Abstract:
A method for high speed spectroscopic constituent verification. The method includes the steps of illuminating a sample with broadband light and measuring two preselected wavelengths of reflected light: a first narrow-range of wavelengths λ1 that is not significantly absorbed by the constituent of interest, and a second narrow-range of wavelengths λ2 that is substantially absorbed by the constituent of interest. Given the two measurements of reflection, upper and lower thresholds are determined (the latter based on a percentage of the measured baseline reflected light of wavelengths λ1). Finally, the presence of the constituent of interest is indicated if the measure of the reflected discriminant wavelength λ1 is within the upper and lower threshold measures of reflected baseline light λ1.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a division of U.S. patent application Ser. No. 09/233,602, filed Jan. 19, 1999 by Wilt et al., now U.S. Pat. No. 6,100,528. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to sensors for process control and, more particularly, to an improved analytical discriminator for high speed spectroscopic constituent analyses. 
     2. Description of the Background 
     Many manufacturing processes involve the high-speed application of glue lines to articles such as paper sheets as they are conveyed past an applicator station. Modern automated gluers also include sensors to check the quality of the glue lines and to provide feedback for process control. For instance, in the context of making cardboard packaging, glue lines are applied to container blanks prior to folding. In such process, it is desirable to provide the process computer continuously with real time electrical signals that each glue line has been applied and is of the proper mix of constituents. However, these sensors must support extremely high-volume throughput (often in excess of 1000 sheets per minute). Thus, any analysis of the applied glue must be accomplished in real time, and the need for speed has greatly limited the types of analyses to simple (and largely unreliable) optical checks for the physical presence of glue. 
     Optical constituent analysis is well-known. For example, U.S. Pat. No. 5,635,402 to Alfano et al. shows a technique for determining whether a cell is malignant or not based on fluorescence. The cell is exposed to a fluorescent dye, it is irradiated, and the intensity of the resulting fluorescence at two wavelengths is compared to known standards to measure the intensity of the fluorescence. However, this fluorescent analyses results in an actual CRT map, and the system is unsuited for any form of high-speed process control or analysis of moving glue lines. 
     Another more promising type of analysis is based on infra-red reflectance. Infra-red reflectance or spectroscopy has proven itself very useful in other industries and is capable of far more accurate and thorough analysis of constituents in a sample. 
     For example, U.S. Pat. No. 4,801,804 to Rosenthal shows a method and apparatus for near infrared reflectance measurement of a non-homogenous sample such as ground sunflower seeds. The sample is quantitatively analyzed by uniformly irradiating with near infrared radiation. A bifurcated optical fiber bundle is used with single source and return paths, and various (at least two) wavelengths of the reflected light are successively measured by an optical “chopper” and detector. The wavelengths are ratioed and compared to known values to give a direct reading of moisture content. 
     Likewise, U.S. Pat. No. 4,840,706 to Campbell shows an infra-red scanning gauge used in measuring the moisture content of a paper-web during manufacture. The scanner employs a measurement channel and a reference channel. U.S. Pat. No. 5,365,067 to Cole et al. shows a method and apparatus for determining surface molecular orientation based on relative intensity of spectral reflectance. The relative intensity is affected by the preferential orientation of the polymeric chains. In both of the above cases, only a fleeting overview of the spectropolarimetry methods are disclosed. There are no structural details, circuit details, or optical details (see Cole &#39;067, column 7, lines 3-6). 
     U.S. Pat. No. 5,813,403 to Soller measures the pH of tissue by irradiating tissue with 2-20 light sources, measuring the reflectance, and applying a least squares analysis to the measured absorption spectra. This is a pre-dispersant system wherein the specimen is illuminated by various wavelengths. The Soller &#39;403 device is clearly not concerned with speed for process control, and it teaches the use of single optical fibers (see column 3, line 52 et seq.). 
     U.S. Pat. No. 5,218,206 to Schmitt et al. shows a method for determining the dryness, wetness, or icing of a road. The method employs reflection measurements of light in the infrared range. The reflected light is measured selectively and simultaneously by a receiver in at least two wavelength regions. A quotient of the detected signals determines the respective condition of the roadway surface. The two wavelength regions are selected to ensure that the quotient is indicative of either dryness, wetness., or icing. 
