WO2017137972A1

WO2017137972A1 - Infrared detection and imaging device with no moving parts

Info

Publication number: WO2017137972A1
Application number: PCT/IL2016/050173
Authority: WO
Inventors: Dario Cabib; Amir Gil
Original assignee: Ci Systems (Israel) Ltd.
Priority date: 2016-02-14
Filing date: 2016-02-14
Publication date: 2017-08-17
Also published as: IL261129A

Abstract

A device images radiation from a scene. A detector is sensitive to the radiation in a first wavelength band. A lens forms an image of the scene on the detector. A filtering arrangement includes two sets of radiation absorbing molecules. A control unit switches the filtering arrangement between two states, in the first state, all of the radiation in the first wavelength band is transmitted to the detector, in the second state, the radiation in a second wavelength band within the first wavelength band is absorbed by the radiation absorbing molecules. The control unit synchronizes the switching of the filtering arrangement with the detector. Each pixel of the image formed on the detector includes two signals. The first signal includes information from the scene radiation in the first wavelength band. The second signal excludes information from the scene radiation absorbed by the filtering arrangement in the second wavelength band.

Description

TITLE

Infrared Detection and imaging Device with No Moving Parts

TECHNICAL FIELD

The present invention relates to the detection and imaging of infrared radiation,

BACKGROUND OF THE INVENTION

The last twenty-five years have seen a very large amount of work done in the field of multispectral and spectral imaging all around the world, in industry and academia. The motivation is to collect spatially and spectrally resolved radiance information of a predefined egion of space. The number and fields of application of such information are innumerable; as a esult, the different optical configurations used in the design of instrumentation built for this purpose have also been innumerable, in order to provide the needed information through a method and format most suitable for the relevant application. Instrumentation has been built for aboratory and field use, industrial and military use, on land, on the sea, from the air and in pace. Such types of instrumentation are based in the visible spectral range (400-750 nanometers), and in the various regions of infrared (the Near Infrared (NIR) range of 750-2500 anometers, the Mid Wave Infrared (MWIR) range of 2500-5000 nanometers, and the Long Wave Infrared (LWIR) of 5000-14000), Instrumentation of spectral imaging has been built for nalysis of microscopic samples in hospital environments, as well as of distant cosmic objectshrough large astronomical telescopes. The size of the analyzed region of space and spatialesolution also vary widely, as well as the spectral resolution, depending on the type of detector sed (i.e. the size, speed, sensitivity and number of resolution elements (pixels) that the detector rovides). All work done in this field allows the acquisition of knowledge of the spatial istribution (the imaging side) of the material constituents (the spectroscopic side) in a redefined region of space. Just one of examples of early commercial spectral imagers ishe microscope mounted SpectraCube 1000 of the early nineties ("Novel spectral imaging ystem combining spectroscopy with imaging applications for biology", Proc. SPIE Vol. 2329, 80 (1995), and U.S. Patent No. 5,539,517), sensitive in the viable ranges invented at CI Systems in 1991 and transferred to Numetrix and later to Applied Spectral Imaging Ltd. for use n medical applications.

The early drive in the 1990's was to make the transition from systems based on imaging at a limited number of wavelengths using bandpass filters (below ten) used on satellites (like Landsat and similar instruments of the 19?(Ts and 1980's), to hyperspectral imaging, providing hundreds of spectral resolution elements at each image pixel, using gratings and interferometers nd sophisticated optics. The latter group, especially through advances in modern cooled or hilled CCD's and cryogenically cooled infrared detector arrays, represents a high end type of nstrumentation. Such high end instrumentation provides large quantities of information, but is sually more suitable for research projects and not for practical day-to-day civilian and industrial pplications, due to exceedingly high costs which may reach hundreds of thousands of dollars ach.

