DISTINCT WAVELENGHT LIGHT REFLECTANCE MEASURING APPARATUS

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 REFLECΓANCE 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. B..c..κro._nd 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 stHges 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 5 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 10 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 15 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 20 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 25 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 30 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. 35 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 whi is capable of providing accurate and precise measurements of light reflectan at a single wavelength or at a plurality of distinct wavelengths under hig adverse environmental conditions. Summary of the Invention Briefly, these objects and other objects and advantages that will realized from a consideration of the following portion of the specification a achieved in an apparatus for measuring the light reflectance of a substance at distinct wavelength. The apparatus comprises: first means for illuminating the substance with a light pulse th occurs at a predetermined time in a predetermined measurement cycle, the lig pulse comprising substantially monochromatic light; and, second means for measuring light that is reflected by the substan in synchronism with the occurrence of the light pulse' during the measureme cycle. Such an apparatus may be adapted for measuring the lig reflectance of a substance at a plurality of distinct wavelengths. When adapted, the first means is operative to illuminate the substance with a pluralit of light pulses that sequentially occur in a predetermined measurement cycl each light pulse comprising substantially monochromatic light having a nomin " wavelength that is distinct from that of any other light pulse, and the secon means is operative to measure light that is reflected by the substance i synchronism with the sequential occurrence of the light pulses during th measurement cycle. In a preferred embodiment, the first means includes: a plurality o light sources, each of the light sources being constructed so as to provid substantially monochromatic light having a distinct nominal wavelength; mean for sequentially energizing the plurality of light sources during the measuremen cycle; and, means for optically coupling the plurality of light sources to th substance. The second means includes: a light sensor arfnpted to provide a electrical output signal related to the intensity of light incident on the ligh 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 i synchronism with the sequential occurrence of the plurality of light pulses durin the measurement cycle. Preferably, the plurality of light sources and the ligh sensor are optically coupled with the substance by respective fiber optic bundles and each light source includes a plurality of light-emitting diodes whos o RIGINAL tempcrature is maintained at a substantially constant value. The . apparatus of the invention may be particularly adapted f measuring the brightness of paper pulp within each of a plurality of distin spectral bands of wavelengths, and as such also comprises: an elongated tube having proximal and distal ends, the proximal e being adapted to be mounted in proximity to the paper pulp; first and second, elongated fiber optic bundles disposed in the tub and each having a proximal end located in proximity to the proximal end of th tube and a distal end located in proximity to the distal end of the tube; and, housing receiving t e distal end of the tube, the distal ends of the first an second fiber optic bundles being arranged so as to extend into the housing. The plurality of light sources are mounted within the housing an arranged so as to illuminate the distal end of the first fiber optic bundle, wit each light source being capable of providing a light output within one of th plurality of distinct spectral bands. The light sensor is also mounted within th housing and arranged so as to receive light from the distal end of the secon " fiber optic bundle. Preferably, mounting means are provided which are disposed withi the housing for locating the plurality of light sources and the light sensor in predetermined spatial relationship with the distal ends of the first and secon fiber optic bundles, respectively. The mounting means includes: a first bloc having first and second, opposing surfaces; first and second recesses formed i the first block and extending therein from the first surface; first and secon apertures formed in the first block and extending from the first and secon recesses, respectively, to the second surface, the distal ends of the first an second fiber optic bundles being received in the first and second apertures respectively; and, a printed circuit board assembly supported on and by the firs block and mounting the plurality of light sources and the light sensor so that th plurality of- light sources are substantially located within the first recess an optically aligned with the distal end of the first fiber optic bundle and so that th light sensor is substantially located within the second recess and optically aligne with the distal end of the second fiber optic bundle. Each light source comprises a plurality of light-emitting diode mounted on the printed circuit board assembly, and heating means are dispose within the first recess for maintaining the temperature of each light-eι"τ-ittin diode at a substantially constant value. The printed circuit board assembly an the first block are encapsulated so as to substantially seal the printed circui board a Brief Description of the Drawings The invention can best be understood by reference to the follow portion of the specification, taken in conjunction with the accompany drawings in which: FIGURE 1 is a pictorial