WO1982003688A1

WO1982003688A1 - Distinct wavelenght light reflectance measuring apparatus

Info

Publication number: WO1982003688A1
Application number: PCT/SE1982/000118
Authority: WO
Inventors: Ab Bonnierfoeretagen; Daniel F Pope; Jon V Hokanson; Robert R Sullivan; Esko J Venalainen
Original assignee: Ab Bonnierfoeretagen
Priority date: 1981-04-13
Filing date: 1982-04-08
Publication date: 1982-10-28
Also published as: EP0076301A1

Abstract

A distinct wavelenght light reflectance measuring apparatus that is particularly adapted for measuring the brightness of paper pulp includes an elongated tube (14) in which are disposed first and second fiber optic bundles (16, 18). The first end of the tube (and of the fiber optic bundles) is mounted in proximity to the paper pulp, and a housing (24) is mounted on the second end of the tube so that the second ends of the fiber optic bundles extend into the housing. A plurality of light sources (each including a plurality of light-emitting diodes) and a light sensor are mounted on a printed circuit board assembly which is supported on and by a block mounted within the housing and receiving the second ends of the first and second fiber optic bundles. A circuit is provided for pulsing the light sources in sequence during each of a plurality of successive measurement cycles and for detecting the electrical output signal from the light sensor in synchronism with the sequential pulsing of the plurality of light source.

Description

DISTINCT WAVELENGTH LIGHT REFLECTANCE MEASURING APPARATUS

Field of the Invention This invention generally relates to the field of light reflectance measurements, and, more particularly, to an apparatus for measuring the light reflectance of a substance at a distinct wavelength.

Background of the Invention

In the production of bleached paper, pulp is taken from a digester and after being refined and washed is transported to a bleaching process wherein the reflectance or "brightness" of the pulp is adjusted to a desired value. In a typical bleaching process, the pulp is pumped through a conduit to a blenching tower and a bleaching chemical such as chlorine, chlorine dioxide, or hypochlorite is injected into and mixed with the pulp as the pulp is being pumped to the bleaching tower. The pulp is allowed to reside within the bleaching tower for a certain period of time and then is transported to subsequent stages in the paper production process.

The pulp brightness that is achieved during the bleaching process is dependent not only on the time that the pulp is allowed to reside in the bleaching tower but also on the amount of the bleaching chemical that is injected and the brightness of the pulp being pumped to the bleaching tower. The bleaching chemicals are also very expensive and very toxic. In order that the bleaching process may precisely control pulp brightness and use only the proper amount of the bleaching chemical, it is accordingly desirable to measure pulp brightness either at a point in the conduit between the point at which the bleaching chemical is injected and the bleaching tower or at the bleaching tower itself. To optimize brightness control, the pulp brightness is measured within each of a plurality of distinct spectral bands, each spectral band including a predetermined range of wavelengths distributed about a nominal wavelength.

Multiple wavelength measurements of pulp brightness have been made in the prior art by illuminating the pulp with panchromatic light from a white light source such as a quartz halogen lamp and by measuring the light reflected from the pulp as transmitted through each of a plurality of filters having the desired spectral bands. In one such apparatus employing this prior art technique, the light source is continuously energized and the filters are mounted on a filter wheel which is selectively rotated to interpose one of the filters between the pulp and a light sensor.

Although the prior art technique and apparatus described are capable of providing the desired brightness measurements, they encounter significant problems during actual use. The light source has a limited, nominal lifetime (e.g., one thousand hours) and must be periodically replaced, and often fails during use due to the adverse environmental conditions of the bleaching process including high temperatures and excessive vibration. As the light source ages and as the temperature thereof varies, the light output of the light source also varies to introduce measurement errors. Since the light source is continuously energized, it is difficult to compensate for varying amounts of ambient light incident on the light sensor. A considerable number of mechanical and electromechanical components are required to control rotation of the filter wheel, all of which are subject to failure during operation. A significant amount of electrical power is required to drive the electromechanical components, to energize the light source, and to cool the light source. Due to the use of a filter wheel, it is difficult to protect the filters of the filter wheel and accompanying optics from dust, thereby resulting in varying amounts of light being transmitted from the pulp to the light sensor and further introducing measurement errors.

It is therefore an object of this invention to provide a distinct wavelength light reflectance measuring apparatus.

It is a further object of this invention to provide such an apparatus which is particularly constructed and arranged for measuring pulp brightness.

It is yet a further object of this invention to provide such an apparatus which is capable of providing accurate and precise measurements of light reflectance at a single wavelength or at a plurality of distinct wavelengths over a relatively long period of time.

It is another object of this invention to provide such an apparatus which can provide multiple wavelength light reflectance measurements without the use of moving mechanical or electromechanical components. It is still another object of this invention to provide such an apparatus which is relatively low in electrical power consumption.

It is an object of this invention to provide such an apparatus which includes a light source having a relatively long lifetime and a stable light output over that lifetime, irrespective of environmental conditions.

It is an object of this invention to provide such an apparatus which is capable of providing accurate and precise measurements of light reflectance at a single wavelength or at a plurality of distinct wavelengths under highly adverse environmental conditions.

Summary of the Invention Briefly, these objects and other objects and advantages that will be realized from a consideration of the following portion of the specification are achieved in an apparatus for measuring the light reflectance of a substance at a distinct wavelength. The apparatus comprises: first means for illuminating the substance with a light pulse that occurs at a predetermined time in a predetermined measurement cycle, the light pulse comprising substantially monochromatic light; and, second means for measuring light that is reflected by the substance in synchronism with the occurrence of the light pulse during the measurement cycle.

Such an apparatus may be adapted for measuring the light reflectance of a substance at a plurality of distinct wavelengths. When so adapted, the first means is operative to illuminate the substance with a plurality of light pulses that sequentially occur in a predetermined measurement cycle, each light pulse comprising substantially monochromatic light having a nominal wavelength that is distinct from that of any other light pulse, and the second means is operative to measure light that is reflected by the substance in synchronism with the sequential occurrence of the light pulses during the measurement cycle.

