Abstract:
A colorimeter provides compensation for changes in the color signature of an object due to lamp aging. A current measuring circuit measures current to the lamp during an initial training of the colorimeter and stores a value I T  indicative of the lamp current at training. When an object is scanned by the colorimeter, lamp current is again sensed and assigned a value I S . Comparison between a sensed color signature and a stored color signature then occurs. Compensation is accomplished by modifying one of the two signatures by a ratio including I S  and I T .

Description:
BACKGROUND OF THE INVENTION 
     This invention is directed towards the field of color signature sensors used in process automation. More specifically, the invention is a method and an apparatus which performs color recognition of objects for the purposes of identification, sorting or matching. 
     Colorimeters are well known devices used for characterizing the color of an object and comparing the color of an object to the color of other objects. A lamp, either in the colorimeter or an external source, provides illumination which is reflected by or transmitted through the object and transmitted back to a device which disperses light into an array of wavelength components. A detector array then converts the array of wavelength components into discrete signals which are representative of a color signature of the object. The discrete signals are then sent to an analog to digital converter and then on to a processor for processing. Processing involves a component by component subtraction of sensed component values from stored component values to produce a relative signature difference. Generally, as long as the relative signature difference falls within predetermined limits, the color of the sensed object will be acceptable. 
     A problem associated with colorimeters is that as the illuminating lamp ages, the color and intensity of the light produced by the lamp can change. This, in turn, causes the color signature of a sensed object to vary with the age of the lamp. This variation in color signature may result in many acceptable pieces being discarded due to erroneous color sensing. 
     One attempted solution to the problem of lamp aging i to increase the lamp voltage as the lamp ages in order to maintain a constant intensity and color output. However, it has been shown that changing the lamp voltage significantly affects lamp life. 
     