A more recent trend is to exploit spectral imaging technology for safety, security and ndustrial applications, and in particular in the application of hazardous gas cloud detection and maging. Spectral imaging technology as applied to such applications can be used, for example, n automatic detection of leaks in industrial installations without the need for manpower ntensive maintenance investigations, and to identify gases liberated to the air in traffic accidents nvolving trucks during transport. The low price and maintenance-free operation required for this ype of instrumentation is a strong motivation to use low price detectors with no moving parts. Both are significant challenges: i) the former, because most hazardous gases in safety pplications are transparent in the visible range and require more expensive infrared detectors, in rder to be detected; ii) the latter, because the need for simultaneous spatial and spectroscopic nformation needed for detection and identification is more easily achieved with a spectral canning method of some kind, usually requiring the movement of some optical component, such s a scanning mirror of an interferometer, a set of band pass filters mounted on a rotating wheel, r other.

Recent technological advances have allowed the development of more cost effectivenfrared detectors and cameras not requiring cooling at all, or at most requiring thermoelectric ooling (for example microbolpmeters LW1R detectors and MWIR PbSe arrays). As a result, the motivation to find optical configurations yielding just the right amount of information for a pecific application, while maintaining l ost, has also become very strong. Compromises inhis respect have been described in the li rature of many different types. Dereniak et al., "Snapshot dual-band visible hyperspectral imaging spectrometer", Optical Engineering Vol. 46(1), 2007) and Kudenov et al., ("Review of snapshot spectral imaging technologies", Optical Engineering 52(9), 090901 (September 2013)), discuss the use of Optical Computed Tomography (OCT) techniques to remove the use of moving parts in spectral imaging at the expense of spatial and spectral resolutions. Other snapshot methods described in Kudenov et al., have been developed. Such snapshot methods still provide a spectral image with intermediate esolution in both wavelength and space parameters, but the optical fabrication of the required exotic components is cumbersome and expensive (such as reformatting coherent fiber bundles, enslet arrays, and multiple mirror facets).

SUMMARY OF THE INVENTION The present invention is a reliable and low cost device for detecting, imaging and quantifying an airborne gas in a specific range of concentration and cloud size. The device uses no moving parts and has an optical system based on a bistatic electronically controlled notch bsorber, absorbing in the same wavelength range as the gas to be detected. The device lternately images a field of view through a bistatic absorber in the notch and out-of notch wavelength ranges, respectively.

According to an embodiment of the teachings of the present invention there is provided, a evice for imaging radiation from a scene, the radiation including at least a first and second wavelength band, the second wavelength band included in the first wavelength band, the device omprising: (a) a detector of the radiation from the scene sensitive to radiation in the first wavelength band; (b) an image forming optical component for forming an image of the scene on he detector; (c) a filtering arrangement including first and second independently controllable luralities of radiation absorbing molecules in the second wavelength band, the filtering rrangement configured to be electronically switched between: (i) a first state, in which all of the adiation in the first wavelength band is transmitted to the detector, and (it) a second state, in which the radiation in the second wavelength band is at least partially absorbed by each of the first and second pluralities of radiation absorbing molecules, and (d) a control unit electrically oupled to the filtering arrangement and the detector for synchronizing the switching of the ltering arrangement with the image forming on the detector, such that, each pixel of the formedmage includes: (i) a first signal including information associated with the scene radiation in the rst wavelength band, and (ii) a second i al including information associated with the scene adiation in the first wavelength band excluding information associated with the sceneadiation absorbed by the filtering arrangement in the second wavelength band. Optionally, when the filtering arrangement is in the first state, the first plurality of radiation absorbing molecules are arranged in a first orientation substantially parallel to a direction of propagation of the radiation from the scene to the detector.

Optionally, when the filtering arrangement is in the second state, the first plurality of radiation absorbing molecules are arranged in a second orientation substantially perpendicular to a direction of propagation of the radiation from the scene to the detector.

Optionally, when the filtering arrangement is in the first state, the second plurality of radiation absorbing molecules is arranged substantially parallel to the first plurality of radiation absorbing molecules.

Optionally, when the filtering arrangement is in the second state, the second plurality of radiation absorbing molecules is arranged substantially perpendicular to the first plurality of radiation absorbing molecules.