In a preferred embodiment, the first means includes: a plurality of light sources, each of the light sources being constructed so as to provide substantially monochromatic light having a distinct nominal wavelength; means for sequentially energizing the plurality of light sources during the measurement cycle; and, means for optically coupling the plurality of light sources to the substance. The second means includes: a light sensor adapted to provide an electrical output signal related to the intensity of light incident on the light sensor; means for optically coupling light from the substance to the light sensor; and, means for detecting the electrical output signal from the light sensor in synchronism with the sequential occurrence of the plurality of light pulses during the measurement cycle. Preferably, the plurality of light sources and the ligh t sensor are optically coupled with the substance by respective fiber optic bundles, and each light source includes a plurality of light-emitting diodes whose temperature is maintained at a substantially constant value.

The apparatus of the invention may be particularly adapted for measuring the brightness of paper pulp within each of a plurality of distinct spectral bands of wavelengths, and as such also comprises: an elongated tube having proximal and distal ends, the proximal end being adapted to be mounted in proximity to the paper pulp; first and second, elongated fiber optic bundles disposed in the tube and each having a proximal end located in proximity to the proximal end of the tube and a distal end located in proximity to the distal end of the tube; and, a housing receiving the distal end of the tube, the distal ends of the first and second fiber optic bundles being arranged so as to extend into the housing.

The plurality of light sources are mounted within the housing and arranged so as to illuminate the distal end of the first fiber optic bundle, with each light source being capable of providing a light output within one of the plurality of distinct spectral bands. The light sensor is also mounted within the housing and arranged so as to receive light from the distal end of the second fiber optic bundle.

Preferably, mounting means are provided which are disposed within the housing for locating the plurality of light sources and the light sensor in a predetermined spatial relationship with the distal ends of the first and second fiber optic bundles, respectively. The mounting means includes: a first block having first and second, opposing surfaces; first and second recesses formed in the first block and extending therein from the first surface; first and second apertures formed in the first block and extending from the first and second recesses, respectively, to the second surface, the distal ends of the first and second fiber optic bundles being received in the first and second apertures, respectively; and, a printed circuit board assembly supported on and by the first block and mounting the plurality of light sources and the light sensor so that the plurality of light sources are substantially located within the first recess and optically aligned with the distal end of the first fiber optic bundle and so that the light sensor is substantially located within the second recess and optically aligned with the distal end of the second fiber optic bundle.

Each light source comprises a plurality of light-emitting diodes mounted on the printed circuit board assembly, and heating means are dispose within the first recess for maintaining the temperature of each light-emitting diode at a substantially constant value. The printed circuit board assembly and the first block are encapsulated so as to substantially seal the printed circui t board assembly and the first and second recesses from the environment. Brief Description of the Drawings The invention can best be understood by reference to the following portion of the specification, taken in conjunction with the accompanying drawings in which: FIGURE 1 is a pictorial view of a multiple wavelength light reflectance measuring apparatus that is particularly adapted for measuring the brightness of paper pulp;

FIGURE 2 is an exploded, pictorial view of a housing forming part of the apparatus in FIGURE 1 and of the components within the housing; FIGURE 3 is an exploded, partial cross-sectional view of certain components within the housing including a block and a printed circuit board assembly;

FIGURE 4 is a pictorial view of the block in FIGURE 3; FIGURE 5 is a plan view of one of the printed circuit boards comprising the printed circuit board assembly in FIGURE 3;

FIGURE 6 is an electrical schematic diagram of the apparatus; and, FIGURE 7 is a timing diugram illustrating the operation of the apparatus.

Description of the Preferred Embodiment Referring now to FIGURE 1, the pulp whose brightness is to be measured passes through a conduit 10 (which may also comprise the vessel of a bleaching tower). At a point where the pulp brightness is to be measured, conduit 10 is provided with an opening surrounded by a sleeve 12 integral with conduit 10. An elongated tube 14 of the measuring apparatus has one end thereof fitted into sleeve 12 and is supported from conduit 10 by means not illustrated.

Tube 14 is preferably composed of a corrosion resistant material such as titanium or stainless steel, and a valve structure (not illustrated) is provided for permitting lube 14 to be removed from sleeve 12 and for permitting sleeve 12 to be closed whenever tube 14 is so removed. A pair of elongated, cylindrical fiber optic bundles 16, 18 are disposed within tube 14 and extend along its length from the end thereof proximate conduit 10 to the end thereof distant from conduit 10. Fiber optic bundles 16, 18 are constructed so as to transmit light between their proximal and distal ends, and their cylindrical surfaces are preferably provided with an opaque coating. The proximal ends of bundles 16, 18 are optically coupled with the interior of conduit 10 through a transparent quartz window mounted in the end of tube 14. A temperature sensor 20 is also mounted withi n tube 14 behind the quartz window so as to sense the temperature of the pulp within conduit 10, and en electrical cable 22 is connected to temperature sensor 20 and extends to the distal end of tube 14. The distal end of tube 14 is received within a corresponding aperture 26 of a housing 24 so that the distal ends of bundles 16, 18 and cable 22 extend into housing 24, and lube 14 is secured to housing 24 by means not illustrated. As described in detail hereinafter with reference to FIGURES 2-7, a plurality of light sources and a light sensor are disposed within housing 24, with the light sources being optically coupled with the distal end of fiber optic bundle 16 and with the light sensor being optically coupled with the distal end of fiber optic bundle 18. Each light source preferably includes a group of light-emitting diodes that are designed to emit substantially monochromatic light, that is, light within a predetermined spectral band of wavelengths distributed about a nominal wavelength. The light sensor is preferably of a type that provides an electrical signal related to the intensity of the light incident thereon, and that is substantially wavelength-insensitive in a range of wavelengths including the spectral bands of the light sources. During a measurement cycle, the plurality of light sources are caused to emit light pulses in a predetermined sequence which are coupled to the interior of conduit 10 by fiber optic bundle 16 to as to illuminate the pulp therein. Light reflected from the pulp is coupled to the light sensor by fiber optic bundle 18, and the output signal from the light sensor is detected in synchronism with the sequential light pulses coupled to fiber optic bundle 16 so as to provide a plurality of output signals each representing the light reflectance of the pulp within one of the plurality of spectral bands. Provision is also made to compensate the output signals for drift, noise and the ambient light incident on the light sensor. Finally, the output signal from temperature sensor 20 is continuously monitored to provide a signal related to the temperature of the pulp within conduit 10.