view of a multiple wavelength li reflectance measuring apparatus that is particularly adapted for measuring brightness of paper pulp; FIGURE 2 is an exploded, pictorial view of a housing forming p of the apparatus in FIGURE 1 and of the components within the housing; FIGURE 3 is an exploded, partial cross-sectional view of cert components within the housing including a block and a printed circuit bo 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 boa comprising the printed circuit board assembly in FIGURE 3; FIGURE 6 is an electrical schematic diagram of the apparatus; a FIGURE 7 is a timing diugram illustrating the operation of t apparatus. Description of the Preferred Embodiment Referring now to FIGURE 1, the pulp whose brightness is to measured passes through a conduit 10 (which may also comprise the vessel of bleaching tower). At a point where the pulp brightness is to be measure conduit 10 is provided with an opening surrounded by a sleeve 12 integral wi conduit 10. An elongated tube 14 of the measuring apparatus has one end there fitted into sleeve 12 and is supported from conduit 10 by means not illustrate Tube 14 is preferably composed of a corrosion resistant material such as titaniu or stainless steel, and a valve structure (not illustrated) is provided f permitting lube 14 to be removed from sleeve 12 and for permitting sleeve 12 t be closed whenever tube 14 is so removed. A pair of elongated, cylindrical fib optic bundles 16, 18 are disposed within tube 14 and extend along its length fro the end thereof proximate conduit 10 to the end thereof distant from conduit 1 Fiber optic bundles 16, 18 are constructed so as to transmit light between thei proximal and distal ends, and their cylindrical surfaces are preferably provide with an opaque coating. The proximal ends of bundles 16, 18 are opticall coupled with the interior of conduit 10 through a transparent quartz windo mounted in the end of tube 14. A temperature sensor 20. is also mounted withi tube 14 behind the quartz window so as to sense the temperature of the pul within conduit 10, and en electrical cable 22 20 and extends to the distal end of tube 14. The distal end of tube 14 is receiv within a corresponding aperture 26 of a housing 24 so that the distal ends bundles 16, 18 and cable 22 extend into housing 24, and lube 14 is secured housing 24 by means not illustrated. As described in detail hereinafter with reference to FIGURES 2- a plurality of light sources and a light sensor are disposed within housing 24, wi the light sources being optically coupled with the distal end of fiber optic bund 16 and with the light sensor being optically coupled with the distal end of fib optic bundle 18. Each light source preferably includes a group of light-emitti diodes that are designed to emit substantially monochromatic light, that is, lig within a predetermined spectral band of wavelengths distributed about a nomin wavelength. The light sensor is preferably of a type that provides an electric signal related to the intensity of the light incident thereon, and that substantially wavelength-insensitive in a range of wavelengths including t spectral bands of the light sources. During a measurement cycle, the plurality light sources are caused to emit light pulses in a predetermined sequence whi are coupled to the interior of conduit 10 by fiber optic bundle 16 to as t illuminate the pulp therein. Light reflected from the pulp is coupled to the lig sensor by fiber optic bundle 18, and the output signal from the light sensor detected in synchronism with the sequential light pulses coupled to fiber opti bundle 16 so as to provide a plurality of output signals each representing the lig reflectance of the pulp within one of the plurality of spectral bands. Provision i also made to compensate the output signals for drift, noise and the ambient ligh incident on the light sensor. Finally, the output signal from temperature senso 20 is continuously monitored to provide a signal related to the temperature o the pulp within conduit 10. Referring now to FIGURE 2, housing 24 has defined therein cavity 30 which is open at the side of housing 24 opposing the side in which i located aperture 26. The distal . ends of bundles 16, 18 and of cable 22 exten above the floor of cavity 30, and a block 32, a block 34 and a printed circui board assembly 36 mounted on block 34 are situated within cavity 30. Block 3 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 throug corresponding apertures 46 in block 32 and received in corresponding threade apertures 48 formed in ledges 42. Threaded fasteners 39 and 41 are received i corresponding threaded apertures (not illustrated) in block 32 and the end thereof bear against bundles 16 and 18 in assembly so as to restrain bundles 1 and 18 from lateral or longitudinal movement. In order to provide a support block 34 (and printed circuit board assembly 36), a plurality of threa apertures 50 are provided in block 32 into which are inserted a plurality spacers 52. Block 34 is secured in position by a plurality of fasteners 54 pass through corresponding apertures 56 in block 34 and received in correspond threaded apertures 58 in spacers 52. The open end of cavity 30 is closed b cover 60 that is secured in position by a plurality of threaded fasteners passing through corresponding apertures 64 in cover 60 and received corresponding threaded apertures 66 in housing 24. With additional reference now to FIGURES 3-5, block 34 has upper surface 34 A and a lower surface 34B, and printed circuit board assembly includes a printed circuit board 70 supported on surface 34A and a printed circ board. 