample to produce a meaningful spe¢trum from the sample.
Another solution to the problem associated with powders or granular samples is to obtain a diffuse reflectance spectrum of the sample. A diffuse 5 reflectance spectrum is obtained by directing an input energy beam onto the surface of the sample, collecting the diffusely reflected energy from the sampleand directing that energy to a detector. Diffusely reflected energy is energy which is defined to be reflected from below the surface of ~he sample. The energy diffusely reflected from a sample does not have a preferred direction of 10 reflection, i.e., the diffusely reflected energy leaves the sample surface in a hemispherical pattern. The diffusively reflected energy has spectral characteristics that uniquely iden$ify the sample compounds and correspond to the spectrum obtained by transmissive means.
In addition to diffuse reflection, however, an energy beam directed 15 against the surface of a sample produces specular reflection. Specular reflection is defined to be incident energy that is reflected from a surface of a sample as opposed to difYusely reflected energy which i9 energy refl~cted from below the sample surface. Spccular reflectance obeys Snell's l.aw Wlllch states that the angle of incidence of the input energy beam equals the angle of 20 reflectance. In other words, energy that is specularly reflected behaves us light refIected from a mirror. Thus, if all the crystals on the surface of a powder sample were oriented so as to present a homogenous reflective face to the incident energy that was parallel to the plane of the sample, the incident energy would reflect off the surface of the sample according to Snell's Law. However, 25 the reflecting surfaces of individual crystals on the surface of a powder sample are somewhat randomly oriented and, therefore, scatter the incident energy over an entire hemisphere as is the case for diffuse reflection. Our experiments and those of others have shown that the crystals on the surface of a sample may often be oriented so as to produce a preferred direction of reflection.
30 Nevertheless, some of the incident energy beam is nearly always specularly reflected over all angles of reflection. A detailed discussion of the spectrometry of powdered samples is found in Griffiths et al, Advances in Infrared and l~amanSpectroscopy, Vol. 9, Chapter 2, (Heyden, London 1981) the disclosure of which is hereby incorporated by reference.
.12SS~
Conventional analysis of diffuse reflectance spectra employs the Kubelka-Munk function. The Kubelka-Munk function states that the strength of an absorption feature in a diffuse reflectance spectrunn is linearly related to the concentration of the compound producing the spectral feature. The function 5 invol~1es a relationship between an absorption coefficient, a scattering coefficient and the ratio of the diffuse reflectance from a sample and that of anon~absorbing powder reference~ The funetion assumes that the sample extends to an infinite optical àepth, i.e., that depth at which the addition of more sample material to the bottom of the sample does not change the amount of energy 10 diffusely reflected. In theory, the Kubelka-Munk function should enable a spectrum obtained by diffuse reflectance to be compared to a spectrum obtained by transmissive means. The transmission spectrum for many compounds is known. ~ence, the ability to identify a compound from its diffuse reflectance spectrum given a known transmission spectrum enables diffuse reflectance 15 spectroscopy to accurately identjfy trace elements present in powdered samples th~t do not lend themselves to direct tl~ansmission spectroscopy. a specific example of the applicatlon of diffuse refleetion spectrosaopy ls In quality control of pharmac~utical tablets to ~liminate the need to grind up the tablets flnd embed them in a non~absorbtive matrix. Moreover, it i9 belleved that it will oe 20 possible to apply spectral substraction routines commonly used in absorption spectroscopy to diffuse reflectance measurements to identiy trace elements in the sample and to accurately establish their concentration in the sample.
The linearity of the Kubelka-Munk relationship for the strength of an absorption feature with concentration, however, breaks down for experimental 25 conditions involving specular reflectance~ Specular reflection alters a diffuse spectrum in a complex manner which is not well understood. The spectrum produced from specular reflection is a complex, nonlinear function dependent on wavelength, particle size, index of refraction of the particular materials present in the sample, the presence or absence of an absorption band in the surface 30 material and the strength of the band. Moreover, the spectrum obtained from adiffusely reflecting sample may change by simply changing the orientation of thesample or by merely brushing the surface of the sample. At best, specular renectance convolves the Kubelka-Munk relationship with nnother slightly nonlinear function. In certain instances, such ~s for inorganic samples at 35 infrared energies, the effect of specular reflectance is more severe, producing S~ 6 complete Inversion of spectral bands, referred to as a reststrahlen bands, or derivative shaped spectral peaks. The effect of specular reflectance is to make quantitative analysis of the diffuse spectrum an extremely complicated and errorprone undertaking. In many cases, qualitatisle interpretation of the dif fuse 5 reflectance spectrum convolved with specular reflection produces erroneous inIormation as to the composition or concentration of a sample. Thereeore, to obtain ac{urate and useful information, it is highly desirable to eliminate the specular reflectance component from the diffuse reflectance spectra.
As noted abo-!e, specular reflection behaves Iike a mirror with incident 10 energy reflected from a powder surface according to Snell's Law without penetrating into the sample. Any specular component should ideally leave the sample with a smaller angular spread than the diffuse component. Although powdered or granular surfaces adhere to Snell's Law for individual granules, thereflection properties of the aggregate surface can be quite different However, 15 surface preparation techniques could be used to orient the surface granules so that Snell's Law reasonably approximates the reflectance off the surface.
Therefore, certain collection angles could, in principle, contain a pure diffuserenection spectrum, and rotation of the collection mirror nway ~rom a symmetrtcal colleation angle would ellminnte the spec~ r compont~nt of thc 20 energy reflected back from th~ snmple.
Our experiments have shown that specular reflection may indeed have d preferential orientation along the direction predicated by Snell's Law. However,some specularly reflected energy has been found at all angles of reflection. Themagnitude of the specular component over a given angle is a function of the 25 manner in which the sample cup is filled and prepared. Standard practice in examining a powder sample has included drawing a straight edge across the powder surface prior to taking a spectrum. This manner of preparing the surface appears to be highly effective at orienting individual crystals and increases the likelihood that a comparable diffuse reflectance spectrum may be 30 obtained from samples that are identical in composition. However, orienting the surface crystals increases the magnitude of energy that is specularly reflected towards the detector without confining the specular reflection to a particular angle. Thus, while this method might improve repeatability inherent distortions are also repeated.
~L~5~ f~
Roughening the surface of the sample reduces the total amount of specular reflectance directed towards the collector. The roughenin~ may take the form of drawing a camel hair brush o~er the sample or placing a piece of adhesive tape in light contact with the top of the sample and subsequently 5 removing the tape. However, roughening the surface to the same degree is difficult and does not completely eliminate the specular component. Indeed, a completely roughened surface having crystals randomly oriented produces specular reflection over all angles of reflection without a known preferred orientation.