Referring now to FIGURE 2, housing 24 has defined therein a cavity 30 which is open at the side of housing 24 opposing the side in which is located aperture 26. The distal ends of bundles 16, 18 and of cable 22 extend above the floor of cavity 30, and a block 32, a block 34 and a printed circuit board assembly 36 mounted on block 34 are situated within cavity 30. Block 32 includes apertures 38 and 40 through which pass bundles 16 and 18, respectively, and the corresponding ends of block 32 rest on opposing ledges 42 of housing 24. Block 32 is secured to housing 24 by a pair of fasteners 44 passing through corresponding apertures 46 in block 32 and received in corresponding threaded apertures 48 formed in ledges 42. Threaded fasteners 39 and 41 are received in corresponding threaded apertures (not illustrated) in block 32 and the ends thereof bear against bundles 16 and 18 in assembly so as to restrain bundles 16 and 18 from lateral or longitudinal movement. In order to provide a support for block 34 (and printed circuit board assembly 36), a plurality of threa ded apertures 50 are provided in block 32 into which are inserted a plurality of spacers 52. Block 34 is secured in position by a plurality of fasteners 54 passing through corresponding apertures 56 in block 34 and received in corresponding threaded apertures 58 in spacers 52. The open end of cavity 30 is closed by a cover 60 that is secured in position by a plurality of threaded fasteners 62 passing through corresponding apertures 64 in cover 60 and received in corresponding threaded apertures 66 in housing 24. With additional reference now to FIGURES 3-5, block 34 has an upper surface 34 A and a lower surface 34B, and printed circuit board assembly 36 includes a printed circuit board 70 supported on surface 34A and a printed circuit board. 72 supported above printed circuit board 70. Substantially rectangular recesses 74, 76 extend into block 34 from upper surface 34A, and substantially cylindrical apertures 78, 80 extend between the floor of recesses 74 and 76, respectively, and surface 34B. Lenses 82 and 84 are situated in apertures 78 and 80 just below the floors of recesses 74 and 76, respectively, and mounted therein by respective retaining rings 86, 88. When block 34 is supported on spneers 52 as previously described, the distal ends of bundles 16 and 18 are located within apertures 78 and 80 and abut lenses 82 and 84, respectively. Printed circuit board 72 is supported on printed circuit board 70 by a plurality of spacers 92, and printed circuit boards 70 and 72 are secured to block 40 by a plurality of fasteners 94 that pass through corresponding apertures 96 in printed circuit board 72, spacers 92, and corresponding apertures 98 in printed circuit board 70, and that are received in corresponding threaded apertures 1 00 in block 34. The light sources consist of a first group 102 of light-emitting diodes (LEDs) and a second group 104 of LEDs mounted. on printed circuit board 70, both extending toward and situated within cavity 74 in assembly, and a third group 106 of LEDs mounted on printed circuit board 72 and extending through corresponding apertures 108 in printed circuit board 70 in assembly. The LEDs in each group are of an identical type and are designed to emit substantially monochromatic light, that is, light within a predetermined spectral band of wavelengths uniformly distributed about a nominal wavelength. Each group has a different nominal wavelength, for example, i535 nanometers for group 102, 700 nanometers for group 104, and 555 nanometers for group 106. Preferably, the

LEDs are comm ercially available types that are available in a wide range of nominal wavelengths. Such commercially available LEDs have differing physical and optical characteristics, so that the number of LEDs in each group and the relative placement of the LEDs in each group must be carefully chosen to ensure that each group will provide substantially the same light output. As illustrated, the LEDs in group 106 are physically larger than the LEDs in either of groups 102 or 104, thereby necessitating the mounting of group 106 on upper printed circuit board 72 so that the light-emitting surfaces of the LEDs in group 106 are substantially coplanar with those of the LEDs in groups 102 and 104 upon assembly. As also illustrated, there are three LEDs in each of groups 104 and

106, and six LEDs in group 102 since each LED in group 102 has a lesser light output than each LED in groups 104 and 106. As best illustrated in FIGURE 5, the LEDs are arranged in a predetermined pattern around a point 110 aligned with the longitudinal axis of fiber optic bundle 16 and a cylindrical light shield 113 (preferably of heatshrinkable tubing) is disposed about the portions of the LEDs in groups 102, 104 and 106 that extend from printed circuit board 70. The pattern of LED arrangement, the disposition of shield 113, and the optical characteristics of lens 82 are chosen so that the light output from each group is uniformly, distributed along the entire distal end of fiber optic bundle 16.

An LED of the type described is capable of providing a light output whose amplitude varies with temperature but is otherwise substantially constant over a considerable period of time, e.g., one or two years. In order to eliminate temperature-caused variations in light output, the LEDs are maintained at a substantially constant temperature by situating the LEDs within recess 74 (whose dashed outline can be seen in FIGURE 5) and by maintaining the temperature of recess 74 at a substantially constant value through the use of a heating means including a transistor 132 and a power transistor 134 also disposed within recess

74 and mounted on printed circuit board 70.