     U.S. Pat. No. 5,220,168 to Adamski et al. shows a method and apparatus for determining moisture content of materials by irradiating a sample with two wavelengths of light having different water absorptive characteristics (which are therefore reflected by varying degrees depending on the surface moisture on the material). The respective reflections are measured by a single common detector, and a value corresponding to the ambient light is subtracted from each measurement. A ratio of the resultant values is then correlated with data derived from precalibration measurements of samples of known moisture content. 
     U.S. Pat. No. 5,424,545 to Block et al. shows a non-invasive non-spectrophotometric method for measuring the blood glucose. A plurality of broad spectrum filters transmit beams of radiation in overlapping portions of the spectrum to the sample. Radiation reflected or transmitted by the sample is detected and decoded. 
     Theoretically, near-infrared reflectance technology is applicable to the context of paper and cardboard packaging as it is capable of substantiating that each glue line has been applied and is of the proper mix of constituents based on the fact that infrared energy is known to be absorbed by typical glues at very specific wavelengths. That is, the absorptivity of infrared energy by glue is known to be dependent on wavelength, Specifically, using the conventional method, paper blanks moving along a conveyor belt would be illuminated under a reflection sensor. The reflected infrared energy power spectrum would be altered according to the characteristics of the glue (such as starch mass). The reflected light would be filtered by two or more narrow band pass filters of different wavelengths, inclusive of a first wavelength that is not readily absorbed by the glue and a second that is absorbed in the glue. By analyzing the relative reflected wavelengths the data is capable of giving a substantive quality check of the glue. 
     Unfortunately, the conventional analyses required to implement infra-red reflectance techniques as shown in the above-described prior art patents is complex and time-consuming, and there have been few successful efforts to adapt such techniques for the purpose of high-speed process control. 
     One known example is U.S. Pat. No. 5,663,565 to Taylor, which shows a system for determining glue-line characteristics, such as temperature, of corrugated board. The output signal of an infrared absorption sensor provides an on-line starch measurement for corrugators. The incremental amount of infrared radiation that is absorbed by starch and/or water in the glue-lines is isolated from the predominant, more random background absorption due to cellulose and water in the paper substrate. The amplitude of the extracted signal component, which reflects only the starch and/or water in the glue, is then converted using empirically derived historical data. 
     Unfortunately, the analysis and implementing hardware used by the Taylor &#39;565 patent is very cumbersome as the data must be compared and converted based on a database of historical data. As shown in the &#39;565 patent, process control speed considerations require that a running average of glue readings be kept over time. The &#39;565 method and device simply is not fast enough to operate in real time to provide a substantive analysis of each running glue line, and it would be greatly desirable to eliminate the need for averaging. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an analytical discriminator for high speed spectroscopic constituent analyses for the industrial process control setting. 
     It is another object to provide an analytical discriminator as described above that is capable of an accurate and thorough analysis of constituents in a sample, for example, by a comparison of the amount of constituent with upper and lower thresholds. 
     It is another object to adapt the technique of near-IR reflectance to the context of high-speed process control in the paper and cardboard sheet gluer context to discriminate the starch and/or water in the glue. 
     It is another object to simplify the analysis and implementing hardware to provide a substantive analysis of each running glue line in real time, thereby eliminating the need for historical data or a running average of glue readings over time. 
     According to the present invention, the above-described and other objects are accomplished by providing a method and device for high speed spectroscopic constituent verification. 
     The method comprises the steps of illuminating a sample with broadband light and measuring two wavelengths of reflected light. The reflected light is measured in a first narrow-range of wavelengths λ1 that is preselected as a baseline that is not significantly absorbed by a constituent of interest. The reflected light is also measured in a second narrow-range of wavelengths λ2 that is preselected as a discriminant which is substantially absorbed by the constituent of interest. Given the two measurements of reflection, upper and lower thresholds are determined based on a percentage of the measured baseline reflected light of wavelengths λ1. Finally, the presence of the constituent of interest is indicated if the measure of the reflected discriminant wavelength λ2 is within the upper and lower threshold measures of reflected baseline light λ1. 