Optionally, the first and second pluralities of radiation absorbing molecules are arranged in series such mat the second plurality of radiation absorbing molecules is interposed between the first plurality of radiation absorbing molecules and the detector.

Optionally, the first plurality of radiation absorbing molecules are positioned within a first cell, and the second plurality of radiation absorbing molecules are positioned within a second cell.

Optionally, the radiation from the scene is unpolarizecL

Optionally, when the filtering arrangement is in the second state, the radiation in the first wavelength band is polarized by the first plurality of radiation absorbing molecules.

Optionally, the first plurality of radiation absorbing molecules is configured to polarize the radiation incident from the scene.

Optionally, the second plurality of radiation absorbing molecules is configured to polarize the radiation incident from the first plurality of radiation absorbing molecules.

Optionally, the image forming optical component has an optical f-number less than approximately 1.5.

Optionally, the filtering arrangement is interposed between the scene and the image forming optical component.

Optionally, the filtering arrangement is interposed between the detector and the image forming optical component.

Optionally, each of the first a cond pluralities of radiation absorbing moleculesncludes liquid crystal molecules. PCML2016/050173

Optionally, the radiation in the first wavelength band includes radiation in the range of 3.2-3.5 micrometers in wavelength.

Optionally, the first and second signals provide quantitative information about the scene, the quantitative information produced by an analytical technique.

Optionally, the scene is a gas cloud.

Optionally, the gas cloud is a hydrocarbon gas cloud,

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic side view illustrating a prior art device for detecting and imaging radiation from a gas cloud;

FIG. 2 is a schematic side view illustrating a device for detecting and imaging radiation from a scene according to an embodiment of the invention;

FIG. 3A is plot of the transmjttance of a filter according to an embodiment of the nvention;

FIG. 3B is a plot of the transmittance of a gas cloud imaged by the device of FIG. 2;

FIG. 4 is a plot of detected radiation for different filter transmittance values as a function of gas cloud absorption;

FIG. 5 is a plot of the absorption characteristics for a group of hydrocarbon gases;

FIG. 6 is a plot of the transmission characteristics of a liquid crystal;

FIG. 7A is an illustration of radiation absorbing molecules in a polarizing state;

FIG. 7B is an illustration of radiation absorbing molecules in an unpolarizing state;

FIG. 8A is an illustration of two cells of radiation absorbing molecules in a second state n which incident light is at least partially absorbed in the absorption wavelength range of the radiation absorbing molecules;

FIG. SB is an illustration of two cells of radiation absorbing molecules in a first state in which incident light is completely transmitted in the detector wavelength range of sensitivity,ncluding the absorption wavelength range of the radiation absorbing molecules.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of the device according to the present invention may be better understood with reference to the drawings and tile accompanying description. Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the examples. The invention is capable of other embodiments or of being practiced or carried out in various ways. Initially, throughout mis document, references are made to directions such as, for example, right, left, and the like. These directional references are exemplary only to illustrate the invention and embodiments thereof.

The present disclosure is of a device for detecting and imaging a specific gas in the air in specific range of concentration and cloud size that maybe among the least expensive to build nd most reliable, by avoiding the use of moving parts, and for measuring the path concentration of the gas in each pixel of the image. This is made possible, as will be explained in the following ections, by a compromise on the number of wavelength ranges combinations used, coming own to only two, by the use of an infrared sensitive camera (preferably un cooled or merinoelecnically cooled such as a PbSe camera sensitive to the 1 to 5.5 micron range, or microbolometer in the 8 to 14 micron range), arid by an optical system based on a bistatic lectronically controlled notch absorber, absorbing in the same wavelength range as the gas to be etected. Other cooled cameras may be used at higher expense. The device alternately images a ield of view through a bistatic absorber in the notch and out-of notch wavelength ranges espectively. A mathematical relation between the two signals (in and out-of-notch) at each mage pixel is men used to calculate the path concentration of the gas making out the hazardous loud present in the corresponding region of space, as will be explained in Section 1 below. . Gas detection and imaging using two-wavelength passive infrared radiometrv:

It has been well known for many years that it is possible to detect the presence of a gas in he air and measure the corresponding path concentration distribution by measuring the infrared elf-emission of the background of the gas cloud in two different wavelengths, one which is bsorbed by the gas and one which is not, provided that the background and gas are not at the ame temperature. The radiance difference R reaching the measuring instrument between the two wavelengths wo (not absorbed) and wc (absorbed by the gas), can be expressed in terms of the ackground radiance B, the gas temperature T₀ (usually equal to the air temperature, and we ssume that it is known by measurement) and the gas transmittance to at the absorbed wavelength. The gas transmittance tc is in turn dependent on the molecular absorption coefficient f the gas in question multiplied by the d thickness and gas molecular concentration at the ixel in question (referred to as the pa ncentration). Therefore, if the gas in question is nown, the gas path concentration can be estimated. The following will clarify the quantitative method of the present disclosure by showing that to can be measured with the present device, which in turn allows for the gas path concentration to be estimated:

where is the Planck function at temperature and wavelength TWO

simplifications are used in equation (1) which are not important for the sake of this explanation because the associated phenomena can both be calibrated out in the more general case: i) atmospheric transmittance is assumed to be 1, and ii) background radiance in and out of the gas absorption band are equal.

It is obvious from equation (1) that in the case that B is equal to

the radiance difference R is equal to zero, irrespective of the value of to, and in this case no information can be inferred on the quantity

However, if B is different than

then equation (1) can be solved for to as follows:

All parameters on the right hand side of equation (2) are known: B is known because it is measured in the non-absorbing wavelength w<>, PI is known because To is measured and w_G is known, and R is measured. Therefore, tc is known from equation (2). If the molecular absorption coefficient, Ar» of the specific gas being monitored is known from the literature at w^, then ½ gives a measure of the product of average gas volume concentration in the cloud, multiplied by the thickness of the cloud itself, or the so called concentration times length (or path concentration) value of the cloud. In fact, by the Lambert-Beer law as follows:

Note that the estimated average concentration n expressed in equation (5) assumes absorption from single molecules, negligible attenuation effects from scattering, reflections, and multiple absorption in the gas cloud. In general, / is not known and Ac is known, so mis method readily provides nl according to equation (4), once to is measured according to equation (2). The purpose of the present invention is to give a solution to the problem of detecting and imaging the concentration rimes path length distribution of an infrared wavelength absorbing gas cloud (or other similar material with an absorbing wavelength) with the minimum use of moving parts and at the same time retaining the best sensitivity possible.

Figure 1 depicts an example of a prior art solution. In such a solution, an objective lens 2 is positioned in front of a detector array 1 and a two-position filter holder or wheel 3 containing two filters (4a,4b), either in front of die objective lens 2 or between the objective lens 2 and the detector array 1. In this way, by recording successive images of the field of view (FOV) of the instrument at the two wavelength bands the needed information is gathered to detect the gas, if present, and in this case, calculate the concentration times path value of the gas in question for every pixel of the scene, as explained above. From this information the image of the cloud is built pixel by pixel and either displayed in an enhanced color, or otherwise used, eg. to issue an alarm if this value becomes higher than a minimum safety value in some predefined spatial criteria; In the example shown in Figure 1 , the filter holder 3 slides in front of the objective lens 2 in a linear motion. Other configurations using rotating wheels, grating, etc. to filter the different wavelengths can be used, but they all use moving parts. The filters are before the objective lens. Only the principal rays of the central, top and bottom pixels of the FOV are shown. The filters can be alternately placed between the lens and the detector or between lenses in a multiple lens system design.