Another method of attemptin~ to eliminate the specular component from a diffuse reflectance spectrum involves diluting the specularly reflective sample in a matrix having no spectral features at the wavelength of the ineidentenergy and no distorted specular reflectance properties. This rnethod is subjectto the limitation of being able to find an inert matrix material that does not have 15 absorptive or reflective properties in the given range of energy. 'I`he method often requires destroying the sample so that It can be mixed with the matri2c materi~l.
'rhe forcgoing discussion demonstrates ~n acknowle~lged need for some means by which to eliminate specular reflection from diffuse reflectance 20 spectra. We have found a particular need for eliminating the distortion caused by specular reflectance in obtaining diffuse reflectance spectra of inorganic compounds. This is because certain inorganic compounds are not suited for infrared analysis by transmissive means and may not be analyzed with any of the foregoing methods due to the extreme distortion of their diffuse reflectance 25 spectrum caused by their inherent specular properties. More generally, there has not hitherto been a simple means for quickly and economically obtaining an undistorted diffuse reflectance spectrum of any sample having specular reflective properties that is not subject to random distortions caused by the orientation of granules on the surface of the sample.
SUMMA~Y OF T~IE INVENTION
The invention is directed to a method and apparatus eor eliminating specular reflectance from the diffuse reflectance spectrum of a reflecting sample. In most general terms9 the method comprises directing energy onto a surface of a sample, positioning a blocking element at the surface of the sample35 to separate specularly reflected energy from diffusely reflected energy so that ~S~ 6 the diffusely reflected energy can be sepurately cs)llected. The apparatus of the invention includes means for providing an input beam of energy, means for focusing and collecting energy onto and from a reference sample and means for eliminating substantially all specularly reflected energy from the energy 5 collected from the sample. More specifically, the invention utilizes a thin blade as the blocking element (also referred to as the blocker) placed in substantial contact with the surface of the sample at the intersection of the surface with the incident energy beam to substantially eliminate the specular re~lection fromthe diffusely reflected energy beam.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic sideview illustrating the light path taken by light impinging upon a diffusely and specularly reflecting sample, Fig. 2 is a bottom view of the focusing ellipsoid in Pig. 1.
Fig. 3 schematically shows a specular blocker devlce according to the 15 present invention in use in conjunction with the apparatus shown in Fig. 1.
Figs, 4A and B show a cross section of an edge of a blocking element contactlng the surface of the sample for use in the arrangement shown in Fig, 3.~ig. 5 show~ a cross section o~ a blocker blade Inserted below the surface of a sampl~.
Figs. ~A and B show an alternative embodiment of the invention.
Fig. 7 represents an alternative embodiment of the blocker shown in Fig. 3.
Figs. 8A and B show a further alternative embodiment of a blocker for use in an arrangement as depicted in Fig. 3.
Figs. 9 and lû show comparison spectra from ground sand showing how the distortions caused by specular reflection are eliminated through using the blocker of the invention.
Fig. 11 provides a comparison of diffusive spectra obtained from silica demonstrating the effect of the blocker according to the present invention at 30 reducing the specular component of diffusive spectra.
Fig. 12 shows a similar comparison to Fig. Il for alumina powder, Fig. 13 shows a spectrum similar to Fig. Il for finely powdered calcite.
Fig. 14 shows comparison spectra with and without the blocker of the present invention for ethylanthroquinone showing how the blocker does not have 35 a substantial effect on the spectrum of a compound lacking a large degree of ~;~S~ 6 specular reflectance.
DETAILED DESCRIPTION OP THE PREFERRED EMBODIMENTS
Figure l snows the effect of diffuse scattering from a sample. An input beam 1 emitted from source ~ reflects off a focusing ellipsoid rnirror 3 onto a 5 sample 5. While infrared light is the most common energy for use in the invention, energy of other wavelengths may be suitable. Reflected energy bounces off sample 5 onto an output half of the ellipsoid rnirror 6 to focus 7 where it is targeted on the detector 8. At most, one-half the input energy reaches the output half of the ellipsoid mirror because the diffusely reflected 10 light from the sample is reflected over all angles.
If sample 5 is replaced with a mirror~ all the reflective energy will fall on region l0 as shown in Pigure 2. Thus, specularly reflected energy from a sample would, under ideal conditions, obey Snells' Law and fall only on re~ion 10.
However, in practice, specularly reflected energy fal1s on both regions l0 and 12 15 of ellipsoid mirror 6 due to random orientation of the crystals on the surface of the sample.
In ~igure 3A blocker 20 acoording to the present invention is positioned over the sarnple 5 which is contained In cup 9. In a preferred cmbodIment of th~invention, the blocker is a straight edged piece of metQl, although any material20 that is opaque to the incident energy may suffice. The blocker should be madefrom material that does not have reflective spectral features in the energy range of the input beam. This requirement is satisfied for the infrared region by goldplating the blocker because gold is completely reflective in the infrared region.
A blocker coated with a material that is completely absorbent of the incident 25 energy could also be employed.
Blocker 20 is positioned relative to sample 5 and the focus of input beam l reflected off focusing ellipsoid mirror 3 to eliminate the specular component of reflected energy from sample 5 by physically blocking the energy reflected from the surface of the sample. Any means for holding the blocker in 30 position can be employed. A preferred means is achieved by mounting the blocker on a pivoting arm which allows it to be moved into position during operation and pivotally out of position to facilitate replacement of the sample.It is important that the blocker contacts the surface of the sample so as to prevent specularly reflected input radiation frorn leaking under the blocker and35 reaching the output half of the ellipsoid 6. The blocker must be separated from S~
the surface by less than a fraction of a wavelength of the incident energy. If the blocker is to prevent specularly reflected input energy from leakin~ under abottom edge of the blocker the physical contact of a metal blocker with the surface of the sample may also advantageously eliminate light waves propagating 5 along the surface of the sample that may form a component of the specularly reflected light~
The intersection of the blocker and the surface of the sample should be located on the sample at the focus of the input energy being reflected off focusing ellipsoid mirror 3. As shown in ~igure 3B, the blocker splits the image10 focal plane of the input beam on the surface of sample 5.
rO obtain maximum efficiency and the closest approximation to the Kubelka-Munk relationship, a blocker should have an edge that is a fraction of the optical depth of the sample. Such a blade may be considered a thin blocker.
