Abstract:
In one embodiment of the invention there is disclosed a device for correcting the output of a light source such that the corrected output conforms to a known standard. The corrected light outputs are proportional to the colors contained in the light impacting the color sensor. In one embodiment, memory contains at least one color matching function pertaining to the specific color sensor and a controller works with the memory to convert the color sensor light outputs to a specific color PCS space based on the matching function. If desired, a user can select a desired PCS protocol.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to currently pending U.S. patent application Ser. No. 11/029,613 entitled “COLOR MEASUREMENTS OF AMBIENT LIGHT”, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The need for color sensors is becoming more prevalent as technology moves away from black/white and gray scale imaging. Such color sensors are necessary for controlling display devices as well as for making color measurements. Many applications now exist where it is necessary to test a product for proper color. These situations range from determining that a proper yarn is being used to biomechanics where, for example, color is used to detect glucose levels.  
         [0003]     Photodetectors, usually operating in conjunction with color filters, are now used to accomplish such color detection. Such photodetectors provide as their output signals (such as R, G and B) representative of the three basic colors (red, green and blue) which are electronically combined to produce color images.  
         [0004]     Typically, the photo detector device specific RGB (or CMYK) outputs do not relate very well to the human eye&#39;s experience with color. Thus, in order to use the device specific color outputs for testing and control purposes it is necessary to convert them to a system that is device independent and universally accepted as being unambiguous. The profile connection space (PCS) is designed to accomplish this function. In the 1930s the Commission International de l&#39;Eclairage (CIE) began to set color space standards in the form of models. Examples of these color space models are the CIELab, CIEXYX and the CIExyY models.  
         [0005]     It is desired to convert device specific outputs (such as RGB and CMYK) to a PCS (commonly called standard) color space. However, before this can be accomplished it is critical to know the behavior of the photo detector device so that the ambiguous outputs from such a device can be converted to unambiguous color signal outputs. To do this, it has become accepted practice to use a color matching engine. The engine must know the device that generated the input signals (RGB or CMYK) and how those signals are going to be used, i.e. the target device. Accordingly, for each new device a calibration must take place and the necessary data stored for subsequent use.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     In one embodiment of the invention there is disclosed a device for correcting the output of a light source such that the corrected output conforms to a known standard. The corrected light outputs are proportional to the colors contained in the light impacting the color sensor. In one embodiment, memory contains at least one color matching function pertaining to the specific color sensor and a controller works with the memory to convert the color sensor light outputs to a specific color PCS space based on the matching function. If desired, a user can select a desired PCS protocol.  
         [0007]     In one embodiment, there is disclosed a method for calibrating a light sensor device by applying a light source to a light sensor with a known calibrated output and recording the calibrated outputs therefrom. The same light source is then applied to the light sensor device which has an uncalibrated output. The output from the uncalibrated light sensor is then recorded. After repeating these recordings for all of the pertinent color spectrum there is derived a CIE matching function for the uncalibrated light sensor by using the recorded outputs. This matching function is specific to the device and is used so that light subsequently applied to the uncalibrated light sensor will yield a calibrated output controlled, at least in part, by the stored matching function.  
         [0008]     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0010]      FIG. 1  shows one embodiment of the color sensor of the present invention;  
         [0011]      FIG. 2  shows one embodiment of the color sensor of  FIG. 1  used in a system for control purposes;  
         [0012]      FIG. 3  shows an alternate embodiment of the light sensor of  FIG. 1 ;  
         [0013]      FIGS. 4, 5A  and  5 B show embodiments of calibration systems;  
         [0014]      FIG. 6  shows an alternative embodiment for use with printed material;  
         [0015]      FIGS. 7 and 8  show embodiments of calibration methods; and  
         [0016]      FIGS. 9A, 9B , and  9 C show embodiments of the prior art. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Prior to beginning a discussion of the inventive concepts, it would be helpful to briefly review the prior art with respect to  FIGS. 9A, 9B  and  9 C.  
         [0018]      FIG. 9A  shows system  90  in which light source  91  impacts photo diode  92  with RGB producing photocurrent outputs.  
         [0019]      FIG. 9B  shows system  91  in which resistor  93  and a trans-impedance amplifier (TIA)  94  is used to convert the RGB current output ( FIG. 9A ) into an RGB analog voltage output.  
         [0020]      FIG. 9C  shows system  92  in which analog digital converter  95  has been added to convert the current output of  FIG. 9B  to an RGB digital output. Since these outputs are device-dependent and are representative of the input intensity of the various three colors depicted, they do not carry with them chromaticity or luminescence information. Thus, for systems which require chromaticity or luminescence information, the outputs of the devices shown in  FIGS. 9A, 9B  and  9 C are not acceptable. Also, as discussed above, these signals do not translate well into the color spectrum that is observable by the human eye.  
         [0021]      FIG. 1  shows one embodiment of color sensor  10  having input light  100  conveyed to color sensor  11 . Voltage signals Vr, Vg, and Vb are output from color sensor  11  on lead  101  and form the input to analog to digital converter (ADC)  12  as discussed above with respect to  FIG. 9C . The output of ADC  12  is a digital signal Vr, Vg and Vb on lead  102 . This digital signal is input to controller  13 . Controller  13  operates to accept the signals from lead  102  and perform an RGB to CIE conversion. This is accomplished by retrieving a CIE matching function from storage device  14 . Various protocols can be used by controller  13 , such as SPI  12   c , etc. Storage device  14  can be any device that stores data and preferably would be an EEPROM. Selector input  105  can be provided so that a user can select a desired matching function, such as, for example, CIE LAB, CIE XYZ or CIE xyY. Color sensor  11  can be a set of photo diodes with filters so that each photo diode would output a voltage or current proportional to the amount of a certain frequency of light.  