It should be noted that the configuration of Figure 1 can be preferably designed with a large numerical aperture of the objective lens 2 to exploit the best possible detector sensitivity (or low f-number, which is in general kept as close to 1 as possible, especially when using uncooled infrared detector arrays). A different configuration, using a dichroic beamsplitter to split the incoming beam into two beams to be filtered separately in the two wavelengths and two separate detectors can be used, but would be more expensive because of the additional detector cost. A further similar configuration using, besides the dichroic filter, an additional beam combiner and chopper may be used to limit the design to the single array detector, but in this case the chopper, needed to switch between the two wavelengths in synchronization with the detector frame capture rate, is a low reliability moving part. These last two configurations require more complicated optics to avoid decreasing the numerical aperture of the focusing optics at the detector and degrade the system sensitivity

2. Bistatic liquid crystal solution:

Refer now to Figure 2, a device 10 of an embodiment of the present invention. The device 10 enables imaging the same region of space or field of view (FOV) alternately through wo electronically controlled filtering states, using a single detector array camera 12 and optical system without mechanical movement of any optical component (the shutter is still needed to carry out Nott Uniformity Corrections, including environmental drifts, fixed pattern and readout noise), while preserving the high optical throughput of the conventional configurations previously described in the Section 1. This is accomplished by a combination of an imaging optics 14 (referred to interchangeably as focusing optics, image forming optics, and objective ens) with f-number close to 1 (high numerical aperture) and a bistatic liquid crystal (LC) based notch filter 16 (referred to interchangeably as LC filter, LC notch, LC notch filter, bistatic LC ilter, bistatic LC notch filter) having a spectral behavior optimized to the gas absorption pectrum in question. The LC filter configurations pair are, as explained in the example below, i) high filter transmittance spectral region, and ii) a notch filter absorbing in a region around a wavelength absorbed by the gas, this wavelength being within the high transmittance wavelength egion. These two Configurations are alternately switched electronically by a controller 18 in ynchronization with the camera frames recording. In this way the device 10 records two signals or each pixel, one through the high transmittance and one through the notch configuration. In Section 3 we show how to calculate tc of equation (1 ) from these two pixel signals.

The controller 18 can be implemented as any number of computer processors including, ut not limited to, a microprocessor, an ASIC, a DSP, a state machine, and a microcontroller. Such processors include, or may be in communication with computer readable media, which tores program code or instruction sets mat, when executed by the processor, cause the processor o perform actions. Types of computer readable media include, but are not limited to, electronic, ptical, magnetic, or other storage or transmission devices capable of providing a processor with omputer readable instructions.

Once to is known in every pixel, equation (4) yields the mformation of concentration mes path length of the gas cloud at every point in the image. From this information one can uild the image in false color or in different intensity levels of the color, according to the nl alue of the gas being monitored at every pixel. Capturing alternating information in these two onfigurations on the same detector 12 according to the present invention also allows the monitoring of faster phenomena than in a conventional configuration, because the electronic witching between the "off* (high transmittance) and "on" (notch transmittance) configurations f the filter can be done at high frequencies of several kHz. This is due to the fast LC molecules esponse to the suitably applied high ency alternating voltage. With the proper digital nalysis, the final gas cloud image can be shown superimposed on a conventional visible image of the same field of view, obtained by a usual visible CCD camera. This helps the operator locate the cloud with respect to other objects in the field.

With continued reference to Figure 2, the edge and central rays of the FOV are depicted passing through the two bistatic filter 16 and focusing optics 14, and forming the scene image on the detector plane. The view in Figure 2 is a cross section from the side of the device 10. The moving filter holder or wheel 3 of Figure 1 is replaced by the electronically controlled bistatic

IX! fitter system (the bistatic LC filter 16 and the controller 18), including a high transmittance configuration in a wavelength range, alternating with a notch absorbing fitter configuration in a sub-range corresponding to the gas absorption wavelength.

3. Gas path concentration measurement:

As explained in Section I with reference to equation (4), if the absorption coefficient of the gas molecules being monitored is known from the literature and the cloud transmittance is measured, then the cloud path concentration can be at least approximately calculated. In this section it is shown how the signals through a bistatic notch filter can be used to calculate the transmittance through the gas cloud.