A thicker blocker, by removing energy that penetrates only a short distance into15 the sample before reflecting, may have a catastrophic ef~ect on efficiency when used with ~ sample having a shallow optical depth because input energy that penetrates to only ~ shQllow depth may contaln the majority of the encrgy difeused ~nto the samplc. This effect ~9 EIS shown In Figura 4A. Lines 15 represent energy rQys from focused input beam 1 which penetrate the sample 5 20 and are diffusely reflected. As shown by this figure, only a small fraction of the energy escapes from the far side of the blocker. A thick blocker may also introduce spectral distortions caused by energy that is once reflected by the sample to the lower surface of the blocker and again reflected from the blocker to the sample before the energy escapes from the far side of the blocker as 25 shown by line 16 in Figure 4A. Energy thus reflected from the surface of the blocker will acquire any reflectance spectral features of the blocker itself and, thus, distort the output spectrum. Therefore, the blade edge of the blocker device should be made as thin as possible to maximize the efficiency of the device as is shown in Figure 4B minimizing surface spectra of the blocker and 30 permitting a great portion of the diffusively reflected energy 15 to escape from the far side of the blocker.
Efficiency also dictates that tne blocker not penetrate the surface of the sample. As shown in Figure 5A, penetration of the blocker into the sample blocks energy 17 that is reflected from a shallow depth and has an effect that is 35 comparable to employing a thick blocker. In practice, however, it is sometimes , . .
L~6 necessary to break the surface of the sample with the blocker to insure that specularly reflected energy will not leak underneath the blocker at some point along the edge of the blocker as shown by ray 18 in Figure 5B.
The need for penetrating the surface of a sample may be eliminated by 5 smoothing the sample prior to moving the blocker and sample into position. Onemeans of accomplishing this is shown in Figures 6A and B. The flattening window is made of transmissive material 25 having two flattened edges 2~ and 27 positioned on either side adjacent to blocker 20~ The window is positioned on the sample so that face 27 lightly presses or touches the surface of the sample 5 so10 as to smooth any surface irregularities and eliminate any leaking of specularly reflected energy under the blocker.
Figures 7A and 7B show a further embodiment of the invention. An input beam 30 is directed downward by a focusing paraboloid mirror 32 onto the sample at a substantially vertical angle. Energy penetrating the surface which is 1~ diffusively reflected exits the sample and is collected by collection mirror 33.
In this embodiment, the blocker 40 assumes the shape of a cone. The tip of the cone contains an e2~it hole having R diameter equal to the diameter oï the inputbe~m. Speculally re1ectcd llght reflects back of ~ the sur~ace of the s~mple and exits the system by either directly refIecting baclc off input paraboloid mlrror 32 20 or by multiple reflections 41 off the blocker thus removing all specularly reflected light from output beam 45.
- Figure 8 shows a further ernbodiment of the invention. Blocker 50 has three edge surfaces. Surfaces 52 and 54 are straight edges. Surface 5G is arcuate-shaped to match the shape of the edge of the input beam when focused 25 on the surface of the sample. The advantages to the arcuate shape will lie inpotentially obtaining greater efficiency of use from the input beam in that the entire beam reaches the surface of the sample rather than having half of the beam reflected off the blocl;er as is the case for the blocker shown in Figure 3.
In general, the ideal optical arrangement for the blocker device has a 30 symmetrical design where no attempt is made to otherwise optically exclude specular energy. Therefore, all energy received by the output half of the - ellipsoid mirror 6 in Figure l should be directed to focus 7. The blocker has proven so effective at removing the specular component of reflected energy that no need exists to discriminate between light received by areas 10 and 12 in Figure 35 2. Experiment has shown that efficiency is a major consideration in obtaining diffuse reflectance spectra using the blocker device. Some samples, particularlyfinely powdered samples, do not permit the input energy beam to penetrate more than a few wavelengths into the sample. A high percentage of the energy reflected from such a sample is specular. Therefore, obtaining a spectrum within5 a reasonable tims period requires a collector to operate at maximum efficiency, and efficiency is enhanced by utilizing aU energy diEfusely reflected from the sample. It is also suggested that high efficiency detectors be employed to measure the resultant output.
It is believed that the blocker will result in the linearization of the 10 output spectrum according to the Kubelka-Munk formula since present theory maintains that deviations of diffuse reflectance spectra from the Kubelka-Munk relationship are produced solely or primarily by specular renection.
Figures 9-14 are comparisons of inîrared diffuse reflectance spectra for various materials obtained with and without the blocker of Figure 3. Figure 9 is15 à comparison of an infrared spectrum of ne:atl ground sand obtained by diffuse reflectance. Curve a i5 a measure of the spectrum obtained without Q blocker device using the cntlre output halî of the ellipsoid mirror 6 of Figuro 2~ Curve b i9 a sp~ctrum obtain~d by maskIng region 10 of the olIlpso~cl m~rror ~i. Curv~ c ~9 the spectrum produced when region 12 is maslced. Specular distortion of the 20 diffuse reflectance spectrum is present in all three cases. Curve d shows that a dramatic reduction in the specular reflectance can be obtained by roughening thesurface of the sample.
Figure 10 shows a comparison between curve a of Figure 9, i.e., the diffuse spectrum employing the full output half of the ellipsoid mirror 6, and a25 diffuse spectrum e obtained using the blocker device. Curve d in Figure 9 obtained by roughening the surface produces markedly less specular distortion.
However, spectrum e of Figure 10 contains substantially less specular distortionthan even curve d. This effect is most noticeable by comparing the peak value ofthe spectral feature labelled generally 100. Spectral feature 100 is pronounced in 30 curve a containing the highest specular component. Even curve d contains someelement of this distortion. Spectral feature 100 is completely absent in curve e.
Thus, spectral feature 100 is an example of the nature of the distortion introduced into a diffuse reflectance spectrum by specular reflection. Note thatIn Figure 10 spectral feature lOI appears to be an example of a band inversion or 35 reststrahlen band.
~5S~
Figure 11 shows another infrared comparison spectrum of silica. The upper line shows the spectrum having a specular component. The lower spec$rum shows the spectru!n obtained with the use of the blocker. Note again llow spectrum feature 105 totally disappears with the use of the blocker device.Sim~larly, Figure 12 shows comparison spectra ~or alumina. Note how specular reflectance feature 110 disappears in the diffuse reflectance spectra obtained with the blocker device.
Figure 13 shows comparison diffuse reflectance spectra for finely powdered calcite. The energy distribution across wavelengths between the two spectra is clearly different.