The light sensor consists of a photocell 112 including a light sensitive surface 112A and is mounted on printed circuit board 70 so as to be situated within recess 76 in assembly. As best illustrated in FIGURE 5, the optical center 114 of light sensitive surface 112A is aligned with the longitudinal axis of fiber optic bundle 18, and the optical characteristics of lens 84 are chosen so that light appearing at the distal end of fiber optic bundle 18 is focused onto light sensitive surface 112A. A number of additional electrical components of the measuring apparatus are mounted on printed circuit board 70 and situated within recess 74, and certain other components of the measuring apparatus (such as a transistor 116) are mounted on printed circuit board 70 and situated within recess 76. To prevent dust and other contaminants from reaching the electrical and optical components of the measuring apparatus, printed circuit boards 70 and 72 are assembled with block 34 as illustrated in FIGURE 3 and the assembly is encapsulated with a suitable potting compound so as to completely seal printed circuit boards 70 and 72 and recesses 74 and 76 from the surrounding environment. As seen in FIGURE 3, a sheet 90 of elastomcric material is adhesively bonded to lower surface 34B of block 34 and surrounds bundles 16 and

18 so as to provide a seal for the distal ends of bundles 16 and 18 and the adjoining portions of lenses 82 and 84.

Referring now to FIGURE 6, the electrical components of the measuring apparatus that are situated within recess 74 include a voltage reference circuit 120, an adjustable resistor 122, a differential amplifier 124, a temperature sensor 126, an integrator 128, transistor 132, power transistor 134, and LED groups 102, 104 and 106. The electrical components of the measuring apparatus that are situated within recess 76 include photocell 112, a prcamp 160, and a filler circuit 162. Except where specifically indicated in FIGURE 6, all of the electrical components of the measuring apparatus are powered by regulated supply voltages V_S and -V_S obtained from a stable voltage source (not illustrated).

The heating means previously described includes a voltage reference circuit 120 that provides an output signal whose voltage is substantially constant and which is coupled to the inverting input of differential amplifier 124 through adjustable resistor 122. The signal provided to the inverting input of differential amplifier 124 from adjustable resistor 122 comprises a current that is related to the desired temperature of recess 74. Temperature sensor 12G provides an output signal to the noninverting input of differential amplifier 124 that is related to the actual temperature of recess 74, whereby the output signal from differential amplifier 124 comprises an error signal that is related to the difference between the desired temperature and the actual temperature. This error signal is applied directly to the noninverting input of differential amplifier 130 and indirectly to the inverting input of differential amplifier 130 through inverting integrator 128. The output signal from differential amplifier 130 is therefore related to the sum of the error signal and of the time integral thereof and is applied to transistor 132 and power transistor 134 connected in a conventional Darlington configuration and powered by an unregulated supply voltage V_B. As the conduction of the Darlington pair varies in response to the output signal from differential amplifier 130, the heat produced by power transistor 134 also varies so as to maintain the temperature of recess 74 (and therefore the temperature of the components therein including LED groups 102, 104 and 106), substantially at the desired tempe ature; established by the signal from adjustable resistor 122.

The output signal from voltage reference circuit 120 is coupled by a resistor 136 to the IN signal input of a multiplexer 138, and is coupled directly to the input of constant current source 140 which responsively provides a constant current to a lead 141 that is connected through a resistor 143 to the inverting input of a differential amplifier 142 and that is connected directly to the 1, 3, 4, 6 and 7 outputs of a multiplexer 146. The supply voltage V_S, is coupled to lead 141 through a resistor 145. As a result of the constant current through resistor 145, the voltage on lead 141 is always a predetermined amount below the supply voltage V_S, and therefore comprises a reference voltage whose use will be described hereinafter.

The output signal from temperature sensor 20 (FIGURE 1) is applied to the noninverting input of differential amplifier 142. The output signal from differential amplifier 142 is applied to the input of an amplifier 144 on whose output appears a signal TEMP. Amplifier 144 is biased by the supply voltage V (by means not illustrated) so that signal TEMP varies between predetermined low and high current values in proportion to the voltage of the output signal from differential amplifier 142 and so that the lower current value is obtained when the voltage of the output signal from differential amplifier 142 is zero. Resistor 143 is chosen so that the voltage of the output signal from differential amplifier 142 is zero when the measured temperature of the pulp, as represented by the output signal from temperature sensor 20, ' has a predetermined value, e.g., 0ºC. Accordingly, the current of signal TEMP is at its lower current value when the measured temperature of the pulp is 0ºC, and varies in proportion to variations of the measured pulp temperature above 0ºC.

Since amplifier 144 is biased from the supply voltage V_S, and since the reference voltage appearing on lead 141 always has a predetermined relation to the value of the supply voltage V_S, it will be seen that the aforementioned relationship between the current value of signal TEMP and the measured pulp temperature is maintained notwithstanding variations in the supply voltage V_S.

In order to provide for sequential emission of light pulses by LED groups 102, 104 and 106 and for synchronous detection of that portion of each light pulse that is reflected by the pulp and incident on photocell 112, a clock source 148 provides a clock signal to the clock input (CL) of a binary ripple counter 150 whose A output is coupled to an INH control input of multiplexer 146 and whose B, C and D outputs are connected to respective B, C and D control inputs of multiplexers 146 and 138. As an example, the frequency of the clock signal from clock source 148 may be 40 kHz, and the frequencies of the signals on the A, B, C and D outputs of counter 150 may be, respectively, 5 kHz, 2.5 kHz, 1.25 kHz, and 0.625 kHz (as illustrated in FIGURE 7). Multiplexers 146 and 138 are constructed so as to connect their IN signal inputs to one of eight signal outputs (labeled 0-7) in response to the various binary levels of the signals on their B, C and D control inputs, and are further constructed so as to inhibit any such signal connection in response to a positive logic level appearing on their INH control inputs. It will be noted that the 1NH control input of multiplexer 138 is connected to ground potential, whereby the aforementioned inhibiting operation is disabled. As illustrated in FIGURE 7, the signals from counter 150 and the construction of multiplexers 146 and 138 establish a plurality of successive lime windows or "channels" in a repetitive measurement cycle, the channels being labeled 0-7 and corresponding to the similarly labeled outputs of multiplexers 146 and 138. It will be further noted that due to the signal applied to the 1NH control input of multiplexer 146, a signal connection between the IN signal input of multiplexer 146 and any of the outputs ^'thereof will be inhibited during the first half of each of channels 0-7.

