Abstract:
An LED-based color measurement instrument including an illumination system and a sensing system. The illumination system includes modulated LEDs and a temperature control system for regulating the temperature of the LEDs, thereby improving the consistency of their performance. The sensing system includes a photodiode, a transimpedance amplifier, and an integrator in the first stage to cancel the effect of ambient light on the output of the first stage. The sensing system also includes a lens system for imaging a target area of the target sample onto the photo sensor in a manner so that the product of the target area times the solid angle captured by the lens system is generally uniform over a selected range of distances, thereby reducing the positional sensitivity of the instrument with respect to the target sample.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 10/669,110, filed Sep. 23, 2003. 
    
    
     BACKGROUND 
     The present invention relates to color measurement instruments, and more particularly to color measurement instruments that include modulated LEDs as illumination sources. 
     A variety of color measurement instruments are well known and widely used in a variety of applications to measure color. Some of these instruments illuminate the target sample sequentially with a plurality of monochrome illuminators, measure the light reflected by the target sample to each of the monochrome illuminators, and determine the color of the target sample based on all of the measurements. 
     Light emitting diodes (LEDs) have been used as the monochrome illuminators. Original instruments included red, green, and blue LEDs. More recent instruments include more than three LEDs; and some include as many as eight. 
     In such instruments, the LEDs are typically modulated at predetermined frequencies so that the sensing circuit can discriminate between light reflected from the LEDs, which is of interest, and light reflected from ambient light, which is not of interest. The sensing circuit can ignore the ambient component by looking only at reflected light received at the predetermined frequencies. 
     Several problems exist in current LED-based instruments. A first problem is that the output of the LEDs varies with the temperature of the LEDs. Specifically, the output changes in terms of intensity, spectral energy distribution, and the spatial distribution of the output. The temperature changes are attributable both to the ambient temperature and the amount of time that the LEDs are illuminated. Unfortunately, this variation in LED output adversely impacts the accuracy of the color measurement. 
     A second problem is that the ambient light component can saturate the transimpedance amplifiers in the sensing circuit and thereby limit dynamic range, particularly in the first stage. Prior artisans have addressed this problem by placing a shunt element in opposition to the photodiode, integrating the output of the transimpedance amplifier at a frequency less than the ambient light frequencies, and using the integrated signal to control the shunt to act as a current sink for the frequencies of the ambient light. While this is an effective way to cancel the effects of an extremely wide dynamic range of ambient light, it also is inherently noisy and sensitive to loop gain and bandwidth issues. Consequently, measurements include errors of an undesirable magnitude. 
     A third problem is that the distance between the instrument and the sample is a critical factor that must be precisely controlled. This critical factor is known as positional sensitivity. Because the optics of such instruments are typically tuned to a precise distance, variations in that distance typically detract from the accuracy of measurements. Unfortunately, positional accuracy is not a practical possibility in industrial applications, where positional repeatability varies to some degree because moving components, such as robotics, cannot always be positioned precisely. 
     SUMMARY 
     The aforementioned problems are overcome in the present invention in which an LED-based color measurement system provides previously unavailable measurement accuracy in an LED-based instrument. As with all LED-based instruments, the instrument includes an illumination system and a sensor system. The illumination system includes a plurality of monochrome LEDs modulated at preselected frequencies for illuminating a target sample. The sensor system includes a light-sensitive device for measuring the strength of the light reflected from the target sample and for separating the signal of interest from the ambient component. 
     In a first aspect of the invention, the illumination system includes an active temperature control system for the LEDs. More specifically, the temperature control system includes a sensor for measuring the temperature of the LEDs, a temperature-changing device (e.g. a heater) for changing the temperature of the LEDs, and a controller for driving the temperature-changing device in response to the temperature-sensing device. The temperature of the LEDs can be held at a relatively constant value; and, therefore, the outputs of the LEDs are constant—both in terms of frequency and intensity. 
     In a second aspect of the invention, the sensing system includes a photo-sensor, a transimpedance amplifier connected across the photo-sensor, and an integrator having an input connected to the amplifier output, and an output and connected to the photo-sensor and to one of the amplifier inputs. The integrator removes the ambient light contribution to the amplifier output by mirroring the signal attributable to the ambient light at the other amplifier input. Therefore the system provides ambient light rejection, but without the noise and sensitivity of prior systems. 
     In a third embodiment of the invention, the sensor system includes optics that reduce the positional sensitivity of the instrument with respect to the target sample. More particularly, a lens system is provided for imaging the target sample onto the photo-sensor. Within a predetermined distance range, the product of 1) the target area imaged onto the photo-sensor times 2) the solid angle captured by the lens is generally uniform. Consequently, the positional sensitivity of the instrument to the target sample is reduced. 
     These and other objects, advantages, and features of the invention will be more fully understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front perspective view of the color measurement instrument; 
         FIG. 2  is a rear perspective view of the color measurement instrument; 
         FIG. 3  is a plan view of the interior of the color measurement instrument; 
         FIG. 4  is a perspective view of the LED illuminator package; 
         FIG. 5  is a top plan view of the LED illuminator package; 
         FIG. 6  is a sectional view of the LED illuminator package; 
         FIG. 7  is an enlarged view of  FIG. 4 ; 
         FIG. 8  is a rear plan view of the header carrying the LEDs; 
         FIG. 9  is a schematic diagram of the temperature control system; 
         FIG. 10  is an illustration of the spectral coverage provided by the LED sets in the illuminator package; 
         FIG. 11  is a schematic diagram of the first stage of the sensor system; 
         FIG. 12  is a schematic illustration of the optics portion of the sensor system; 
         FIG. 13  is an illustration of the positional flexibility of the present instrument; 
         FIG. 14  is an illustration of the positional sensitivity of the instrument without the disclosed optics; and 
         FIG. 15  is an illustration of the illumination zone and the detector view zone of the instrument. 
     