     The device that implements the above-described method includes a base unit with an enclosure for housing a circuit board, and a near-IR discriminator circuit on the circuit board. The discriminator circuit has a pair of selective light sensors each responsive to a particular wavelength. A light source is positioned in the housing, and the base unit also includes a receptacle on the enclosure for completing at least three fiber optic couplings, two of the couplings leading to the respective light sensors and one to the light source. A sensor unit connects to the base unit. The sensor unit includes a flexible neck with a connecting block attached at one end for mating with the receptacle on said base unit enclosure and thereby completing the three fiber optic couplings. A hood assembly is attached at the other end of the flexible neck, the hood assembly encloses a light collecting and transmitting lens. The sensor unit also includes an optical fiber bundle for transmitting light through the flexible neck via a plurality of optical fibers. A first subset of the optical fibers in the bundle are coupled between the light source in the housing through said lens for illumination of the sample. A second and third subset of the optical fibers in the bundle are coupled between the lens and the light sensors for transmitting light reflected back from the sample to the base unit. The device carries out the above-described method whereby the near-IR discriminator circuit indicates the presence or absence of the constituent based on a difference in the reflected light received at the pair of selective light sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof when taken together with the accompanying drawings in which: 
     FIG. 1 is front perspective view of the analytical quantification and process control system according to one embodiment of the present invention. 
     FIG. 2 is a side cross-section of the hood assembly  30 . 
     FIG. 3 is a bottom perspective view of the connecting block  24 . 
     FIG. 4 is a side cross-section of the connecting block  24 . 
     FIG. 5 is a top view of the base assembly  10 . 
     FIG. 6 is a side cross-section of the base assembly  10 . 
     FIG. 7 is a schematic diagram of the discriminator circuitry on circuit board  100 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As stated above, the basic design concept embodied in the invention is an analytical discriminator and process control system that is physically as small an unobtrusive as possible, with a high-speed functional capability to make real-time go/nogo process control decisions based on the presence of a constituent in a sample within a range of acceptable thresholds. The present device is highly compact and flexible, and it can be retrofit to any existing equipment. Moreover, the device supports extremely high-volume throughput (often in excess of 1000 sheets of paper per minute, each with multiple glue lines). These advantages result from a number of improvements in optical layout, mechanical arrangement, method of discrimination, and electronics therefor. 
     The compactness of the unit is a key advantage. The combination of a miniature base unit (approximately 8″×3″×1″) with a combined source/collector hood mounted on a flexible neck allows the retrofit application of the device to any existing line equipment. This is accomplished by a novel arrangement in which all the electronics are included on a single circuit board within a single housing. Three separate optical couplings are completed through the housing and directly to the circuit board inside. In addition, a halogen light source is incorporated directly in the compact housing, and the housing itself serves as a heat sink while balancing all other design constraints. As will be seen, all the electronics are designed and laid out on a single board, inclusive of source and power supply, sensors, and high-speed discriminator circuitry, again while balancing other design constraints such as the tight tolerances of the optical couplings. 
     FIG. 1 is front perspective view of the analytical discriminator and process control system  2  according to one embodiment of the present invention. The discriminator  2  generally includes a lower base  10  for housing a light source and the discriminating circuitry (to be described), a hood assembly  30  for housing a light collecting and transmitting lens  32  (to be described), and a flexible neck assembly  20  mounted on a connecting block  24  for adjustably supporting hood assembly  30  a short distance from base  10 . Flexible neck assembly  20  houses an optical fiber bundle that connects the hood assembly  30  to connecting block  24 . Connecting block  24  in turn plugs directly into the lower base  10  to complete the requisite optical couplings to the light source, sensors and the discriminating circuitry housed therein. Physically, the dimensions of the lower base  10  measure approximately 2″ by 1.2″ by 6″, the flexible neck  20  measures 18″, and the hood assembly  30  is approximately 3″ long with a ⅞″ diameter. 