         (life.sub.actual /life.sub.design)=(V.sub.design /V.sub.actual).sup.10˜14 
    
     This means that a voltage increase of five percent will result in an approximate reduction in life of fifty percent. 
     Another possible solution to the problem of lamp aging is frequent recalibration of the colorimeter. However, this is only feasible in a laboratory environment. In commercial environments, access to the colorimeter may be difficult. Further, frequent recalibration increases the costs of scanning. 
     Thus, it is an object of the present invention to provide colorimetric sensing which compensates for lamp aging without adversely affecting lamp life. It is a further object of the present invention to provide colorimetric sensing which compensates for lamp aging without requiring frequent recalibration. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for eliminating the effects of lamp aging in colorimetric sensing. The inventive colorimeter comprises a lamp, a current sensor for determining the electrical current used by the lamp, a diffraction grating for separating light into its component wavelengths, a detector array for sensing the component wavelengths and a processor which includes a compensation means. The processor compares sensed component values with stored component values to produce a signature difference or DELTA. The compensation means causes the stored component value to be adjusted by a factor based on a ratio of the sensed lamp current with a stored lamp current. The ratio of sensed lamp current and stored lamp current can be raised to a predetermined exponent for optimum results. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of the inventive colorimeter and an object to be analyzed. 
     FIG. 2 is a functional diagram of the processor used by the inventive colorimeter. 
     FIG. 3 is comparison between a stored color signature and the sensed color signature with and without compensation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the inventive colorimeter is shown in FIG. 1. The colorimeter 10 is used to detect the color signature of an object 5. 
     Lighting means 15 is used to illuminate object 5. In one implementation, lighting means 15 comprises a lamp 20, a mirror 30 and optical fiber 22. Lamp 15 is generally a halogen incandescent lamp. Power supply 40 is connected to lamp 15 to provide electrical power. 
     First optical fiber 22 is used to transmit light from lamp 20 to object 5. First optical fiber 22 has two ends, a light receiving end 25 positioned near lamp 20 and a light exiting end 35 positioned near a location where objects such as object 5 are to be analyzed. 
     Mirror 30 is positioned near lamp 20 opposite light receiving end 25. Mirror 30 has a concave side 32 which faces the lamp 20 and focuses light on the light receiving end of optical fiber 22. Use of such a mirror and first optical fiber is optional, but by using the mirror and first optical fiber, an increased light intensity may be transmitted to the object. Collectively, the mirror and the first optical fiber are known are light focusing means. 
     Object 5 will generally reflect a portion of the light transmitted to it by illumination means 15. Alternatively, object 5 may transmit light therethrough as where the object being measured is a liquid. The amount of and composition of the reflected or transmitted light depends upon the surface and color of the object. The present embodiment could be used equally as well in measuring either reflected or transmitted light, however, the focus in this description will be on reflected light. The reflected light will be carried by a second optical fiber 42 from object 5 to a dispersing element 55. Second optical fiber 42 has two ends, light receiving end 45 positioned near the object and a light exiting end 50. Reflected light leaving light exiting end 50 strikes dispersing element 55. Dispersing element 55 may be a diffraction grating. The reflected light is then broken into its component wavelengths and reflected to detector array 60. For this embodiment, dispersing element 55 disperses and provides a flat field focus of the spectrum (400 nm to 800 nm) on detector array 60. The focused spectrum strikes array detector 60 with 400 nm light at side C and 800 nm light at side D. 
     The light dispersed and reflected by dispersing element 55 is directed toward a detector array 60. Detector array 60 may be comprised of a linear sequence of N photodetectors. Here, N equals 120. Each photodetector is adapted to produce an electrical signal when light of a predetermined frequency impinges thereon. The magnitude of the signal is directly proportional to the intensity of the light which strikes the photodetectors. A convenient shorthand notation for the second optical fiber, the dispersing element and the detector array is a sensing means 55. 
     The detector array produces an analog signal indicative of the color signature of object 5. The analog signal is amplified by amplifier 70 and then digitized by A/D converter 75 thus creating a 1XN array of sensed component values. After digitization, the array of sensed values is sent to processor 80 for processing. 
     Processing of the digital array involves a comparison between each of the components of the array and each component of a stored component array. The stored component array is a base line color signature to which all sensed objects will be compared. The colorimeter is &#34;trained&#34; before it is used by having the sensing means sense, digitize and store color information from a sample object. The sample object must have the surface and color desired of all the objects to be analyzed. The stored component array is stored in memory 85 and is made up of N stored component values. Selection of a mode of operation for the colorimeter can be controlled by input/output controller 90. 
     Lastly, housing 110 is used to enclose some of the parts of the colorimeter. 
     As was already suggested, each of the components of the sensed wavelength component array is compared with a corresponding component from the stored component array. This can be better understood with reference to FIG. 3. In FIG. 3, curve 301 is representative of the color signature which is stored during the training of the sensor. Curve 302 is the sensed component wavelength array from an object that has been scanned. Looking at one representative component, processor 80 begins by determining the difference between each of the stored component values and the sensed component wavelength values. For example, the processor 80 would subtract the sensed component values E 3S  from stored component values E 3T  to produce a DELTA. If the calculated DELTA fell outside predetermined limits, the object which produced this color signature would be deemed to have an unacceptable color and thus would be discarded. However, this DELTA may in part be due to aging of the lamp. 
     In another scheme, the DELTA for each pair of elements is calculated, then the DELTAS are summed. If the sum of the DELTAS then falls outside predetermined limits, the object is deemed unacceptable. 
     To account for the problem of aging of the lamp when determining the DELTA, current monitor 95 is placed in the power supply lines for lamp 20. The output of the current supply monitor is sent to amplifier 100 and then on to analog to digital converter 105. The digitized current signal is then passed onto microprocessor 80 and is called the sensed current value. The sensed current value is then used in a ratio with a trained current value to eliminate the effects of lamp aging from a color signature of a scanned object. During the aforementioned training of the sensor, the current used by lamp 20 is also determined and stored in memory 85. For convenience, this trained current will be called I T  or the trained current value. 
     Through experiment, it has been determined that light intensity is related to lamp current through the equation: 
     
         (I.sub.C /I.sub.T).sup.1.85 =(L.sub.C /L.sub.T).sup.0.313 
    
     Using the above equation, we can compensate for lamp aging in the calculation of DELTA. There are two ways of doing this. First, the stored component values could be multiplied by (I C  /I T ) 5 .907. Thus the equation for DELTA would be E C  -E T  (I C  /I T ) 5 .907 =DELTA. 
     The second way to correct for lamp aging mathematically is by multiplying the sensed wavelength component values by (I T  /I C ) 5 .907. Thus, DELTA would be calculated by E T  -E C  (I T  /I C ) 5 .907 =DELTA. 
     Once DELTA has been calculated, there are at least two ways in which it can be used. First, each DELTA can be reviewed to determine if it falls within predetermined limits. Second, the absolute values of the DELTAS may be summed, and the sum then compared to predetermined limits. 
     Referring once again to FIG. 3, the effects of using a compensation factor in the calculation of DELTA can be shown. Curve 301 is representative of the stored component values. Curve 302 is representative of the sensed wavelength component values. Curve 303 represents a color signature after compensation has been applied. Note that the compensated curve is closer to the trained curve. 
     FIG. 2 shows some parts of processor 80. In FIG. 2, compensation means 81 is used to calculate the correction factor used in the above equations. Comparison means 82 takes the compensation factor, the stored component value and the sensed wavelength component value and produces a DELTA therefrom. Training means 83 is used to create a string of stored component values representative of a color signature. One processor which could be used is an Intel 80C 196 processor. 
     The foregoing has been a description of a novel and non-obvious colorimeter which provides output compensation for lamp aging. The inventor does not intend to be limited to only the embodiments shown and described in the application. Instead, the scope of the applicant&#39;s invention can be determined by the claims appended hereto.