The 0, 2 and 5 outputs of multiplexer 138 are connected, respectively, to the inputs of LED drivers 152, 154 and 156. LED driver 152 has a pair of outputs, one of which is coupled to the positive supply voltage V_S through the series connection of three LEDs in group 102 and the other of which is coupled to the negative supply voltage -V_S through the remaining three LEDs in group 102. LED drivers 154 and 156 each have a single output, with the output of LED driver 154 being coupled to the negative supply voltage -V_S through the series connection of the three LEDs in group 104 and with the output of LED driver 156 being coupled to the negative supply voltage -V_S through the series connection of the three LEDs in group 106. Each of the LED drivers 152, 154 and 156 is constructed so as to provide a constant voltage drop across the seriesconnected LEDs coupled thereto in response to a constant current signal applied to the input of the LED driver. Referring also to FIGURE 7, the signal on the IN signal input of multiplexer 138 (which has a constant current level due to the constant voltage appearing on the output of voltage reference circuit 120) is sequentially coupled to the inputs of LED drivers 152, 154 and 156, thereby resulting in the emission of substantially constant amplitude light pulses by LED groups 102, 104 and 106 during sequential channels 0, 2 and 5. Each of these light pulses is optically coupled to the pulp within conduit 10 (FIGURE 1) by fiber optic bundle 16, and that portion of each light pulse reflected by the pulp is optically coupled lo photocell 112 by fiber optic bundle 18. The output signal from photocell 112 is preamplified in preamplifier 160 and then filtered in filter circuit 162. The output signal from filter circuit 162 is further amplified in an amplifier 164, and the output signal from amplifier 164 is coupled to the IN signal input of multiplexer 146 through the series connection of a resistor 165 and a capacitor 166. The 0, 2 and 5 outputs of multiplexer 146 are connected directly to the inputs of amplifiers 182, 184 and

186 which provides respective output signals CH0, CH2 and CH5, and are also connected in common to lead 141 by respective capacitors 172, 174 and 176.

The output signal from photocell 112, in addition to including components presenting the reflected portions of each light pulse, also includes low frequency noise such as that provided by incandescent light sources, medium frequency noise such as that provided by fluorescent light sources, and high frequency noise such as switching transients in the light pulses provided by LED groups 102, 104 and 106. In order to substantially reduce the amplitude of both low frequency and high frequency noise appearing in the output signal from amplifier 164, filter circuit 162 is constructed so as to provide both a low frequency and a high frequency roll-off. The output signal from photocell 112, as it passes through preamplifier 160, filter circuit 162 and amplifier 164, is also subject to varying amounts of dc offset which are substantially eliminated by providing ac coupling between preamplifier 160 and filter circuit 162 and between filter circuit 162 and amplifier 164.

The output signal from amplifier 164 comprises a voltage which is seen in FIGURE 7 to comprise a plurality of sequential pulses" occurring during channels 0, 2 and 5. The baseline of the output signals from amplifier 164 is substantially constant, due to the elimination of dc offset as previously described. It will be seen that each pulse rises from this baseline at a certain rate (which is determined by the time constants of photocell 112, preamplifier 160, filter circuit 162 and amplifier 164) to a value that is related to that portion of the corresponding light pulse that is reflected by the pulp. Although not illustrated in FIGURE 7, the output signal from amplifier 164 also has superimposed thereon a ripple representing the medium frequency noise present in the output signal from photocell 112 and the low frequency and high frequency noise present in the output signal from photocell 112 that have not been eliminated by filter circuit 162.

At times during the measurement cycle (e.g., during the second half of each of channels 1, 3, 4, 6 and 7) preceding the channel during which a pulse in. the output signal from amplifier 164 should appear (e.g., during channels

0, 2 and 5), multiplexer 146 couples the reference voltage appearing on lead 141 to the side of capacitor 166 connected to the IN signal input of multiplexer 146. Under the assumption that there is no light incident on photocell 112 during these times, it will be seen that the voltage across capacitor 166 is equal to the reference voltage at the initiation of channels 0, 2 and 5. Let it now be assumed that there is no light incident on photocell 112 during channels 0, 2 and 5, e.g., the reflectance of the pulp is zero and there is no noise present. As multiplexer

146 connects its IN signal input to its 0, 2 and 5 outputs during the second half of channels 0, 2 and 5 in the measurement cycle, it will be seen that the voltage across each of capacitors 172, 174 and 176 will be zero. Amplifiers 182, 184 and 186, in a manner similar to amplifier 144, are each biased by the supply voltage V_S and are constructed so that their respective output signal varies between predetermined low and high current values. When the voltage applied to the input of each of amplifiers 182, 184 and 186 is zero, the output signal therefrom is at the low current value, and, as the voltage applied to the input of each of amplifiers 182, 184 and 186 increases from zero, the output signal therefrom proportionally increases from the low current value. Under the assumption that the light reflectance of the pulp is zero and that there is no noise present, each of the output signals CH0, CH2 and CH5 will accordingly be at the low current value.