     FIG. 2 is a side cross-section of the hood assembly  30 . Hood assembly  30  includes a hollow cylindrical casing  306  for housing and protecting a lens  302  seated therein. Casing  306  is open at one end for exposing lens  302 . The optical fiber bundle  202  is routed through flexible neck  20 , enters casing  306  through the other end, and is terminated a predetermined distance  1  from lens  302  as will be described. 
     The fiber bundle  202  itself is a random (incoherent) fiber bundle adapted for illumination with broadband light for post-dispersive collection of light at two different near-IR wavelengths. An incoherent bundle is a collection of fibers with random relative positioning along the length of the bundle. The random fiber bundle increases the uniformity of light application and collection and increases the sensitivity as is absolutely necessary in a high-speed system trying to discriminate non-homogenous products. In contrast, a coherent bundle consists of numerous individuals fibers such that the relative position of any given fiber is maintained along the length of the bundle. Coherent fiber bundles are commonly used for the transmittance of an image. However, if light is collected with single fibers only a small portion of the sample is seen. Where spatial proportions such as height are an issue, there is a large propensity for error. In the context of glue lines, the actual height of the glue line is most definitely an issue as it directly affects reflectance. Consequently, a random trifurcated bundle is used in the present invention to increase the throughput of light for increased speed of processing. Optical fiber bundle  202  may be a commercially available bundle of 30-300 optical fibers (more is generally better, although as few as thirty will function, the preferred embodiment employs approximately three hundred). The fiber bundle is made up of small cylindrical fibers packed together. In a random bundle the individual fibers are haphazardly located in the input and output (this is also known as “salt and pepper” fibers). Smaller fibers are more effective for the present application than larger core fibers. 
     The flexible neck  20  is terminated at a threaded collar  22  that mates with cylindrical casing  306 . The entire optical fiber bundle  202  also terminates at the casing  306 , the fibers being terminated and bonded by a stainless steel ferrule, and the ends being terminated (cut and polished flush) with a ferrule. The other end of fiber bundle  202  is trifurcated to effect three-way beam splitting, thereby providing an illumination path and two return paths from/to lower-base  10 . 
     Lens  302  is a preferably a bi-convex lens for imaging the sample to the fiber bundle. Proper imaging of the sample to the fiber bundle  202  to requires a choice of lens with two constraints. First, the focal length of the lens should be equal to both ½ the spacing  1  and ½ the distance from the lens to the intended sample. This ensures that the unmagnified sample image will be focused onto the end of the fiber bundle  202 . Second, the numeric aperture of the fibers in bundle  202  should be matched to the numeric aperture of the lens  302  as closely as possible. For the present application it is desirable to image a spot-size of ≦1 mm. To accomplish this, it is necessary to employ at least  10  fibers in each of the beam-split paths, and the fibers should be approximately 150 micron core fibers. For lens  302 , a 25 mm lens works well with a 0.5 mm spacing within the cylindrical casing  306 , and an expected a 0.5 mm expected distance from lens to sample. 
     FIG. 3 is a bottom perspective view and FIG. 4 is a side cross-section of the connecting block  24 . Connecting block  24  includes a generally rectangular connector shell  248 . Optical fiber bundle  202  is routed through flexible neck  20  and enters shell  248  through one end. The flexible neck  20  is terminated at a threaded collar  240  that mates with connector shell  248 . Once inside connector shell  248 , the fibers of bundle  202  are trifurcated and randomly divided into three groups. In the preferred embodiment, each of the beam split paths comprises a substantially equal number of individual fibers. One group continues through to a transmissive optical coupling  242  which transmits light from an illumination source in lower base  10  through lens  302  for illuminating the specimen. The other two groups are directed into side-by-side reflective optical couplings  242 ,  244 ,  246  which return reflective light from the specimen that is captured by lens  302 . It is noteworthy that the even division of fibers is not necessary because inequalities can easily be compensated for simply by altering circuit parameters to ratio the return reflective light from the specimen. 