    
    
     DETAILED DESCRIPTION 
     A color measurement instrument constructed in accordance with a preferred embodiment of the invention is illustrated  FIGS. 1–3  and generally designated  10 . The instrument includes a housing  12 , an illumination system  14  ( FIG. 3 ), a sensor system  16  ( FIG. 3 ), and communication/power ports  18 . 
     The housing  12  is constructed using conventional techniques to provide a protective enclosure for the color measurement instrument  10 . The housing  12 , and the contents to be described, are designed and built to withstand the rigors of an industrial environment. Suitable housings  12  will be readily apparent to those skilled in the art. The communication/power ports  18  provide communication and power ports for the instrument  10 . The ports  18  also are well known to those skilled in the art. 
     I. Illumination System 
     The illumination system includes an illuminator  20  ( FIGS. 4–7 ), an anti-reflective tube  21  ( FIG. 3 ), and lens  15  ( FIGS. 1 and 3 ). 
     The illuminator package  20  is illustrated in  FIGS. 4–7  and includes eight sets of different wavelength die-mounted LEDs on a TO-8, 12-pin header or “can” that enables each set of LEDs to be individually addressable. A total of twenty-seven LEDs is mounted on the header as follows: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Nominal 
                 Typical Peak 
                   
                 Typical Vf 
                 Radiometric 
               
               
                 Wavelength 
                 Wavelength 
                 # of LED Die 
                 (V) @ 
                 Flux (mW) @ 
               
               
                 (nm) 
                 (nm) 
                 Within Set 
                 20 mA/die 
                 20 mA/die 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 405 
                 400 
                 2 
                 7.37 
                 5.79 
               
               
                 470 
                 469 
                 3 
                 11.77 
                 8.07 
               
               
                 505 
                 508 
                 2 
                 7.44 
                 3.66 
               
               
                 527 
                 519 
                 3 
                 11.18 
                 5.89 
               
               
                 570 
                 575 
                 10 
                 12.03 
                 3.87 
               
               
                 590 
                 592 
                 3 
                 5.93 
                 4.71 
               
               
                 630 
                 628 
                 3 
                 5.51 
                 4.56 
               
               
                 660 
                 661 
                 1 
                 1.86 
                 4.14 
               
               
                   
               
             
          
         
       
     
     The LED die are placed on an Alumina substrate inside of the TO-8 can. The substrate is adequately thermally conductive to enable uniform temperature distribution. A serpentine resistor or heater  26  (see  FIG. 8 ) wraps across the back side of the LED substrate in on which the LEDs are mounted on the front side of the substrate. The heater  26  has a resistance (480 ohms in the current embodiment) that allows heating of the entire can from 0EC to 45EC in a reasonable waiting period. A thermistor  32  is mounted on the front side of the LED substrate and reports the temperature of the substrate and therefore of the LED die. Current is pulsed through the serpentine resistor  26  to keep the thermistor  32  at its target temperature. 
     The pin layout for the illuminator  20  is as follows: 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Pin # 
                 Function 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Common Cathode “−” 
               
               
                 2 
                 480 Ohm Serpentine “+”* 
               
               
                 3 
                 470 nm Anode “+” 
               
               
                 4 
                 570 nm Anode “+” 
               
               
                 5 
                 570 nm Anode “+” 
               
               
                 6 
                 405 nm Anode “+” 
               
               
                 7 
                 1K Ohm Thermistor “−”* 
               
               
                 8 
                 1K Ohm Thermistor “+”* 
               
               
                 9 
                 660 nm Anode “+” 
               
               
                 10 
                 630 nm Anode “+” 
               
               
                 11 
                 590 nm Anode “+” 
               
               
                 12 
                 505 nm Anode “+” 
               