The foregoing figures illustrate the ubiquitous presence of specular reflection distortion in infrared diffuse reflectance spectroscopy and how this distortion can be eliminated through use of the blocker device.
Not all diffusely reflecting compounds produce specular distortion.
Figure I~L is a comparison spectrum of aqlleous ethylanthroquinona. ~his solution, as is true ~or most organlo compounds, does not exhibit spectral di~tortlons ca~ls~d by sp~culllr re~l~ction du~ to the ~bsence of strong reYonance bands at infrared frequencies. Thus, the blocker does not significlntly ch~nge the spectrum obtained through unblocked diffuse reflectance spectroscopy.
Operation of the blocker device according to the present invention has been descibed by way of example as applied to infrared spectroscopy. As noted, the blocker has particular utility in the field of infrared diffuse spectroscopy of inorganic compounds. ~Iowever, the foregoing specification enables one of ordinary skill in the art to apply the principles of the blocker device to any 25 instance where an energy beam consisting of a particle demonstrating significant quantum mechanical wave properties is incident on the surface of a material thatreflects the incident beam at both the surface of the material and from below the surface. Therefore, the invention which is intended to be protected herein should not be construed as limited to the particular forms described, as these are 30 to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of thepresent invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not as limiting to the scope and spirit of the invention as set forth in the appended claims.
._ FROM A DIFFU~E REFLECTIC~N SPECTRUM
Field of the Invention This invention deals with a method and apparatus for obtaining diffuse reflectance spectra, wherein the specular component of the diffuse spectra is substantially eliminated. Distortions in the diffuse refl~ctance spectr~ ca~lsed5 by the specular component are thus eliminRted.
Back~rround of the Invention _.. . . _ It is kr1own thnt partlcul~r compounds possess uniqu~ spectral signatures. One method to obtain a spectrum indicative of a particular compound is transmission spectroscopy. A transmission spectrum can be 10 obtained by transmitting an energy beam of known intensity and frequency through an at least partially transmissive sample and recorcling the intensity of the energy transmitted through the sample at various incident wRvelengths.
This method works quite well for a wide range of compounas having known transmission spectra. More recent technology for obtaining infrared spectra 15 uses interferometers and computers in what is commonly called Fourier Transform Infrared (FTIE~) spectroscopy~ This technology has proved to have significant advantages over prior art methods of obtaining infrared spectra. A
transmission spectrum cannot, generally, be obtained for a comp~und composed of powder grains or small size granules, particularly when the powder is 20 substantially opaque to the frequencies of the incident energy at common granule thickness. One solution in such an instance is to embed the powder or granules in a matrix not having spectral features in the frequency range of the incident energy beam. This method only works so long as a suitable matrix compound can be found and the powder or granules are not so opaque as to ~bsorb all input ~...
~2~ 6 energy when the matrix contains a sufficient density of the sample to produce a meaningful spe¢trum from the sample.
Another solution to the problem associated with powders or granular samples is to obtain a diffuse reflectance spectrum of the sample. A diffuse 5 reflectance spectrum is obtained by directing an input energy beam onto the surface of the sample, collecting the diffusely reflected energy from the sampleand directing that energy to a detector. Diffusely reflected energy is energy which is defined to be reflected from below the surface of ~he sample. The energy diffusely reflected from a sample does not have a preferred direction of 10 reflection, i.e., the diffusely reflected energy leaves the sample surface in a hemispherical pattern. The diffusively reflected energy has spectral characteristics that uniquely iden$ify the sample compounds and correspond to the spectrum obtained by transmissive means.
In addition to diffuse reflection, however, an energy beam directed 15 against the surface of a sample produces specular reflection. Specular reflection is defined to be incident energy that is reflected from a surface of a sample as opposed to difYusely reflected energy which i9 energy refl~cted from below the sample surface. Spccular reflectance obeys Snell's l.aw Wlllch states that the angle of incidence of the input energy beam equals the angle of 20 reflectance. In other words, energy that is specularly reflected behaves us light refIected from a mirror. Thus, if all the crystals on the surface of a powder sample were oriented so as to present a homogenous reflective face to the incident energy that was parallel to the plane of the sample, the incident energy would reflect off the surface of the sample according to Snell's Law. However, 25 the reflecting surfaces of individual crystals on the surface of a powder sample are somewhat randomly oriented and, therefore, scatter the incident energy over an entire hemisphere as is the case for diffuse reflection. Our experiments and those of others have shown that the crystals on the surface of a sample may often be oriented so as to produce a preferred direction of reflection.
30 Nevertheless, some of the incident energy beam is nearly always specularly reflected over all angles of reflection. A detailed discussion of the spectrometry of powdered samples is found in Griffiths et al, Advances in Infrared and l~amanSpectroscopy, Vol. 9, Chapter 2, (Heyden, London 1981) the disclosure of which is hereby incorporated by reference.
.12SS~
Conventional analysis of diffuse reflectance spectra employs the Kubelka-Munk function. The Kubelka-Munk function states that the strength of an absorption feature in a diffuse reflectance spectrunn is linearly related to the concentration of the compound producing the spectral feature. The function 5 invol~1es a relationship between an absorption coefficient, a scattering coefficient and the ratio of the diffuse reflectance from a sample and that of anon~absorbing powder reference~ The funetion assumes that the sample extends to an infinite optical àepth, i.e., that depth at which the addition of more sample material to the bottom of the sample does not change the amount of energy 10 diffusely reflected. In theory, the Kubelka-Munk function should enable a spectrum obtained by diffuse reflectance to be compared to a spectrum obtained by transmissive means. The transmission spectrum for many compounds is known. ~ence, the ability to identify a compound from its diffuse reflectance spectrum given a known transmission spectrum enables diffuse reflectance 15 spectroscopy to accurately identjfy trace elements present in powdered samples th~t do not lend themselves to direct tl~ansmission spectroscopy. a specific example of the applicatlon of diffuse refleetion spectrosaopy ls In quality control of pharmac~utical tablets to ~liminate the need to grind up the tablets flnd embed them in a non~absorbtive matrix. Moreover, it i9 belleved that it will oe 20 possible to apply spectral substraction routines commonly used in absorption spectroscopy to diffuse reflectance measurements to identiy trace elements in the sample and to accurately establish their concentration in the sample.