     FIG. 5 is a top view of the base assembly  10 . Base assembly  10  includes a hollow elongate rectangular cabinet  11  which is pre-drilled to mount an array of three optical couplings  142 ,  144  and  146  at one end for mating with the corresponding optical couplings  242 ,  244  and  246  of connecting block  24 . Cabinet  11  is also pre-drilled to expose a conventional D 9  connector  148  near the other end, the D 9  connector being resident on a circuit board  100  that is attached to the bottom of cabinet  11 . The D 9  connector  148  provides an electrical connection as necessary to the discriminating circuitry to provide logic outputs to the user&#39;s existing computer or programable logic controllers (PLCs) in order to provide feedback for process control. 
     FIG. 6 is a side cross-section of the base assembly  10 . Base assembly  10  is five-walled and closed at the bottom by a circuit board containing discriminator circuitry  100 . The discriminator circuit board  100  fits within a shallow recess in the bottom of cabinet  11 . An illumination source  132  is mounted at one end of cabinet  11  (this can be mounted on the circuit board  100 ). In the preferred embodiment, the illumination source  132  is a lensed-in +5 v, 5 watt (maximum) halogen bulb, although other bulbs may serve equally well. The illumination source  132  is preferably powered by a switched DC supply that can be resident on the circuit board. More specifically, a raw AC power input is taken from a power cord or through the D 9  connector  148 . This is rectified in a known manner, and the rectified DC output is switched and applied to the illumination source  132 . This use of a switching power supply to drive the illumination source  132  power stabilizes the lamp and prevents flicker. 
     As can be seen, the end of the rectangular cabinet  11  proximate to illumination source  132  is configured as a heat sink to dissipate the heat generated by the bulb. The illumination source  132  is positioned beneath optical coupling  142  and is coupled thereto to transmit broadband illuminating light through the transmission third of the optical fibers and outward through the hood  30  onto the sample of interest. Reflected light from the sample returns through the reflection fibers. The two sets of reflection fibers are coupled directly into the two corresponding optical couplings  144 ,  146 . The D 9  connector  148  is preferably mounted directly on the circuit board  100 , and a fitted aperture is provided through the top wall of rectangular cabinet  11 . This way, the D 9  connector protrudes upward through the aperture when the circuit board  100  is attached (by screws or the like) to the bottom of the rectangular cabinet  11 . 
     Yet another key feature of the present invention that contributes to its high throughput is its electronics. A high-speed comparator-based circuit design for implementing the discrimination method herein is capable of providing real-time discriminate analysis of the presence or absence of glue spot sizes as small as 0.5×0.5 mm wide and 0.5 mm high in less than 100 microseconds. 
     FIG. 7 is a schematic diagram of the discriminator circuitry that is resident on the circuit board  100 . Two identical sensing photodiodes  102   a  &amp;  102   b  are mounted behind two selective filters  104 ,  106 , respectively. Both filters  104 ,  106  are conventional narrow band-pass filters that pass a selected wavelength of near- infra-red reflected light in the 600 nm to 2000 nm range. The particular band-pass characteristics of filters  104 ,  106  are chosen in accordance with the constituent to be discriminated. Specifically, the band-pass characteristics of filter  104  is chosen to be a first wavelength λ 1  in the near-IR range that is not significantly absorbed by the constituent of interest. The first wavelength λ 1  serves as a baseline wavelength. On the other hand, the band-pass characteristics of filter  106  is chosen to be a second wavelength λ 2  in the near-IR range that is significantly absorbed by the constituent of interest. The second wavelength λ 2  serves as the discriminator wavelength. Reflected light of the baseline wavelength λ 1  is passed by filter  104  and illuminates photodiode  102   a , thereby generating a baseline signal. Reflected light of the discriminator wavelength λ 2  is passed by filter  106  and illuminates photodiode  102   b , thereby generating a discriminator signal. Photodiodes  102   a  and  102   b  may be any suitable commercially available near-IR sensitive photodiodes with high speed sensing capability. 
     The baseline and discriminator signals are fed to the inverting inputs of detector amplifiers  110  and  112 , respectively. The detector amplifiers  110  and  112  may both be commercially available op-amps (quad low-noise JFET-input op amps are suitable) set to run in transimpedance mode with feedback through resistor-capacitor bridges  111 . The feedback resistors should be selected to optimize the dynamic range. The feedback capacitors should be selected to provide a 3 dB roll-off at the frequency of interest (approximately 10 kHz). The resistor-capacitor bridges  111  produce a low-pass filter. Consequently, both detector amplifiers  110  and  112  produce an output voltage that is proportional to the respective baseline wavelength λ 1  and discriminator wavelength λ 2  sensed by photodiodes  102   a  and  102   b . The baseline and discriminator