         [0022]     Storage  14  will confirm one or more pre-stored CIE matching functions, but prior to device  10  being calibrated, the output  104  for any light arriving at sensor  11  will be ambiguous. Calibration of device  10  will be discussed hereinafter with respect to  FIGS. 4, 5A ,  5 B,  7  and  8 . At this point it is sufficient to note that once a calibration has been performed on a specific device, the calibration need not be repeated unless a user has a reason to believe the device is out of calibration. If desired, the calibration can be performed periodically to ensure continued reliability.  
         [0023]      FIG. 2  shows one embodiment  20  of color sensor  10  used in a system for control purposes. Ambient light  21  impacts input  100  of device  10 . Output  104  from device  10  is an unambiguous color space representation of the colors contained in ambient light  21  as discussed with respect to  FIG. 1  (assuming device  10  has been calibrated). This color space representation of the ambient light is provided to controller  23  (in one embodiment, a signal processor) which receives inputs from display  22  via lead  201 . The inputs from display  22  are compared and a signal is provided to processor  24  which uses the ambient chromaticity as well as brightness information as inputs and provides the information back to display  22  where display  22  can change its dynamic color and brightness under control of processor  24 . This then allows display  22  (which could be a TV or other display device) to adjust its color balance based on the color spectrum of the ambient light. Note that, of course, signal processor  23  and processor  24  can be combined into one unit as well as this unit can also be combined with display  22 . In one embodiment display  22  is a PDP display.  
         [0024]      FIG. 3  shows an alternate embodiment  30  of light sensor  10  ( FIG. 1 ) with optical lens  31  as part of input  100 . Lens  31  functions to gather ambient light, such as ambient light  21  or any other light source into the input of device  10 . Output  104  then, as discussed, provides unambiguous light output signals in the color space.  
         [0025]      FIG. 4  illustrates an embodiment of a system and method for calibrating device  10  of  FIG. 1 . In operation, selected light from light source  41  is received by CIE camera  42 . The light is measured therein and the outputs of the CIE camera, which by definition yield very accurate color space outputs, are used to code the output for a particular light input.  
         [0026]     Light from source  41  is also provided to color sensor  10  and the outputs are recorded in storage  43 . While a single database is shown, different or multiple databases can be utilized. The outputs from camera  42  are compared (frequency by frequency) against the outputs stored from color sensor  10  in order to derive a matching function. Once the matching function has been derived, this function is stored in device  10  via lead  403 . Process  711  controls the storage of the function in EEPROM  14  within color sensor  10  via lead  403 .  
         [0027]     Note that the data recorded from CIE camera  42  is an unambiguous known entity and therefore the output from color sensor  10  which is ambiguous to begin with is then compared to the output of CIE camera  42  based upon the same light input from selected light source  41 . The difference then over a broad spectrum is determined and becomes part of the matching function which is then stored in color sensor  10  (for example, in an EEPROM) so that in subsequent operations the output from color sensor  10  is corrected to be an accurate and unambiguous statement of the colors contained in a light source.  
         [0028]      FIGS. 5A and 5B  show a similar calibration system and method for reflective light. As shown in  FIG. 5A , light source  52  reflects light from a chart, such as a MacBeth chart, into CIE camera  54  which records data in storage  55 .  
         [0029]      FIG. 5B  shows light source  52  reflecting light from chart  53  into color sensor  10 , the output of which is recorded in storage  56 . The data in storage  55  is compared to the data in storage  56  to devise a proper matching function unique to device  10  and chart  53 .  
         [0030]      FIG. 6  shows alternative embodiment  10  for use with printed material which typically uses the CMYK color values. In the embodiment of  FIG. 6 , light source, such as LED  61 , shines upon a material printed with CMYK ink. The reflection is then picked up by lens  100  of device  10  and output  104  is then converted into the CIE color space for presentation to a control system or for testing purposes, or to display the colors that are shown in the printed material on sheet  62 .  
         [0031]      FIG. 7  illustrates one embodiment  70  of a system and method for calibrating device  10 . In operation, process  701  selects a color and process  702  or process  704  provides the selected color to both the camera ( 702 ) or the color sensor ( 704 ). The respective outputs are then stored, in processes  703  and  705 . Process  706  determines if all the outputs have been recorded.  
         [0032]     Since there could be several different light frequencies that need to be tested it is important to be sure they are each passed through the system for calibration and thus process  707  insures that all frequencies are tested. If some frequencies have not been tested, then process  708  changes the peak wavelength and process  70  repeats.  
         [0033]     Once process  707  determines that there are no further wavelengths to record, process  709  controls the comparison of the recorded outputs from both the CIE camera and the color sensor in order to derive a matching function under process  710 . Once the matching function has been derived, process  711  and process  712  control the storage of the function, for example, in an EEPROM within device  10 .  
         [0034]      FIG. 8  shows process  80  for selecting a light frequency to be corrected using a reflective light system. In operation, process  801  selects a color to apply to the chart and process  802  or process  804  provides the light either to the CIE camera or to the color sensor. This can be done sequentially or in parallel. The outputs are recorded by either process  803  or  805 . Processes  806 - 812  are the same as discussed with respect to  FIG. 7  process  706 - 712 . The difference being that when a chart is being used there are certain colors in the chart that must be cycled through. This is controlled by process  807 . For example, with the MacBeth chart, there are  24  colors which would be cycled in order to achieve the proper matching function.  
         [0035]     The matching function is derived in many ways, one of which is a n×3 matrix containing XYZ outputs, for example, from the CIE camera which equals a coefficient times three×n matrix containing RGB outputs from the light sensor. The CIE matching function is derived by solving the linearly correlated equation.  
         [0036]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.