In Figure 3A the transmtttances of the bistatic LC filter 16 in the off and on configurations are shown. The dotted line is the "off configuration (i.e. no notch region). In the

"off configuration, there is 100% transmittance in the range between λι and λ₂ and 0% transmittance outside of that range). In the "on" configuration, the transmittance is a notch centered at λ« and width w. The transmittance in the notch region is z.

In Figure 3B, the gas cloud transmittance in a pixel of path concentration nl is shown.

The absorption wavelength range is centered in the same wavelength λο as the LC notch and has the same width w. The transmittance in the absorption range is x and 100% otherwise.

The detector array 12 is assumed to be sensitive only in the wavelength region between λι and λ₂. Now assume that the spectral radiance of the cloud background is B and is a constant function of wavelength. Then, in view of Figures 3A and 3B, the average radiance reaching the detector array 12 in the wavelength range between X_t and λ₂ can be expressed for four cases.

Defining

where w is defined above, the four cases are defined as follows:

First case: gas present (G), notch on (N):

Equations (8) and (9) are special cases of equations (6) and (7) respectively for x- l . This should be apparent, as x-l implies that the gas cloud is absent In this case the right hand side of equation (6) is the same as in equation (8) and the right hand side of equation (7) becomes equation (9). Similarly, for a very high concentration gas cloud, x=0, and equations (6) and (7) give the same result. This is because the total signal is B multiplied by the product of the two transmittanee functions in Figures 3A and 3B. If the gas cloud absorbs all radiation within the notch range the signal is independent on whether the LC notch is in the on or off configuration.

A ratio, F, of the difference between "on" and "off and the "off signal (or normalized signal contrast) can be calculated from equations (6) and (7). The ratio F can be expressed as follows:

Equation (10) is the basic relation between the pixels' signals in the LC on and off positions and the quantity x, which is equal to to of equation (3) above, in turn the parameter related to the path concentration of the gas cloud in question according to equation (4).

The ratio F of equation (10) can be plotted as function of the total gas cloud absorption

of equation (4)) for different values of y and z. Example plots are shown in Figure 4.

The plotted values of Figure 4 are calculated for y^OJJ. (y can change by definition between 0 and 1 , but these two values are limiting values that have no physical meaning for our purpose). In fact, y=0 is not physical and for y=l , F does not depend on x< The exact values of y and z should be selected according to the design of the device 10. In practice, the values of y and z are typically measured separately in the laboratory or known from the literature. Once y and z are known, the function F of equation 10 is plotted as a function of x for these values of y and z. x=to is then determined by determining the value of ln(l/x) corresponding to the measured value of F for the particular pixel in question on the plotted graph. This can be done, since the plotted functions of F, as seen in Figure 4, are monotonic, and therefore have one-to-one relationships. Alternatively, equation (10) can be inverted to express x as function of F, y and z. Again, once F is measured with the device 10 and y and z are known independently, x=tc (and therefore nl) is also known. It is noted that the above example refers to the case in which the background of the gas cloud is at higher temperature than the cloud itself, so that the gas appears as absorbing. However, the same arguments can be shown to hold in the opposite case of background being cooler than the gas cloud, when the gas appears as an emitter.

4. Liquid crystal implementation:

In this section, implementation of the bistatic notch LC filter 16, in order to provide the ype of signals as described above without moving parts, is discussed. This section also addresses the integration of the bistatic notch LC filter 16 in the device 10, in order to detect and mage a gas cloud. The implementation and integration discussion is applied, as a non-limiting example, to a group of hydrocarbons that all have an absorption around the same wavelength range, between 3.2 and 3.5 microns, and to a particular detector type that is sensitive in that range. As should be apparent to one of ordinary skill m the art, many other combinations of materials to be detected, LC filters materials, and different detectors may be used according to he teachings of the present embodiments described herein.