If it is now assumed that the reflectance of the pulp is not zero, it will be seen that the voltage across capacitor 166 varies as the voltage of the output signal from amplifier 164 varies during each sequential pulse therein. During the first half of each of channels 0, 2 and 5, multiplexer 146 is inhibited from connecting its IN signal input to its 0, 2 and 5 outputs so as to allow the voltage across capacitor 166 to rise to a value related to the reflectance of the pulp. During the second half of each of channels 0, 2 and 5, multiplexer 146 connects its IN signal input to its 0, 2 and 5 outputs, whereby the voltage across capacitor 166 is sequentially sampled and stored in capacitors 172, 174 and 176. Since the voltages across capacitors 172, 174 and 176 are zero when no light is incident on photocell 112 as previously described, it will be seen that the voltages stored across capacitors 172, 174 and 176 during sampling are proportional to the reflectance of the pulp within the respective spectral bands established by LED groups 102, 104 and 106. The respective voltages across capacitors 172, 174 and 176 accordingly cause amplifiers 182, 184 and 186 to adjust the current of their respective output signals CH0, C H2 and CH5 so that those currents are proportional to the reflectance of the pulp within each of the spectral bands. Preferably, the time constants associated with capacitors 172, 174 and 176 and their associated amplifiers 182, 184 and 186 is chosen so that capacitors 172, 174 and 176 do not substantially discharge between successive measurement cycles. Accordingly, output signals CH0, CH2 and CH5 have substantially constant current values that are updated once each measurement cycle. Since amplifiers 182, 184 and 185 are biased from the supply voltage V_S and since the reference voltage appearing on lead 141 and applied to capacitors 172, 174 and 176 always has a predetermined relation to the supply voltage V_S, it will be seen that the aforementioned relationship between the current value of output signals CHO, CH2 and CH3 and the measured pulp reflectance is maintained notwithstanding variations in the supply voltage V_S.

As previously noted, the output signal from amplifier 164 will contain medium frequency noise and uncompensated low frequency and high frequency noise. Those noise components arc substantially eliminated as follows. Resistor 165, capacitor 166, the internal resistance of multiplexer 146, and capacitors 172, 174 and 176 establish a predetermined time constant which limits the rate at which charge can be transferred to any of capacitors 172, 174 and 176. Accordingly, further filtering is provided for certain high frequency noise.

The remaining noise generally will be either aperiodic or asynchronous with the detection afforded by multiplexer 146. Accordingly, such aperiodic and asynchronous noise is substantially averaged over succeeding measurement cycles. While the invention has been described with reference to a preferred embodiment, it is to be clearly understood by those skilled in the art that the invention is not limited thereto. For example, the apparatus can be readily adapted to provide multiple wavelength measurements of the light reflectance of substances other than paper pulp. Therefore, the scope of the invention is to be interpreted only in conjunction with the appended claims.

Claims

1. An apparatus for measuring the light reflectance of a substance at a distinct wavelength, said apparatus comprising: first means for illuminating the substance with a light pulse that occurs at a predetermined time in a predetermined measurement cycle, said light pulse comprising substantially monochromatic light; and, second means for measuring light that is reflected by the substance in synchronism with the occurrence of said light pulse during said measurement cycle.

2. An apparatus as recited in Claim 1 which is adapted to measure the light reflectance of a substance at a plurality of distinct wavelengths, wherein said first means is operative to illuminate the substance with a plurality of light pulses that sequentially occur in said measurement cycle, each said light pulse comprising substantially monochromatic light having a nominal wavelength that is distinct from that of any other said light pulse; and, wherein said second means is operative to measure light that is reflected by the substance in synchronism with the sequential occurrence of said light pulses during said measurement cycle.

3. An apparatus as recited in Claim 2, wherein said first means is operative to illuminate the substance with said plurality of light pulses in each of a plurality of successive and periodic measurement cycles; and wherein sai d second means is operative to measure light reflected by the substance during each of said plurality of measurement cycles.

4. An apparatus as recited in Claim 2, wherein said first means includes.: a plurality of light sources, each of said light sources being constructed so as to provide substantially monochromatic light having a distinct nominal wavelength; means for sequentially energizing said plurality of light sources during said measurement cycle; and, means for optically coupling said plurality of light sources to the substance.

5. An apparatus recited in Claim 4, wherein each said light source is constructed so as to provide light having a predetermined spectral band of wavelengths distributed about said distinct nominal wavelength.

6. An apparatus as recited in Claim 5, wherein each said light source includes at least one light-emitting diode.

7. An apparatus as recited in Claim 6, wherein each said light source includes a plurality of light-emitting diodes.

8. An apparatus as recited in Claim 7, wherein emitting diodes comprising each said light source are of an identical type; and, wherein the number of the light-emitting diodes comprising each said light source are chosen so that each said light source provides substantially the same light output.

9. An apparatus as recited in Claim 6, wherein said first means includes heating means for maintaining the temperature of said light-emitting diodes at a substantially constant value.

10. An apparatus as recited in Claim 4, wherein said opticallycoupling means includes an elongated fiber optic bundle having a proximal end disposed in proximity to the substance and a distal end disposed in proximity to said plurality of light sources.

11. An apparatus as recited in Claim 10, wherein said opticallycoupling means further includes a lens disposed between said distal end of said fiber optic bundle and said plurality of light sources, said lens and said plurality of light sources being constructed and arranged so as to distribute the light from each of said plurality of light sources uniformly across said distal end of said fiber optic bundle.

12. An apparatus as recited in Claim 2, wherein said second means includes: a light sensor adapted to provide an electrical output signal related to the intensity of light incident on said light sensor; means for optically coupling light from the substance to said light sensor; and, means for detecting said electrical output signal from said light sensor in synchronism with the sequential occurrence of said plurality of light pulses during said measurement cycle.

13. An apparatus as recited in Claim 12, wherein said opticallycoupling means includes an elongated fiber optic bundle having a proximal end disposed in proximity to the substance and a distal end disposed in proximity to said light sensor.

14. An apparatus as recited in Claim 13, wherein said opticallycoupling means further includes a lens disposed between said distal end of said fiber optic bundle and said light sensor, said lens and said light sensor being constructed and arranged so as to focus light from said distal end of said fiber optic bundle on said light sensor.