               
                   
               
               
                 *“+/−” Arbitrary for Resistor 
               
             
          
         
       
     
     The heater control system is illustrated in  FIG. 9 . The system includes the thermistor  32 , the serpentine resistor  26 , and a control  34 . The control  34  is operatively coupled to both the thermistor  32  and the heater  26  to control the heater. The control  34  periodically samples the thermistor  32 , which n the current embodiment is every 50 milliseconds. If the temperature is below a predetermined target temperature, the heater is turned on. If the temperature is above the predetermined target temperature, the heater  26  is turned off. 
     There are three options in controlling temperature. The first is to heat the LEDs to some point above the operational ambient temperature range. The second is to cool the LEDs to some point below the operational ambient temperature range. The third is to ignore ambient temperature range and implement both heating and cooling. Each option has its advantages and disadvantages. Heating has the advantage of lowest cost, but the disadvantage that the LEDs are less efficient at higher temperatures. Cooling has the opposite advantages and disadvantages. The third option has the advantage of providing an ideal temperature, but the disadvantage of greatest cost. Based on the present ambient temperature range and economics, the present invention implements the first option of heating only. 
     The ambient temperature range of the current instrument is 0EC to 40EC. The target temperature for the header  22 , and therefore the LEDs in the header, is selected to be 45EC so that the temperature of the header will always be above the temperature of the operating environment. Using the described control methodology, the temperature can be maintained within 0.1EC of the target temperature. Different target temperatures can be selected depending on the ambient range. Maintaining the LEDs at a uniform temperature enhances the uniformity of the output of the illuminator, including the intensity, the spectral energy distribution, and the spatial distribution of the output. While the temperature control concept has been described in conjunction with LEDs, the concept is applicable and adaptable to other temperature-sensitive illuminators. 
     Each of the LED sets is modulated at approximately 24 KHz, using techniques well know to those skilled in the art. This frequency is substantially above the frequency of virtually all known sources of ambient light. 
     The spectral output of the eight sets of LEDs is illustrated in  FIG. 10 . Each of the eight spectral curves corresponds to one of the sets of LEDs. As can be seen, the eight LED sets provide thorough coverage of the visible spectrum (i.e. between 400 nm and 700 nm) the particularly selected LEDs provide an appropriate compromise between spectral coverage and cost. 
     The anti-reflective tube  21  is of a type generally known to those skilled in the art. The tube includes internal saw-tooth circumferential ribs having a black matte finish. 
     The lens  15  is selected so that the illumination system  14  provides spatially uniform illumination or irradiance, particularly at the target distance from the sample. Spatially uniform means that the flux is uniform throughout the cross section of the illumination beam. In the current embodiment, the lens is a single bi-convex lens. Other suitable lenses are known to those skilled in the art. 
     II. Sensor System 
     A. First Stage 
     The first stage  40  of the sensor system  16  is illustrated in  FIG. 11 . The first stage includes a photodiode  42 , a transimpedance amplifier  44 , an integrator  46 , and a high-pass filter  48 . The first stage  40  is designed 1) to produce a signal proportional to the light reflected from the target sample (from both the modulated LEDs and from ambient light sources), 2) to amplify the signal, and 3) to cancel the ambient light component from the signal. 
     The photodiode  42  provides a current output proportional to the amount of light directed onto the photodiode. The transimpedance amplifier  44  includes a pair of inputs  45  connected across the photodiode  42 . The output of the amplifier  44  is connected to the input of the integrator  46 . The output of the integrator  46  is connected to both the photodiode  42  and to one of the inputs  45  of the transimpedance amplifier  44 . In the current embodiment, the integrator  46  has a cutoff frequency of 250 Hz. This frequency is above the dominant frequency of most ambient light sources. These relatively low frequencies are fed back to the input of the transimpedance amplifier  44  opposite the photodiode  42  so that the effect of ambient light is canceled at the amplifier output. Consequently, the current output of the photodiode  42  is attributable only to light reflected from the modulated LEDs within the illuminator  20 . 
     The first stage  40  has at least two benefits. First, at direct current (DC) and ambient light frequencies, the signal across the photodiode is essentially zero (i.e. the photodiode is essentially bootstrapped); and the output of the transimpedance amplifier at ambient light frequencies also is essentially zero. Because ambient light rejection occurs in the first stage of signal processing, extended dynamic range is enabled in later stages. Second, cancellation of the ambient light effect is performed away from the sensitive input of the transimpedance amplifier. 
     The output of the amplifier  44  is fed to a high-pass filter  48 , whose cutoff frequency is approximately 1000 Hz. The high pass filter removes most of the residual ambient light component from the signal. 
     B. Sensing System Optics 