The absorption characteristics for the non-limiting example group of hydrocarbons are shown in Figure 5. In the particular wavelength range of hydrocarbons pertinent to the present non-limiting example, there exist liquid crystals, such as, for example, E-7 and BDH-E7. Such iquid crystals present absorption in this wavelength range. The transmission characteristics of such example liquid crystals are shown in Figure 6 which is an excerpt from the Handbook of Optics, McGraw-Hill Inc., Sponsored by the Optical Society of America, Volume II, page 14.6, 995.

As described, for example, by C.L. Mulder et al., in the publication Dye alignment in uminescent solar concentrators: I. Vertical alignment for improved waveguide coupling, Optics Express Vol. 18, No. SI, p. A79, 2010, there are at least two ways to obtain the same desired effect. One is that the LC itself preferentially absorbs infrared radiation polarized along the long molecular direction and does not appreciably absorb radiation polarized perpendicular to that direction, including when the molecule is aligned parallel to the incoming infrared light propagation direction. The second way is to have a long absorbing molecule embedded in the LC matrix, such that its polarization properties are controlled in a similar way by the LC molecules lignment. In both cases a cell containing the LC itself or the molecular mixture mentioned here, can be electronically induced to absorb radiation anisotropically, as shown in Figure 7 A.

In Figure 7A, unpolarized light 7 ncident from the left on an LC 72 or absorbing long molecule oriented perpendicularly to the light propagation direction. The transmitted light at wavelengths within the gas and LC absorption wavelength region is polarized perpendicularly to he long axis of the molecule. Light outside this band is not absorbed and is unpolarized. As a result, in this non-limiting example, molecules oriented perpendicularly to the light propagation direction have the following effect: they absorb the portion of light around the 3.5 microns range which is polarized parallel to their long molecular axis, while transmitting the same wavelength range polarized in a direction perpendicular to their long molecular axis (depicted as a ransmitted polarized light beam 74 around the gas absorption wavelength); in addition, light outside the 3.5 micron wavelength range is completely transmitted.

Alternately, when these molecules are placed in a cell and the appropriate voltage is applied to it so that their long axis is aligned with the propagation direction of the incoming ight, they do not absorb any light and in this situation all radiation is transmitted through the cell, as shown in Figure 7B.

In Figure 7B, theunpolarized light 70 is incident on an LC long molecule 76 whose long axis is oriented in the direction of the light propagation direction. As a result, all light is ransmitted and is unpolarized (depicted as a transmitted unpolarized light beam 78). The configuration depicted in Figure 7B is the "off' filter configuration of the LC filter 16.

Now consider, as in Figures 8A and 8B, a system composed of two equal LC cells in eries (a first LC cell 80 and a second LC cell 82), where the molecules 81 in the first LC cell 80 nd the molecules 83 in the second LC cell 82 are oriented with their long axis in two perpendicular directions with respect to each other and to the light propagation direction. An unpolarized light beam 84 incident from the left is completely (or partially) absorbed in the bsorption wavelength range of the molecule and gas by the pair of cells (this is the "on" filter tate of the LC filter 16 described in Section 3 above). This is due to the fact that the second LC ell 82 absorbs the residual polarized light 85 transmitted by the first LC cell 80 (functioning as ross polarizers with respect to this radiation). All light outside that range is transmitted. When he molecules of both cells are switched electronically (via the control unit 18) to be oriented arallel to the light propagation direction as in Figure 8B, no light is absorbed (this is the "off" tate of the LC filter 16 described in Section 3 above). Specifically, in Figure 8B, the unpolarized ght heam 84 is transmitted unpolarized (and shown as 85) after passing through the first LC cell 0, and likewise by LC cell 82. As a result, all light incident on the first LC cell 80 passeshrough the second LC cell 82 within the whole wide band region between λι and λ₂ of Figure A.

As should be apparent from the di sion above, the device 10 (as shown in Figure 2) in which the filtering element 16 is a bistat er as described above in Section 4, provides images f the scene in which each pixel's radiation is known according to the four equations (6), (7), (8), nd (9). Accordingly, a procedure as described in Section 3 can be followed to detect a gas cloudaving the above described absorption properties, if present in the air, and map the pathoncentration of the gas cloud pixel by pixel.