15. An apparatus as recited in Claim 12, wherein said light sensor is substantially wavelength-insensitive in a range of wavelengths encompassing the distinct nominal wavelengths of said plurality of light sources.

16. An apparatus as recited in Claim 12, wherein said means for detecting includes: sampling means having an input and being adapted to sarnple and hold the magnitude of an electrical signal applied to said input; and, means for coupling said electrical output signal from said light sensor to said input of said sampling means; and, wherein said apparatus further comprises a control means for causing said first means to illuminate the substance with said plurality of light pulses during said measurement cycle and for causing said sampling means to sequentially sample and hold the magnitude of the electrical signal applied t o said input thereof in synchronism with the sequential occurrence of said plurality of light pulses during said measurement cycle.

17. An apparatus as recited in Claim 16, wherein said means for coupling said electrical output signal from said light sensor to said input of said sampling means includes a first capacitance storage means; and, wherein said sampling means includes a plurality of second capacitance storage means, said sampling means being controlled by said control means to connect said first capacitance storage means to a distinct one of said plurality of second capacitance storage means in synchronism with the corresponding one of said plurality of light pulses during said measurement cycle.

18. An apparatus as recited in Claim 17, wherein said means for detecting further includes: a source of a stable reference voltage; means coupling said reference voltage to said plurality of second capacitance storage means; and, wherein said sampling means is operative to couple said reference voltage to said first capacitance storage means at sequential times during said measurement cycle preceding the occurrence of each of said plurality of light pulses.

19. An apparatus as recited in Claim 16, wherein said control means is operable to cause said first means and said sampling means to operate during each of a plurality of successive measurement cycles that occur at a predetermined frequency.

20. An apparatus as recited in Claim 19, wherein said means for coupling said electrical output signal from said light sensor to said sampling means includes filter means having a low-frequency roll-off and a high-frequency roll-off respectively below and above the predetermined frequency of said plurality of successive measurement cycles.

21. An apparatus as recited in Claim 2, further comprising third means for establishing a plurality of successive and periodic measurement cycles and a plurality of sequential time windows during each of said plurality of measurement cycles; wherein said first means is responsive to said third means to illuminate the substance with a distinct one of said plurality of light pulses during a selected and corresponding one of said plurality of sequential window; and, wherein said second means is responsive to said third means to measure the light reflected from the substance during each said selected and corresponding one of said plurality of sequential windows.

22. An appara tus as recited in Claim 21, wherein said plurality of quential windows have equal durations.

23. An apparatus as recited in Claim 21, wherein the duration of each said light pulse is substantially equal to the duration of its corresponding sequential window.

24. An apparatus as recited in Claim 23, wherein the time during which the light reflected by the substance is measured during each said selected and corresponding sequential window is substantially less than the duration of said selected and corresponding sequential window.

25. An apparatus as recited in Claim 24, wherein the light reflected by the substance is measured only during a terminal portion of each said selected and corresponding sequential window.

26. An apparatus as recited in Claim 1, wherein said first means includes: a light source that is constructed to provide substantially monochromatic light having a distinct nominal wavelength; means for energizing said light source at said predetermined time during said measurement cycle; and, means for optically coupling said light source to the substance.

27. An apparatus as recited in Claim 26, wherein said light source includes at least one light-emitting diode.

28. An apparatus as recited in Claim 27, wherein said light source includes a plurality of light-emitting diodes.

29. An apparatus as recited in Claim 27, wherein said first means includes heating means for maintaining the temperature of said lightemitting diode at a substantially constant value.

30. An apparatus as recited in Claim 26, wherein said opticallycoupling means includes an elongated fiber optic bundle having a proximal end disposed in proximity to the substance and a distal end disposed in proximity to said light source.

31. An apparatus as recited in Claim 30, wherein said opticallycoupling means further includes a lens disposed between said distal end of said fiber optic bundle and said light source, said lens and said light source being constructed and arranged so as to distribute the light from said light source uniformly across said distal end of said fiber optic bundle.

32. An apparatus as recited in Claim 1, wherein said second means includes: a light sensor adapted to provide an electrical output signal related to the intensity of light incident on said light sensor; means for optically coupling light from the substance to said light sensor; and, means for detecting said electrical output signal from said light sensor in synchronism with the occurrence of said light pulse during said measurement cycle.

33. An apparatus as recited in Claim 32, wherein said opticallycoupling means includes an elongated fiber optic bundle having a proximal end disposed in proximity to the substance and a distal end disposed in proximity to said light sensor.

34. An apparatus as recited in Claim 33, wherein said opticallycoupling moans further includes a lens disposed between said distal end of said fiber optic bundle and said light sensor, said lens and said light sensor being constructed and arranged so as to focus light from said distal end of said fiber optic bundle on said light sensor.

35. An apparatus as recited in Claim 32, wherein said means for detecting includes: sampling means having an input and being adapted to sample and hold the magnitude of an electrical signal applied to said input; and, means for coupling said electrical output signal from said light sensor to said input of said sampling means; and, wherein said apparatus further comprises a control means for causing said first means to illuminate the substance with said light pulse at said predetermined time during said measurement cycle and for causing said sampling means to sample and hold the magnitude of the electrical signal applied to said input thereof in synchronism with the occurrence of said light pulse during said measurement cycle.

36. An apparatus as recited in Claim 35, wherein said means for coupling said electrical output signal from said light sensor to said input of said sampling means includes a first capacitance storage means; and, wherein said sampling means includes a second capacitance storage means, said sampling means being controlled by said control means to connect said first capacitance storage means to said second capacitance storage in synchronism with said light pulse during said measurement cycle.

37. An apparatus as recited in Claim 36, wherein said means for detecting further includes: a source of a stable reference voltage; means coupling said reference voltage to said second capacitance storage means; and, wherein said sampling means is operative to couple said reference voltage to said first capacitance storage means at a time during said measurement cycle preceding the occurrence of said light pulse.