72 supported above printed circuit board 70. Substantially rectangul recesses 74, 76 extend into block 34 from upper surface 34A, and substantial cylindrical apertures 78, 80 extend between the floor of recesses 74 and 7 - respectively, and surface 34B. Lenses 82 and 84 are situated in apertures 78 a 80 just below the floors of recesses 74 and 76, respectively, and mounted there by respective retaining rings 86, 88. When block 34 is supported on spneers 52 previously described, the distal ends of bundles 16 and 18 are located withi apertures 78 and 80 and abut lenses 82 and 84, respectively. Printed circu board 72 is supported on printed circuit board 70 by a plurality of spficers 92, an printed circuit boards 70 and 72 are secured to block 40 by a plurality fasteners 94 that pass through corresponding apertures 96 in printed circuit boar 72, spacers 92, and corresponding apertures 98 in printed circuit board 70, an that are received in corresponding threaded apertures 1 00 in block 34. .The light sources consist of a first group 102 of light-emittin diodes (LEDs) and a second group 104 of EDs' mounted. on printed circuit bo..r 70, both extending toward and situated within cavity 74 in assembly, and a thir group 106 of LEDs mounted on printed circuit board 72 and extending throu corresponding apertures 108 in printed circuit board 70 in assembly. The LEDs i each group are of an identical type and are designed to emit substantiall monochromatic light, that is, light within a predetermined spectral band o 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 o'ptϊcal_.characteristics, so that the number of LEDs in each 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 heat- shrinkable 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 112 A. 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, 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 g and -Vg 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 voltnge 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 noninverling 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 t e 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 . 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 j _ai__ir_§; BAD ORIGINA X^^^ establϊshed 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 responsϊvely 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-, 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_, 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 noninvertϊng 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., 0 β C. Accordingly, the current of signal TEMP is at its lower current value when the measured lemperature of the pulp is 0 β C, and varies in proportion to variations of the measured pulp temperature above 0 β C. Since amplifier 144 is biased from the supply voltage V_, and since the reference voltage appearing on lead 141 always has a predetermined relation to the value of the supply voltage V g , 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„. 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 * bf the____ώμ____ls BAD ORIGINAL f ^W^ Έ ^ ^ ' ' - ¬ on the A, B, C and D outputs of counter 150 may be, respectively, 5 kH 2.5 kHz, 1.25 kHz, and 0.625 kHz (as illustrated in FIGURE 7). Multiplexers 14 and 138 are constructed so as to connect their IN signal inputs to one of eig signal outputs (labeled 0-7) in response to the various binary levels of the signal on their B, C and D control inputs, and are further constructed so as to inhibi any such signal connection in response to a positive logic level appearing on thei INH control inputs. It will be noted that the 1NH control input of multiplexer 13 is connected to ground potential, whereby the aforementioned inhibitin operation is disabled. As illustrated in FIGURE 7, the signals from counter 15 and the construction of multiplexers 146 and 138 establish a plurality o successive lime windows or "channels" in a repetitive measurement cycle, th channels being labeled 0-7 and corresponding to the similarly labeled outputs o multiplexers 146 and 138. It will be further noted that due to the signal applie to the INH control input of multiplexer 146, a signal connection between the I signal input of multiplexer 146 and any of the outputs ' thereof will be inhibite 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 g through the series connection of three LEDs in group 102 and the other of which is coupled to the negative supply voltage -V„ 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„ 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_ 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 series- connected 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 circui 162.is further amplified in an amplifier 164, and the output signal from amplifie 164 is coupled to the IN signal input of multiplexer 146 through the scrie connection of a resistor 165 and a capacitor 166. The 0, 2 and 5 outputs o multiplexer 146 are connected directly to the inputs of amplifiers 182, 184 an 186 which provides respective output signals CHO, CH2 and CH5, and are als connected in common to lead 141 by respective capacitors 172, 174 and 176. The output signal from photocell 112, in addition to includin components presenting the reflected portions of each light pulse, also include low frequency noise such as that provided by incandescent light sources, mediu frequency noise such as that provided by fluorescent light sources, and hig frequency noise such as switching transients in the light pulses provided by LE groups 102, 104 and 106. In order to substantially reduce the amplitude of bot 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. BAD ORIGINAL $TTR__X 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 essumed 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 - . . •■• - » BAD OMPI -14- measurement cycles. Accordingly, output signals CHO, CH2 and CH5 hav substantially constant current values that are updated once each measuremen cycle. Since amplifiers 182, 184 and 185 are biased from the supply voltage V and since the reference voltage appearing on lead 141 and applied to capacitor 172, 174 and 176 always has a predetermined relation to the supply voltage V g , i will be seen that the aforementioned relationship between the