The linearity of the Kubelka-Munk relationship for the strength of an absorption feature with concentration, however, breaks down for experimental 25 conditions involving specular reflectance~ Specular reflection alters a diffuse spectrum in a complex manner which is not well understood. The spectrum produced from specular reflection is a complex, nonlinear function dependent on wavelength, particle size, index of refraction of the particular materials present in the sample, the presence or absence of an absorption band in the surface 30 material and the strength of the band. Moreover, the spectrum obtained from adiffusely reflecting sample may change by simply changing the orientation of thesample or by merely brushing the surface of the sample. At best, specular renectance convolves the Kubelka-Munk relationship with nnother slightly nonlinear function. In certain instances, such ~s for inorganic samples at 35 infrared energies, the effect of specular reflectance is more severe, producing S~ 6 complete Inversion of spectral bands, referred to as a reststrahlen bands, or derivative shaped spectral peaks. The effect of specular reflectance is to make quantitative analysis of the diffuse spectrum an extremely complicated and errorprone undertaking. In many cases, qualitatisle interpretation of the dif fuse 5 reflectance spectrum convolved with specular reflection produces erroneous inIormation as to the composition or concentration of a sample. Thereeore, to obtain ac{urate and useful information, it is highly desirable to eliminate the specular reflectance component from the diffuse reflectance spectra.
As noted abo-!e, specular reflection behaves Iike a mirror with incident 10 energy reflected from a powder surface according to Snell's Law without penetrating into the sample. Any specular component should ideally leave the sample with a smaller angular spread than the diffuse component. Although powdered or granular surfaces adhere to Snell's Law for individual granules, thereflection properties of the aggregate surface can be quite different However, 15 surface preparation techniques could be used to orient the surface granules so that Snell's Law reasonably approximates the reflectance off the surface.
Therefore, certain collection angles could, in principle, contain a pure diffuserenection spectrum, and rotation of the collection mirror nway ~rom a symmetrtcal colleation angle would ellminnte the spec~ r compont~nt of thc 20 energy reflected back from th~ snmple.
Our experiments have shown that specular reflection may indeed have d preferential orientation along the direction predicated by Snell's Law. However,some specularly reflected energy has been found at all angles of reflection. Themagnitude of the specular component over a given angle is a function of the 25 manner in which the sample cup is filled and prepared. Standard practice in examining a powder sample has included drawing a straight edge across the powder surface prior to taking a spectrum. This manner of preparing the surface appears to be highly effective at orienting individual crystals and increases the likelihood that a comparable diffuse reflectance spectrum may be 30 obtained from samples that are identical in composition. However, orienting the surface crystals increases the magnitude of energy that is specularly reflected towards the detector without confining the specular reflection to a particular angle. Thus, while this method might improve repeatability inherent distortions are also repeated.
~L~5~ f~
Roughening the surface of the sample reduces the total amount of specular reflectance directed towards the collector. The roughenin~ may take the form of drawing a camel hair brush o~er the sample or placing a piece of adhesive tape in light contact with the top of the sample and subsequently 5 removing the tape. However, roughening the surface to the same degree is difficult and does not completely eliminate the specular component. Indeed, a completely roughened surface having crystals randomly oriented produces specular reflection over all angles of reflection without a known preferred orientation.
Another method of attemptin~ to eliminate the specular component from a diffuse reflectance spectrum involves diluting the specularly reflective sample in a matrix having no spectral features at the wavelength of the ineidentenergy and no distorted specular reflectance properties. This rnethod is subjectto the limitation of being able to find an inert matrix material that does not have 15 absorptive or reflective properties in the given range of energy. 'I`he method often requires destroying the sample so that It can be mixed with the matri2c materi~l.
'rhe forcgoing discussion demonstrates ~n acknowle~lged need for some means by which to eliminate specular reflection from diffuse reflectance 20 spectra. We have found a particular need for eliminating the distortion caused by specular reflectance in obtaining diffuse reflectance spectra of inorganic compounds. This is because certain inorganic compounds are not suited for infrared analysis by transmissive means and may not be analyzed with any of the foregoing methods due to the extreme distortion of their diffuse reflectance 25 spectrum caused by their inherent specular properties. More generally, there has not hitherto been a simple means for quickly and economically obtaining an undistorted diffuse reflectance spectrum of any sample having specular reflective properties that is not subject to random distortions caused by the orientation of granules on the surface of the sample.
SUMMA~Y OF T~IE INVENTION
The invention is directed to a method and apparatus eor eliminating specular reflectance from the diffuse reflectance spectrum of a reflecting sample. In most general terms9 the method comprises directing energy onto a surface of a sample, positioning a blocking element at the surface of the sample35 to separate specularly reflected energy from diffusely reflected energy so that ~S~ 6 the diffusely reflected energy can be sepurately cs)llected. The apparatus of the invention includes means for providing an input beam of energy, means for focusing and collecting energy onto and from a reference sample and means for eliminating substantially all specularly reflected energy from the energy 5 collected from the sample. More specifically, the invention utilizes a thin blade as the blocking element (also referred to as the blocker) placed in substantial contact with the surface of the sample at the intersection of the surface with the incident energy beam to substantially eliminate the specular re~lection fromthe diffusely reflected energy beam.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic sideview illustrating the light path taken by light impinging upon a diffusely and specularly reflecting sample, Fig. 2 is a bottom view of the focusing ellipsoid in Pig. 1.
Fig. 3 schematically shows a specular blocker devlce according to the 15 present invention in use in conjunction with the apparatus shown in Fig. 1.
Figs, 4A and B show a cross section of an edge of a blocking element contactlng the surface of the sample for use in the arrangement shown in Fig, 3.~ig. 5 show~ a cross section o~ a blocker blade Inserted below the surface of a sampl~.
Figs. ~A and B show an alternative embodiment of the invention.
Fig. 7 represents an alternative embodiment of the blocker shown in Fig. 3.
Figs. 8A and B show a further alternative embodiment of a blocker for use in an arrangement as depicted in Fig. 3.
Figs. 9 and lû show comparison spectra from ground sand showing how the distortions caused by specular reflection are eliminated through using the blocker of the invention.
Fig. 11 provides a comparison of diffusive spectra obtained from silica demonstrating the effect of the blocker according to the present invention at 30 reducing the specular component of diffusive spectra.
Fig. 12 shows a similar comparison to Fig. Il for alumina powder, Fig. 13 shows a spectrum similar to Fig. Il for finely powdered calcite.
Fig. 14 shows comparison spectra with and without the blocker of the present invention for ethylanthroquinone showing how the blocker does not have 35 a substantial effect on the spectrum of a compound lacking a large degree of ~;~S~ 6 specular reflectance.