signals from detector amplifiers  110 ,  112 , respectively, are input to comparators  118 ,  116  (commercially available comparators are suitable). The baseline signal from amplifier  110  is also used to provide an indication both visually and as data output for process control, both confirming the presence of baseline signal that is within low and high tolerances. The visual indication is accomplished by connecting the output of amplifier  110  through a series zener diode  140  (approximately 12 v is suitable) and LED  142  to ground. Thus, the LED  142  indicates an over-illumination fault condition (too much light coming back into detector  102   a ) by illuminating upon the zener  140  breaking down at 12 v. The over-illumination data output is accomplished by connecting the zener  140 /LED  142  junction to a general purpose driver  154  through resistor  126  (a conventional open-collector logic driver is suitable). The over-illumination data output /2HI is ORed with an under-illumination output /2LO (to be described), and the combined /NOSIG data is output to the D 9  connector  148 . 
     The circuit also checks the baseline against an under-illumination threshold. This is accomplished by connecting the output of amplifier  110  to the (−) input of a second identical comparator  118 . The (+) input of comparator  118  is connected to an adjustable threshold setting circuit comprised of a series-connected resistor with zero-adjust  120  and fixed resistor  124  connected to a +15 v rail of the power source. This preferably establishes an under-illumination threshold less than or equal to 10% of full-scale power. The (+) input of comparator  118  is connected to filter capacitor  121 . The output of comparator  118  is then connected to another general purpose driver  166  through resistor  162 . A visual low-signal indication is accomplished by connecting the output of comparator  118  through LED  172  to ground. Thus, the LED  172  illuminates to indicate an under-illumination fault condition. 
     As mentioned above, a single combined fault data output is accomplished by connecting the output of driver  166  with the output of driver  154 . This effectively ORs the outputs of LED  172  and LED  142  to provide a single /NOSIG data output line indicative of either a high or low baseline fault condition. The combined /NOSIG data is output over the D 9  connector  148 . 
     The actual discrimination of sample is accomplished by using the baseline current from detector amplifier  110  to set-up a reference baseline threshold at comparator  116 . The output of amplifier  110  is connected in parallel with variable sensitivity-adjust resistor  122 , and in series with resistor  130  to the (+) input of comparator  116 . Resistors  130 ,  132  and comparator  116  define a hysteresis threshold of operation. Typical hysteresis thresholds of 3-15 mV help to stabilize the measurement process. Thus, the baseline current input to comparator  118  is compared to an adjustable threshold to provide an output indicative of whether or not there is a reflected baseline wavelength λ 1 . For purposes of the present invention, comparator  116  is preferably set to fire only when the reflected discriminator wavelength λ 2  exceeds approximately one-half the reflected baseline wavelength λ 1 . It has been found that this comparison gives a fast and accurate indication of the presence or absence of sample. 
     This should be contrasted to other spectroscopic analyzers currently on the market which try to quantify the results based on historical data. These require a complex comparison of baseline-adjusted discriminator wavelength to a database of values. The analysis is very time-consuming (and prevents real-time discrimination as with the present invention). The circuitry described above is capable of providing real-time discriminate analysis of the presence or absence of glue spot sizes as small as 0.5×0.5 mm wide and 0.5 mm high in less than  100  microseconds. Thus, the invention is fast enough to be used for real time process control of high-speed industrial glue applicators. 
     The output of comparator  116  is connected to a general purpose driver  168  through resistor  164 , and driver  168  outputs a /GLUE signal for process control. A visual low-signal indication is accomplished by connecting the output of comparator  116  through LED  170  to ground. Thus, the LED  170  illuminates to indicate the presence of the constituent of interest (e.g., starch) in the sample (e.g., glue). Note that the output of drivers  150  and  156  (the over-illumination data output /2HI and under-illumination output /2LO) are also connected to the output of comparator  116 . This disables the /GLUE signal and invalidates the output whenever the /NOSIG data output line indicates a high or low baseline fault condition, thereby preventing erroneous readings. 
     The operation of the analytical quantification and process control system according to the present invention will now be described with reference to FIGS. 1-7. 