current value o output signals CHO, CH2 and CH3 and the measured pulp reflectance i maintained notwithstanding variations in the supply voltage V g . As previously noted, the output signal from amplifier 164 wil contain medium frequency noise and uncompensated low frequency and hig frequency noise. Those noise components arc substantially eliminated as follows Resistor 165, capacitor 166, the internal resistance of multiplexer 146, an capacitors 172, 174 and 176 establish a predetermined time constant which limit the rate at which charge can be transferred to any of capacitors 172, 174 an 176. Accordingly, further filtering is provided for certain high frequency noise The remaining noise generally will be either aperiodic or asynchronous with th detection afforded by multiplexer 146. Accordingly, such aperiodic an asynchronous noise is substantially averaged over succeeding measuremen 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: Claims 1. An apparatus for measuring the light reflectance of substance at a distinct wavelength, said apparatus comprising: first means for illuminating the substance with a light pulse th occurs at a predetermined time in a predeter ined measurement cycle, said lig pulse comprising substantially monochromatic light; and, second means for measuring light that is reflected by the substan in synchronism with the occurrence of said light pulse during said measureme cycle. 2. An apparatus as recited in Claim 1 which is adapted measure the light reflectance of a substance at a plurality of distin wavelengths, wherein said first means is operative to illuminate the substan with a plurality of light pulses that sequentially occur in said measurement cycl each said light pulse comprising substantially monochromatic light having nominal wavelength that is distinct from that of any other said light pulse; an wherein said second means is operative to measure light that is reflected by th substance in synchronism with the sequential occurrence of said light pulse during said measurement cycle. 3. An apparatus as recited in Claim 2, wherein said first mean is operative to illuminate the substance with said plurality of light pulses in eac of a plurality of successive and periodic measurement cycles; and wherein sai second means is operative to measure light reflected by the substance durin each of said plurality of measurement cycles. 4. An apparatus as recited in Claim 2, wherein said first mean 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 t - 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 optϊcally- • coupling 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 optically- coupling 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 w th the sequential occurrence of said plurality of light pulses during said measurement cycle. 13. An apparatus as recited in Claim 12, wherein said optically- coupling 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 optically- coupling 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 encom¬ passing 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 adaple^to sarnple and hold the magnitude of an electrical signal applied to said input; and, mea for coupling said electrical output signal from said light sensor to said input said sampling means; and, wherein said apparatus further comprises a control means f causing said first means to illuminate the substance with said plurality of lig pulses during said measurement cycle and for causing said sampling means t sequentially sample and hold the magnitude of the electrical signal applied t said input thereof in synchronism with the sequential occurrence of said pluralit of light pulses during said measurement cycle. 17. An apparatus as recited in Claim 16, wherein said means fo coupling said electrical output signal from said light sensor to said input of sai sampling means includes a first capacitance storage means; and, wherein sai sampling means includes a plurality of second capacitance storage means, sai sampling means being controlled by said control means to connect said firs capacitance storage means to a distinct one of said plurality of secon capacitance storage means in synchronism with the corresponding one of sai plurality of light pulses during said measurement cycle. 18. An apparatus as recited in Claim 17, wherein said means fo 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 windows and, wherein said second means is responsive to said third means to measure th light reflected from the substance during each said selected and correspondin one of said plurality of sequential windows. 22. An appara tus as recited in Claim 21, wherein said pluralit of sequential windows have equal durations. \ 23. An apparatus as recited in Claim 21, wherein the duration o each said light pulse is substantially equal to the duration of its correspondin sequential window. 0 24. An apparatus as recited in Claim 23, wherein the tim during which the light reflected by the substance is measured during each sai selected and corresponding sequential window is substantially less than th 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 monochro¬ matic 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 light- emitting diode at a substantially constant value. 30. An apparatus as recited in Claim 26, wherein said optically- coupling 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 optically- coupling 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 cans includes: a light sensor adapted to provide an electrical output sig related to the intensity of light incident on said light sensor; means for optica coupling light from . the substance to said light sensor; and, means for detecti said electrical output signal from said light sensor in synchronism with t occurrence of said light pulse during said measurement cycle. 33. An apparatus as recited in Claim 32, wherein said opticall I coupling means includes an elongated fiber optic bundle having a proximal e disposed in proximity to the substance and a distal end disposed in proximity said light sensor. 