It will be appreciated that the above descriptions are intended only to serve as examples,nd that many other embodiments are possible within the scope of the present invention asefined in the appended claims.

Claims

WHAT IS CLAIMED IS;

1 , A device for imaging radiation from a scene, the radiation including at least a first and second wavelength band, the second wavelength band included in the first wavelength band,he device comprising:

(a) a detector of the radiation from the scene sensitive to radiation in the first wavelength band;

(b) an image forming optical component for forming an image of the scene on the detector;

(c) a filtering arrangement including first and second independently controllable pluralities of radiation absorbing molecules in the second wavelength band, the filtering arrangement configured to be electronically switched between:

(i) a first state, in which all of the radiation in the first wavelength band is transmitted to the detector, and

(ii) a second state, in which the radiation in the second wavelength band is at least partially absorbed by each of the first and second pluralities of radiation absorbing molecules, and

(d) a control unit electrically coupled to the filtering arrangement and the detector for synchronizing the switching of the filtering arrangement with the image forming on the detector, such that, each pixel of the formed image includes:

(i) a first signal including information associated with the scene radiation in the first wavelength band, and

(ii) a second signal including information associated with the scene radiation in the first wavelength band and excluding information associated with the scene radiation absorbed by the filtering arrangement in the second wavelength band.

2. The device of claim 1 , wherein when the filtering arrangement is in the first state,he first plurality of radiation absorbing molecules are arranged in a first orientation substantiallyarallel to a direction of propagation of the radiation from the scene to the detector.

3. The device of claim 1, w n when the filtering arrangement is in the second ate, the first plurality of radiation absorbing molecules are arranged in a second orientation substantially perpendicular to a direction of propagation of the radiation from the scene to the detector.

4. The device of claim 1 , wherein when the filtering arrangement is in the first state, he second plurality of radiation absorbing molecules is arranged substantially parallel to the first plurality of radiation absorbing molecules.

5. The device of claim l, wherein when the filtering arrangement is in the second tate, the second plurality of radiation absorbing molecules is arranged substantially perpendicular to the first plurality of radiation absorbing molecules.

6. The device of claim 1, wherein the first and second pluralities of radiation absorbing molecules are arranged in series such that the second plurality of radiation absorbing molecules is interposed between the first plurality of radiation absorbing molecules and the detector.

7. The device of claim 1 , wherein the first plurality of radiation absorbing molecules are positioned within a first cell, and the second plurality of radiation absorbing molecules are positioned within a second cell.

8. The device of claim 1 , wherein the radiation from the scene is unpolarized.

9. The device of claim 8, wherein when the filtering arrangement is in the second tate, the radiation in the first wavelength band is polarized by the first plurality of radiation bsorbing molecules.

10. The device of claim 1, wherein the first plurality of radiation absorbing molecules s configured to polarize the radiation incident from the scene.

Π. The device of claim 10 wherein the second plurality of radiation absorbing molecules is configured to polarize the ation incident from the first plurality of radiation bsorbing molecules.

12. The device of claim 1, wherein the image forming optical component has an optical f-number less than approximately 1.5.

13. The device of claim 1, wherein the filtering arrangement is interposed between he scene and the image forming optica) component.

14. The device of claim 1. wherein the filtering arrangement is interposed betweenhe detector and the image forming optical component.

15. The device of claim 1 , Wherein each of the first and second pluralities of radiation absorbing molecules includes liquid crystal molecules.

16. The device of claim 1 , wherein the radiation in the first wavelength band includesadiation in the range of 3.2-3.5 micrometers in wavelength.

17. The device of claim 1, wherein the first and second signals provide quantitativenformation about the scene, the quantitative information produced by an analytical technique.

18. The device of claim 1 , wherein the scene is a gas cloud.

The device of claim 18, wherein the gas cloud is a hydrocarbon gas cloud.