DETAILED DESCRIPTION OP THE PREFERRED EMBODIMENTS
Figure l snows the effect of diffuse scattering from a sample. An input beam 1 emitted from source ~ reflects off a focusing ellipsoid rnirror 3 onto a 5 sample 5. While infrared light is the most common energy for use in the invention, energy of other wavelengths may be suitable. Reflected energy bounces off sample 5 onto an output half of the ellipsoid rnirror 6 to focus 7 where it is targeted on the detector 8. At most, one-half the input energy reaches the output half of the ellipsoid mirror because the diffusely reflected 10 light from the sample is reflected over all angles.
If sample 5 is replaced with a mirror~ all the reflective energy will fall on region l0 as shown in Pigure 2. Thus, specularly reflected energy from a sample would, under ideal conditions, obey Snells' Law and fall only on re~ion 10.
However, in practice, specularly reflected energy fal1s on both regions l0 and 12 15 of ellipsoid mirror 6 due to random orientation of the crystals on the surface of the sample.
In ~igure 3A blocker 20 acoording to the present invention is positioned over the sarnple 5 which is contained In cup 9. In a preferred cmbodIment of th~invention, the blocker is a straight edged piece of metQl, although any material20 that is opaque to the incident energy may suffice. The blocker should be madefrom material that does not have reflective spectral features in the energy range of the input beam. This requirement is satisfied for the infrared region by goldplating the blocker because gold is completely reflective in the infrared region.
A blocker coated with a material that is completely absorbent of the incident 25 energy could also be employed.
Blocker 20 is positioned relative to sample 5 and the focus of input beam l reflected off focusing ellipsoid mirror 3 to eliminate the specular component of reflected energy from sample 5 by physically blocking the energy reflected from the surface of the sample. Any means for holding the blocker in 30 position can be employed. A preferred means is achieved by mounting the blocker on a pivoting arm which allows it to be moved into position during operation and pivotally out of position to facilitate replacement of the sample.It is important that the blocker contacts the surface of the sample so as to prevent specularly reflected input radiation frorn leaking under the blocker and35 reaching the output half of the ellipsoid 6. The blocker must be separated from S~
the surface by less than a fraction of a wavelength of the incident energy. If the blocker is to prevent specularly reflected input energy from leakin~ under abottom edge of the blocker the physical contact of a metal blocker with the surface of the sample may also advantageously eliminate light waves propagating 5 along the surface of the sample that may form a component of the specularly reflected light~
The intersection of the blocker and the surface of the sample should be located on the sample at the focus of the input energy being reflected off focusing ellipsoid mirror 3. As shown in ~igure 3B, the blocker splits the image10 focal plane of the input beam on the surface of sample 5.
rO obtain maximum efficiency and the closest approximation to the Kubelka-Munk relationship, a blocker should have an edge that is a fraction of the optical depth of the sample. Such a blade may be considered a thin blocker.
A thicker blocker, by removing energy that penetrates only a short distance into15 the sample before reflecting, may have a catastrophic ef~ect on efficiency when used with ~ sample having a shallow optical depth because input energy that penetrates to only ~ shQllow depth may contaln the majority of the encrgy difeused ~nto the samplc. This effect ~9 EIS shown In Figura 4A. Lines 15 represent energy rQys from focused input beam 1 which penetrate the sample 5 20 and are diffusely reflected. As shown by this figure, only a small fraction of the energy escapes from the far side of the blocker. A thick blocker may also introduce spectral distortions caused by energy that is once reflected by the sample to the lower surface of the blocker and again reflected from the blocker to the sample before the energy escapes from the far side of the blocker as 25 shown by line 16 in Figure 4A. Energy thus reflected from the surface of the blocker will acquire any reflectance spectral features of the blocker itself and, thus, distort the output spectrum. Therefore, the blade edge of the blocker device should be made as thin as possible to maximize the efficiency of the device as is shown in Figure 4B minimizing surface spectra of the blocker and 30 permitting a great portion of the diffusively reflected energy 15 to escape from the far side of the blocker.
Efficiency also dictates that tne blocker not penetrate the surface of the sample. As shown in Figure 5A, penetration of the blocker into the sample blocks energy 17 that is reflected from a shallow depth and has an effect that is 35 comparable to employing a thick blocker. In practice, however, it is sometimes , . .
L~6 necessary to break the surface of the sample with the blocker to insure that specularly reflected energy will not leak underneath the blocker at some point along the edge of the blocker as shown by ray 18 in Figure 5B.
The need for penetrating the surface of a sample may be eliminated by 5 smoothing the sample prior to moving the blocker and sample into position. Onemeans of accomplishing this is shown in Figures 6A and B. The flattening window is made of transmissive material 25 having two flattened edges 2~ and 27 positioned on either side adjacent to blocker 20~ The window is positioned on the sample so that face 27 lightly presses or touches the surface of the sample 5 so10 as to smooth any surface irregularities and eliminate any leaking of specularly reflected energy under the blocker.
Figures 7A and 7B show a further embodiment of the invention. An input beam 30 is directed downward by a focusing paraboloid mirror 32 onto the sample at a substantially vertical angle. Energy penetrating the surface which is 1~ diffusively reflected exits the sample and is collected by collection mirror 33.
In this embodiment, the blocker 40 assumes the shape of a cone. The tip of the cone contains an e2~it hole having R diameter equal to the diameter oï the inputbe~m. Speculally re1ectcd llght reflects back of ~ the sur~ace of the s~mple and exits the system by either directly refIecting baclc off input paraboloid mlrror 32 20 or by multiple reflections 41 off the blocker thus removing all specularly reflected light from output beam 45.
- Figure 8 shows a further ernbodiment of the invention. Blocker 50 has three edge surfaces. Surfaces 52 and 54 are straight edges. Surface 5G is arcuate-shaped to match the shape of the edge of the input beam when focused 25 on the surface of the sample. The advantages to the arcuate shape will lie inpotentially obtaining greater efficiency of use from the input beam in that the entire beam reaches the surface of the sample rather than having half of the beam reflected off the blocl;er as is the case for the blocker shown in Figure 3.
