Industrial colorimeter having lamp aging compensation means

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.sub.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.sub.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.sub.S and I.sub.T.

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. 
EQU (life.sub.actual /life.sub.design)=(V.sub.design 
/V.sub.actual).sup.10.about.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.

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 "trained" 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.sub.3S from stored component values E.sub.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.sub.T or the trained current value. 
Through experiment, it has been determined that light intensity is related 
to lamp current through the equation: 
EQU (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.sub.C /I.sub.T).sup.5.907. Thus 
the equation for DELTA would be E.sub.C -E.sub.T (I.sub.C 
/I.sub.T).sup.5.907 =DELTA. 
The second way to correct for lamp aging mathematically is by multiplying 
the sensed wavelength component values by (I.sub.T /I.sub.C).sup.5.907. 
Thus, DELTA would be calculated by E.sub.T -E.sub.C (I.sub.T 
/I.sub.C).sup.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's invention can be 
determined by the claims appended hereto.