38. An apparatus as recited in . Claim 37, wherein said control means is operable to cause said first means and said sampling means to operate during each of a plurality of successive measurement cycles that occur at a predetermined frequency.

39. An apparatus as recited in Claim 38, wherein said means for coupling said electrical output signal from said light sensor to said sampling means includes filter means having a low-frequency roll-off and a high-frequency roll-off respectively below and above the predetermined frequency of said plurality of successive measurement cycles.

40. An apparatus that is particularly adapted for measuring the brightness of paper pulp within each of a plurality of distinct spectral bands of wavelengths, said apparatus comprising: an elongated tube having proximal and distal ends, said proximal end being adapted to be mounted in proximity to the paper pulp; first and second, elongated fiber optic bundles disposed in said tube and each having a proximal end located in proximity to said proximal end of said tube and a distal end located in proximity to said distal end of said tube; a housing receiving said distal end of said tube, said distal ends of said first and said second fiber optic bundles being arranged so as to extend into said housing; a plurality of light sources mounted within said housing and arranged so as to illuminate said distal end of said first fiber optic bundle, each of said plurality of light sources being capable of providing a light Output within one of the plurality of distinct spectral bands; and, a light sensor mounted within said housing and arranged so as to receive light from said distal end of said second fiber optic bundle.

41. An apparatus as recited in Claim 40, further comprising a temperature sensor disposed in said tube in proximity to said distal end thereof for measuring the temperature of the paper pulp.

42. An apparatus as recited in Claim 40, further comprising mounting means disposed within said housing for locating said plurality of light sources and said light sensor in a predetermined spatial relationship with said distal ends of said first and second fiber optic bundles, respectively.

43. An apparatus as recited in Claim 42, wherein said mounting means includes: a first block having first and second, opposing surfaces; first and second recesses formed in said first block and extending therein from said first surface; first and second apertures formed in said first block and extending from said first and second recesses, respectively, to said second surface, said distal ends of said first and second fiber optic bundles being received in said first and second apertures, respectively; and, a printed circuit board assembly supported on and by said first block and mounting said plurality of light sources and said light sensor so that said plurality of light sources are substantially located within said first recess and optically aligned with said distal end of said first fiber optic bundle and so that said light sensor is substantially located within said second recess and optically aligned with snid distal end of said second fiber optic bundle.

44. An apparatus as recited in Claim 43, wherein each of said plurality of light sources comprises a plurality of light-emitting diodes mounted on said printed circuit board assembly.

45. An apparatus as recited in Claim 44, further, comprising heating means disposed within said first recess for maintaining the temperature of each said light-emitting diode at a substantailly constant value.

46. An apparatus as recited in Claim 45, wherein said heating means is mounted on said printed circuit board assembly.

47. .An apparatus as recited in Claim 46, wherein said heating means comprises a power transistor mounted on said printed circuit board assembly.

48. An apparatus as recited in Claim 46, wherein said healing means comprises a temperature sensor mounted on said printed circuit board assembly.

49. An apparatus as recited in Claim 44, wherein said printed circuit board assembly includes at least one printed circuit board supported on and by said first surface of said first block.

50. An appara tus as recited in Claim 49, wherein said printed circuit board assembly includes a first printed circuit board supported on and by said first surface of said first block and having the light-emitt ing diodes comprising certain ones of said plurality of light sources mounted thereon, and a second printed circuit board supported on and by said first printed circuit board and having the light-emitting diodes comprising the remaining ones of said plurality of light sources mounted thereon, said first printed circuit board being provided with apertures therein through which the light-emitting diodes mounted on said second printed circuit board extend,

51. An apparatus as recited in Claim 44, further comprising a lens supported within said first aperture and disposed between said distal end of said first fiber optic bundle and said plurality of light sources, said lens and the plurality of light-emitting diodes comprising each said light source being constructed and arranged so as to distribute the light from each said light source uniformly across said distal end of said first fiber optic bundle.

52. An apparatus as recited in Claim 43, wherein said light sensor includes a light-sensitive surface that is optically aligned with said distal end of said second fiber optic bundle.

53. An apparatus as recited in Claim 52, further comprising a lens supported within said second aperture and disposed between said distal end of said second fiber optic bundle and said light-sensitive surface, said lens and said light-sensitive surface being constructed and arranged so as to focus light from said distal end of said second fiber optic bundle on said light-sensitive surface.

54. An apparatus as recited in Claim 43, wherein said light sensor is substantially wavelength-insensitive within a range of wavelengths encompassing the distinct spectral bands of said plurality of light sources.

55. An apparatus as recited in Claim 54, wherein said ligh t sensor comprises a photocell.

56. An apparatus as recited in Claim 43, wherein said printed circuit board assembly and said first block are encapsulated so as to substantially seal said printed circuit board assembly and said first and second recesses from the environment.

57. An apparatus as recited in Claim 43, further comprising a sheet of elastomeric material secured to said second surface of said first block and surrounding said first and second fiber optic bundles so as to seal said distal ends thereof from the environment.

58. An apparatus as recited in Claim 43, wherein said mounting means includes means secured to said housing for supporting the portions of said first and second fiber optic bundles, that are adjacent said distal ends thereof in spaced, parallel relationship.

59. An apparatus as recited in Claim 58, wherein said means secured to said housing includes a second block having formed therein and extending therethrough first and second apertures through which respectively pass said first and said second fiber optic bundles.

60. An apparatus as recited in Claim 59, wherein said first block is supported on and by said second block.

61. An apparatus as recited in Claim 40, wherein said light sensor is adapted to provide an electrical output signal related to the intensity of light incident thereon; and, further comprising means for pulsing said plurality of light sources in sequence during each of a plurality of successive measurement cycles and for detecting said electrical output signal from said light sensor in synchronism with the sequential pulsing of said plurality of light sources during each said measurement cycle.