In general, the ideal optical arrangement for the blocker device has a 30 symmetrical design where no attempt is made to otherwise optically exclude specular energy. Therefore, all energy received by the output half of the - ellipsoid mirror 6 in Figure l should be directed to focus 7. The blocker has proven so effective at removing the specular component of reflected energy that no need exists to discriminate between light received by areas 10 and 12 in Figure 35 2. Experiment has shown that efficiency is a major consideration in obtaining diffuse reflectance spectra using the blocker device. Some samples, particularlyfinely powdered samples, do not permit the input energy beam to penetrate more than a few wavelengths into the sample. A high percentage of the energy reflected from such a sample is specular. Therefore, obtaining a spectrum within5 a reasonable tims period requires a collector to operate at maximum efficiency, and efficiency is enhanced by utilizing aU energy diEfusely reflected from the sample. It is also suggested that high efficiency detectors be employed to measure the resultant output.
It is believed that the blocker will result in the linearization of the 10 output spectrum according to the Kubelka-Munk formula since present theory maintains that deviations of diffuse reflectance spectra from the Kubelka-Munk relationship are produced solely or primarily by specular renection.
Figures 9-14 are comparisons of inîrared diffuse reflectance spectra for various materials obtained with and without the blocker of Figure 3. Figure 9 is15 à comparison of an infrared spectrum of ne:atl ground sand obtained by diffuse reflectance. Curve a i5 a measure of the spectrum obtained without Q blocker device using the cntlre output halî of the ellipsoid mirror 6 of Figuro 2~ Curve b i9 a sp~ctrum obtain~d by maskIng region 10 of the olIlpso~cl m~rror ~i. Curv~ c ~9 the spectrum produced when region 12 is maslced. Specular distortion of the 20 diffuse reflectance spectrum is present in all three cases. Curve d shows that a dramatic reduction in the specular reflectance can be obtained by roughening thesurface of the sample.
Figure 10 shows a comparison between curve a of Figure 9, i.e., the diffuse spectrum employing the full output half of the ellipsoid mirror 6, and a25 diffuse spectrum e obtained using the blocker device. Curve d in Figure 9 obtained by roughening the surface produces markedly less specular distortion.
However, spectrum e of Figure 10 contains substantially less specular distortionthan even curve d. This effect is most noticeable by comparing the peak value ofthe spectral feature labelled generally 100. Spectral feature 100 is pronounced in 30 curve a containing the highest specular component. Even curve d contains someelement of this distortion. Spectral feature 100 is completely absent in curve e.
Thus, spectral feature 100 is an example of the nature of the distortion introduced into a diffuse reflectance spectrum by specular reflection. Note thatIn Figure 10 spectral feature lOI appears to be an example of a band inversion or 35 reststrahlen band.
~5S~
Figure 11 shows another infrared comparison spectrum of silica. The upper line shows the spectrum having a specular component. The lower spec$rum shows the spectru!n obtained with the use of the blocker. Note again llow spectrum feature 105 totally disappears with the use of the blocker device.Sim~larly, Figure 12 shows comparison spectra ~or alumina. Note how specular reflectance feature 110 disappears in the diffuse reflectance spectra obtained with the blocker device.
Figure 13 shows comparison diffuse reflectance spectra for finely powdered calcite. The energy distribution across wavelengths between the two spectra is clearly different.
The foregoing figures illustrate the ubiquitous presence of specular reflection distortion in infrared diffuse reflectance spectroscopy and how this distortion can be eliminated through use of the blocker device.
Not all diffusely reflecting compounds produce specular distortion.
Figure I~L is a comparison spectrum of aqlleous ethylanthroquinona. ~his solution, as is true ~or most organlo compounds, does not exhibit spectral di~tortlons ca~ls~d by sp~culllr re~l~ction du~ to the ~bsence of strong reYonance bands at infrared frequencies. Thus, the blocker does not significlntly ch~nge the spectrum obtained through unblocked diffuse reflectance spectroscopy.
Operation of the blocker device according to the present invention has been descibed by way of example as applied to infrared spectroscopy. As noted, the blocker has particular utility in the field of infrared diffuse spectroscopy of inorganic compounds. ~Iowever, the foregoing specification enables one of ordinary skill in the art to apply the principles of the blocker device to any 25 instance where an energy beam consisting of a particle demonstrating significant quantum mechanical wave properties is incident on the surface of a material thatreflects the incident beam at both the surface of the material and from below the surface. Therefore, the invention which is intended to be protected herein should not be construed as limited to the particular forms described, as these are 30 to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of thepresent invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not as limiting to the scope and spirit of the invention as set forth in the appended claims.
Claims (20)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for obtaining a diffuse reflectance spectra comprising:
means for providing an incident energy beam;
means for directing the incident energy beam to a sample;
means for collecting energy that is diffusely reflected from the sample;
blocking means positioned on or adjacent to the sample to substantially eliminate energy that is specularly reflected off the sample.
means for providing an incident energy beam;
means for directing the incident energy beam to a sample;
means for collecting energy that is diffusely reflected from the sample;
blocking means positioned on or adjacent to the sample to substantially eliminate energy that is specularly reflected off the sample.
2. The apparatus of claim 1, wherein:
the means for providing an incident energy beam to the sample includes means for focusing the incident energy beam substantially at a surface of the sample; and the means for collecting the energy that is diffusely reflected from the sample includes means for focusing the reflected energy onto a detector means.
the means for providing an incident energy beam to the sample includes means for focusing the incident energy beam substantially at a surface of the sample; and the means for collecting the energy that is diffusely reflected from the sample includes means for focusing the reflected energy onto a detector means.
3. The apparatus of claim 2, wherein:
the incident energy beam contains infrared radiation; and the detector is used for obtaining a diffuse reflectance spectrum of the sample.
the incident energy beam contains infrared radiation; and the detector is used for obtaining a diffuse reflectance spectrum of the sample.
4. The apparatus of claim 2, wherein the blocking means on or adjacent to the sample does not penetrate the surface of the sample.
5. The apparatus of claim 2, wherein said blocking means is separated from a surface of the sample by a distance that is less than a wavelength of the energy of the incident energy beam.
6. The apparatus of claim 2, wherein said blocking means has a thickness that is not substantially greater than the average penetration of the incident energy beam into the sample.
7. The apparatus of claim 2, wherein said blocking means is a straight-edged object that extends substantially beyond edges of the incident energy beam.
8. The apparatus of claim 2, wherein:
said blocking means is generally conical in shape;
and the means for directing the incident energy beam to the sample is positioned such that the incident energy impinges substantially normal to the sample through an aperture in the blocking means.
said blocking means is generally conical in shape;
and the means for directing the incident energy beam to the sample is positioned such that the incident energy impinges substantially normal to the sample through an aperture in the blocking means.
9. The apparatus of claim 2, wherein the blocking means is arcuate-shaped to substantially match an outer edge of at least one-half of a shape of the incident energy beam on the surface of the sample.