     The positional sensitivity of a color measurement instrument is a critical parameter due to the combinatorial effect of two basic factors. First, the distance between the instrument and the target sample varies. This is attributable to the positional repeatability error of robotic fixtures and other industrial equipment. Additionally, the target area on the target sample may vary positionally from piece to piece. The second factor relates to the inverse square law. Specifically, the intensity of light radiating at multiple angles from a point decreases in intensity by the inverse square of the distance. Consequently, even at a nominal target distance of 1.5 inches, a variation of even 0.1 or 0.2 inch in the distance of the instrument from the target sample will introduce significant error into the measurement. The chart illustrated in  FIG. 14  illustrates the variation in signal strength of a conventional instrument at various distances around a nominal target distance of 1.5 inches. 
     The optical portion  50  of the sensing system  16  is schematically illustrated in  FIG. 12 . The preferred target sample position is illustrated at  52 , and the acceptable target sample range extends between the extreme positions  54  and  56 . The optical system  50  includes a lens system  17 , whose focal length is the distance between the lens system and the preferred target sample position  52 . The lens system  17  is between the detector  42  and the target sample TS. In the current embodiment the lens system  17  is a two-piece lens assembly including a plano-convex lens  17   a  and an aspheric lens  17   b . Such a lens system is considered optically “fast”; it enhances light gathering efficiency; and it therefore improves signal strength. Alternatively, a single lens could work under the appropriate circumstances. The selection of the particular lens system  17  in view of this description would be routine by one skilled in the art. 
     The optical system  17  images the desired target area TA (see also  FIG. 15 ) of the target sample TS onto a fixed-area  57  of the image plane of the photodiode  42 . The size and shape of the fixed area are defined by a mask  59  adjacent to and/or on the image plane of the photodiode  42 . The mask  59  provides a crisp edge to the fixed area. Preferably, the mask is thin and opaque to enhance the crispness of the edge; and the front of the mask is dark (e.g. black matte) to absorb stray light. In the current embodiment, the mask is a metal foil. Alternatively, the mask could be coated on, or otherwise applied to, the photodiode  42 . 
     As illustrated in  FIG. 12 , the solid angle  60   a  collected by the lens system  17  is relatively large when the target sample TS is in the closest position  56 . Conversely, the solid angle  60   b  is the smallest when the target sample TS is in its farthest position  54 . In the closest position  56 , the target area of the target sample imaged onto the detector  42  is smaller than the target area when the sample is in the preferred position  52 , and the target area of the target sample TS imaged onto the detector when the target sample TS is in the farthest position  54  is relatively large. As long as the distance between the extreme sample positions  54  and  56  is small in comparison with the other distances, such as the focal distance of the lens system  17 , then these two effects (solid angle and target area) will essentially cancel each other, providing a constant detector signal for samples with the same radiance. In other words, the product of the target area and the solid angle captured by the lens is the same in all cases between positions  54  and  56 . The radiance of the samples is the same because the illumination field irradiance is the same (as discussed above) due to the search light illumination of the target sample TS. While the viewed area of the target (the target area) will vary slightly between positions  54  and  56 , the tradeoff has been found to be acceptable. 
       FIG. 13  illustrates the improved positional insensitivity (or positional flexibility) provided by the present design. When compared to  FIG. 14 , it will be noted that the measurement error at all LED frequencies is significantly reduced in the present design. 
       FIG. 15  illustrates both the illumination optics and the sensor optics. These optics are interrelated by the issues of instrument geometry and mechanical packaging. In the current instrument, the angle between the axis of illumination and the axis of detection is approximately 30°. This geometry provides an appropriate balance and compromise among the following objectives and considerations: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1. 
                 Small package size 
               
               
                 2. 
                 Large measurement spot size 
               
               
                 3. 
                 “Fast” optics for good light gathering efficiency 
               
               
                 4. 
                 Depth of field 
               
               
                 5. 
                 Cost 
               
               
                 6. 
                 Target discrimination 
               
               
                 7. 
                 Positional insensitivity (or flexibility) 
               
               
                 8. 
                 Easy targeting and setup 
               
               
                   
               
             
          
         
       
     
       FIG. 15  illustrates the current optical geometry. The outer perimeter  60  of the target area TA when the target sample TS is in the focal plane  52  (also known as the in focused detector image) is generally square. The outer perimeter  62  of the target area TA when the target sample TS is in either of the positions  54  or  56  (also known as the defocused detector image) is also generally square, but somewhat larger than the in focus detector image. The distance between the lens  17  and the desired position or focal plan  52  is 38.1 mm (1.5 inch), and the distance range is from 33.0 mm (1.3 inch) to 43.2 mm (1.7 inch). This results in a positional range of 13.2 mm (0.4 inch). 
     The above description is that of a preferred embodiment of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as set forth in the appended claims, which are to be interpreted in accordance with the principles of patent law including the Doctrine of Equivalents.