     For set up, the connecting block  24  is plugged directly into the lower base  10  to complete the requisite optical couplings  242 ,  244 ,  246 . To apply power and to transfer data for process control, the cable of a conventional programable logic controllers (PLC) is connected to the D 9  connector on lower base  10 . The flexible neck  20  should be adjusted with respect to the sample such that the. light collecting lens  302  is exposed at an angle relative to the sample. The angle of the lens  302  tends to maximize diffusely reflected light energy whilst minimizing directly reflected light energy, thereby maximizing the measurable characteristics of the sample. This effectively makes the sensor unit a diffuse reflectance probe. The illuminated halogen light source  132  sends light through optical coupling  142  into a randomized one-third of the fibers of bundle  202 . The transmitted light is transmitted through the lens  302  of hood  30  to the spot-sensing area (e.g., focussed on paper blanks moving along a conveyor). The present embodiment is intended to yield a 1 mm optical sensing spot size, and the sample is illuminated with the broadband light. Reflected light travels back through the lens  302  and is split by coupling it into the two remaining groups of fibers in bundle  202 . The reflected light travels through the fiber bundle  202  and optical couplings  144 ,  146 , and is filtered by the respective filters  104 ,  106 . Filter  104  passes the baseline wavelength λ 1 , while filter  106  passes the discriminator wavelength λ 2 . Reflected light of the baseline wavelength λ 1  (that is passed by filter  104 ) illuminates photodiode  102   a , thereby generating a baseline signal. Reflected light of the discriminator wavelength λ 2  is passed by filter  106  and illuminates photodiode  102   b , thereby generating the discriminator signal. Both of the photodiodes  102   a  and  102   b  are connected to the inverting inputs of detector amplifiers  110  and  112 , respectively. The baseline and discriminator signals from detector amplifiers  110 ,  112 , respectively, are input to comparators  1   18 ,  116 . In addition, the baseline current from detector amplifier  110  is used to set-up a reference baseline threshold at comparator  116 . In essence, the baseline wavelength λ 1  (that passed by filter  104 ) modulates the comparator  116  threshold value which discriminates wavelength λ 2  (passed by filter  106 ) in direct proportion to the recovered (reflected) illumination energy. The energy of the recovered discriminator wavelength λ 2  (for the constituent of interest) is then compared against the λ 1  modulated value to determine presence or absence of the constituent of interest in the sample. The discriminator circuitry yields three visual outputs: if LED  170  is on there is glue present; if LED  172  is on there is a low signal level fault condition; if LED  142  is on there is a high signal fault condition. For real time process control, the outputs of LED  172  and LED  142  are ORed together and output on a single INOSIG data output line. The output of LED  170  is output on a /GLUE data output line. The data outputs are conveyed to the user&#39;s existing off-board programable logic controller (PLC), computer or other controller through the D 9  link from lower base  10 . 
     A matrix of output states for each fault condition follows: 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Sample 
                   
               
             
          
           
               
                   
                 Light 
                   
                   
               
               
                 Glue 
                 Condi- 
                 LEDs 
                 Outputs 
               
             
          
           
               
                 Presence 
                 tion 
                 /2HI LED 
                 /2LO LED 
                 /GLUE 
                 /NOSIG 
                 /GLUE 
               
               
                   
               
               
                 n/a 
                 too little 
                 off 
                 on 
                 off 
                 1 
                 0 
               
               
                 n/a 
                 too 
                 on 
                 off 
                 off 
                 1 
                 0 
               
               
                   
                 much 
                   
                   
                   
                   
                   
               
               
                 0 
                 nominal 
                 off 
                 off 
                 off 
                 0 
                 0 
               
               
                 1 
                 nominal 
                 off 
                 off 
                 off 
                 0 
                 1 
               
               
                   
               
             
          
         
       
     
     Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. For instance, the method and apparatus can easily be adapted to discriminate on the basis of transmitted light through a sample rather than reflected light. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.