34. An apparatus as recited in Claim 33, wherein said opticall coupling moans further includes a lens disposed between said distal end of sa fiber optic bundle and said light sensor, said lens and said light sensor bei constructed and arranged so as to focus light from said distal end of said fib optic bundle on said light sensor. 35. An apparatus as recited in Claim 3 ' 2, wherein said means f detecting includes: sampling means having an input and being adapted to sampl and hold the magnitude of an electrical signal applied to said input; and, mean for coupling said electrical output signal from said light sensor to said input o said sampling means; and, wherein said apparatus further comprises a control means fo causing said first means to illuminate the substance with said light pulse at sai predetermined time during said measurement cycle and for causing said samplin means to sample and hold the magnitude of the electrical signal applied to sai input thereof in synchronism with the occurrence of said light pulse during sai 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 s.-.id 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. NAL 38. An apparatus as recited in . Claim 37, wherein said contro means is operable to cause said first means and said sampling means to operat during each of a plurality of successive measurement cycles that occur at predetermined frequency. 39. An apparatus as recited in Claim 38, wherein said means fo coupling said electrical output signal from said light sensor to said samplin means includes filter means having a low-frequency roll-off and a high-frequenc roll-off respectively below and above the predetermined frequency of sai plurality of successive measurement cycles. 40. An apparatus that is particularly adapted for measuring th brightness of paper pulp within each of a plurality of distinct spectral bands o 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 lό 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 a d 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, ends of said first and second fiber optic bundles being received in said first an second apertures, respectively; and, a printed circuit board assembly supporte on and by said first block and mounting sa d plurality of light sources and sai light sensor so that said plurality of light sources are substantially locate 5 within said first recess and optically aligned with said distal end of said firs fiber optic bundle and so that said light sensor is substantially located within sai second recess and optically aligned with snid distal end of said second fiber opti bundle. 44. An apparatus as recited in Claim 43, wherein each of sai 0 plurality of light sources comprises a plurality of light-emitting diodes mounte on said printed circuit board assembly. . 45. An apparatus as recited in Claim 44, further, comprisin heating means disposed within said first recess for maintaining the temperature of each said light-emitting diode at a substantailly constant value. 5 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 bourd 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 constructed and arranged so as to distribute the light from each said light sour uniformly across said distal end of said first fiber optic bundle. 52. An apparatus as recited in Claim 43, wherein said lig sensor includes a light-sensitive surface that is optically aligned with said dist end of said second fiber optic bundle. 53. An apparatus as recited in Claim 52, further comprising lens supported within said second aperture and disposed between said distal en of said second fiber optic bundle and said light-sensitive surface, said lens an said light-sensitive surface being constructed and arranged so as to focus ligh from said distal end of said second fiber optic bundle on said light-sensitiv surface. 54. An apparatus as recited in Claim 43, wherein said ligh sensor is substantially wavelength-insensitive within a range of wavelength encompassing the distinct spectral bands of said plurality of light sources. • 55. An apparatus as recited in Claim 54, wherein said ligh sensor comprises a photocell. 56. An apparatus as recited in Claim 43, wherein- said printe circuit board assembly and said first block are encapsulated so as to substantiall seal said printed circuit board assembly and said first and second recesses fro the environment. 57. An apparatus as recited in Claim 43, further comprising sheet of elastomeric material secured to said second surface of said fir'st bloc and surrounding said first and second fiber optic bundles so as to seal said dista ends thereof from the environment. . 58. An apparatus as recited in Claim 43, wherein said mountin means includes means secured to said housing for supporting the portions of sai first and second fiber optic bundles, that are adjacent said distal ends thereof i spaced, parallel relationship. ' - • - . 59. An apparatus as recited in Claim 58, wherein said mean secured to said housing includes a second block having formed therein an extending therethrough first and second apertures through which respectivel pass said first and said second fiber optic bundles. 60. An apparatus as recited in Claim 59, wherein said first bloc is supported on and by said second block. 61. An apparatus as recited in Claim 40, wherein said ligh sensor is adapted to provide an electrical output signal related to the intensity o light incident thereon; and, further comprising means for pulsing said plurality o light sources in sequence during each of a plurality of successive measurement " ■ • . GINM. 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. SHEET OMPI

Inventors: HOKANSON JON V, POPE DANIEL F, SULLIVAN ROBERT R, VENALAINEN ESKO J
Applicants: BONNIERFOERETAGEN AB
Application year: 1982
Main classification: G01N21
IPC codes: G01N21