10. An apparatus responsive to a distribution of energy in a reflectance spectra, comprising:
means for directing an incident energy beam to a sample;
means for receiving energy reflected from the sample; and blocking means positioned on or adjacent to the sample to block out energy specularly reflected off the sample.
means for directing an incident energy beam to a sample;
means for receiving energy reflected from the sample; and blocking means positioned on or adjacent to the sample to block out energy specularly reflected off the sample.
11. The apparatus of claim 10, wherein:
the means for receiving and responding to energy reflected from the sample includes detector means;
the detector means receives energy containing the reflectance spectra of the sample and the sample specularly reflects energy which produces distortion of the distribution of energy in the reflectance spectra of the sample; and the blocking means substantially eliminates the distortion in the reflectance spectra received by the detector means by blocking out the energy specularly reflected off the sample.
the means for receiving and responding to energy reflected from the sample includes detector means;
the detector means receives energy containing the reflectance spectra of the sample and the sample specularly reflects energy which produces distortion of the distribution of energy in the reflectance spectra of the sample; and the blocking means substantially eliminates the distortion in the reflectance spectra received by the detector means by blocking out the energy specularly reflected off the sample.
12. The apparatus of claim 11, wherein:
the blocking means is at a distance from the sample such that the specularly reflected energy from the sample is substantially eliminated without also substantially eliminating energy from the sample which is not specularly reflected; and the detector means is used in obtaining a diffuse reflectance spectra from at least a portion of the energy not specularly reflected from the sample.
the blocking means is at a distance from the sample such that the specularly reflected energy from the sample is substantially eliminated without also substantially eliminating energy from the sample which is not specularly reflected; and the detector means is used in obtaining a diffuse reflectance spectra from at least a portion of the energy not specularly reflected from the sample.
13. The apparatus of claim 11, wherein the substantial elimination of the distortion in the reflectance spectra received by the detector causes at least a portion of the energy in the spectra to vary linearly with a concentration of a composition of matter in the sample producing the absorption feature.
14. The apparatus of claim 11, wherein the substantial elimination of the distortion in the reflectance spectra received by the detector produces a reproducible spectra of the sample that is indicative of a composition of or a concentration of a substance contained in the sample.
15. A method for blocking out specularly reflected energy, comprising:
directing energy to a sample;
positioning a blocking element on or adjacent to the sample so that the specularly reflected energy from a surface of the sample is separated from energy diffusely reflected from the sample;
collecting the energy diffusely reflected from the sample.
directing energy to a sample;
positioning a blocking element on or adjacent to the sample so that the specularly reflected energy from a surface of the sample is separated from energy diffusely reflected from the sample;
collecting the energy diffusely reflected from the sample.
16. The method of claim 15, wherein:
the directing of the energy to the sample comprises focusing a beam of energy onto the surface of the sample;
the positioning of the blocking element includes positioning an edge of the blocking element at a distance from the surface of the sample not substantially greater than a wavelength of the energy;
the collecting of the diffusely reflected energy includes focusing the diffusely reflected energy to a detector.
the directing of the energy to the sample comprises focusing a beam of energy onto the surface of the sample;
the positioning of the blocking element includes positioning an edge of the blocking element at a distance from the surface of the sample not substantially greater than a wavelength of the energy;
the collecting of the diffusely reflected energy includes focusing the diffusely reflected energy to a detector.
17. The method of claim 16, wherein the detector is used to obtain a diffuse reflectance spectrum of the sample.
18. The method of claim 17, wherein the energy is at infrared wavelengths.
19. The method of claim 16, wherein the positioning of the blocking element on or adjacent to the surface of the sample does not cause the edge of the blocking element to penetrate into the sample.
20. The method of claim 16, wherein:
the positioning of the blocking element is determined by a predetermined positioning of the collecting means; and the positioning of the collecting means determines a positioning of the sample.
the positioning of the blocking element is determined by a predetermined positioning of the collecting means; and the positioning of the collecting means determines a positioning of the sample.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/705,201 US4661706A (en) | 1985-02-25 | 1985-02-25 | Blocker device for eliminating specular reflectance from a diffuse reflection spectrum |
| US705,201 | 1985-02-25 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1255946A true CA1255946A (en) | 1989-06-20 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000501488A Expired CA1255946A (en) | 1985-02-25 | 1986-02-10 | Blocker device for eliminating specular reflectance from a diffuse reflection spectrum |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4661706A (en) |
| EP (1) | EP0200302B1 (en) |
| JP (1) | JPS61196138A (en) |
| CN (1) | CN86101254A (en) |
| CA (1) | CA1255946A (en) |
| DE (1) | DE3663307D1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4764676A (en) * | 1986-10-20 | 1988-08-16 | Laser Precision Corporation | Apparatus for spectral analysis of chromatographic fractions |
| US4853542A (en) * | 1987-06-08 | 1989-08-01 | Nicolas J. Harrick | Collecting hemispherical attachment for spectrophotometry |
| US4965454A (en) * | 1988-01-21 | 1990-10-23 | Hitachi, Ltd. | Method and apparatus for detecting foreign particle |
| US4859064A (en) * | 1988-05-09 | 1989-08-22 | Spectra-Tech, Inc. | Diffuse reflectance spectroscopy system and method |
| US5058982A (en) * | 1989-06-21 | 1991-10-22 | Orbot Systems Ltd. | Illumination system and inspection apparatus including same |
| JPH064841B2 (en) * | 1989-06-28 | 1994-01-19 | 日本鋼管株式会社 | Method of manufacturing infrared scattering agent |
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-
1985
- 1985-02-25 US US06/705,201 patent/US4661706A/en not_active Expired - Lifetime
-
1986
- 1986-02-10 CA CA000501488A patent/CA1255946A/en not_active Expired
- 1986-02-24 DE DE8686301307T patent/DE3663307D1/en not_active Expired
- 1986-02-24 EP EP86301307A patent/EP0200302B1/en not_active Expired
- 1986-02-25 CN CN198686101254A patent/CN86101254A/en active Pending
- 1986-02-25 JP JP61040151A patent/JPS61196138A/en active Pending
Also Published As
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|---|---|
| DE3663307D1 (en) | 1989-06-15 |
| US4661706A (en) | 1987-04-28 |
| JPS61196138A (en) | 1986-08-30 |
| EP0200302B1 (en) | 1989-05-10 |
| CN86101254A (en) | 1986-10-22 |
| EP0200302A1 (en) | 1986-11-05 |
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