Abstract:
A computer program product and method for calibrating and characterizing a color display perform calibrating and characterizing steps. A light source is operated in order to emit light from one or more light emitters on the light source. A color capture device, e.g., a digital camera, is calibrated and characterized based on the emitted light. Then, color images are displayed on the color display and captured on the color capture device. The color display is calibrated and characterized based on the captured color images. Computer program instructions are recorded on the computer readable medium, and are executable by a processor, for performing the calibrating and characterizing steps. A method for generating a controlled light source includes displaying light source selections to a user and receiving a user light source selection. Selected light emitters produce a light output matching the user light source selection.

Description:
BACKGROUND 
       [0001]    Desktop imaging solutions are being increasingly used for both personal photography and to obtain image content for low-budget printed documents in small businesses. The typical desktop imaging environment consists of a digital camera, a computer, a color display device (CRT monitor or flat panel LCD) and image editing and organization software. Image and document editing tasks are often undertaken using the available monitor or display with little regard for its color accuracy or state of calibration. Unforeseen or unsatisfactory results can be obtained if the color monitor or display is poorly calibrated. Unfortunately, such failures are not noticed until the document is viewed on a different monitor or printed, by which time considerable editing effort may have been wasted. Calibrating and characterizing the color display can remedy these problems. The cost for hardware and software for such calibration is a significant expenditure for personal use or in a small business. Furthermore, the calibration system is a poor value proposition for many persons and organizations because it has no use beyond display calibration. Color display characterization is a process of deriving the relationship between digital input RGB values driving the display and the resulting colors emanating from the display as perceived by the human visual system. The latter is typically measured using instruments such as a calorimeter or spectroradiometer. Such instruments can be expensive, and require a certain level of skill on the part of the user. 
         [0002]    An alternative is to replace costly color measurement instrumentation with an inexpensive consumer camera to measure displayed colors. However, since most digital cameras do not produce calorimetric data by default, they must themselves first be characterized. The standard camera characterization process once again requires color measurement instrumentation, and therefore does not reduce the cost or burden to the user. 
         [0003]    Thus, there exists a need for a relatively low-cost, multipurpose system for calibration and characterization of both color displays and digital cameras. 
       INCORPORATION BY REFERENCE 
       [0004]    The following references are incorporated herein in their entireties, by reference: 
         [0005]    U.S. Pat. No. 6,784,995 issued Aug. 31, 2004, entitled “Colorimeter,” by Merle, et al. discloses a colorimeter for measuring a color of light, including a color sensing device, a hanging means, and a means for reducing color distortion. 
         [0006]    U.S. Pat. No. 6,816,262 issued Nov. 9, 2004, entitled “Colorimeter having field programmable gate array,” by Slocum, et al. discloses a colorimeter capable of calibrating color monitors, whether having cathode ray tube or liquid crystal (LCD) displays, by a photometric array of photodetector and optical filter pairs. 
         [0007]    DiCarlo, Jeffrey M., Glen Eric Montgomery and Steven W. Trovinger, “Emissive chart for imager calibration,”  Proceedings of the  12 th Color Imaging Conference: Color Science and Engineering , Scottsdale, Ariz., USA, Nov. 9-12, 2004 discloses a calibration instrument (emissive calibration chart) based on emissive narrow-band light sources arranged in a grid pattern or chart configuration. 
         [0008]    Digital Color Imaging Handbook, G. Sharma, Ed. Boca Raton, Fla.: CRC. 2003, ch. 5, “Device characterization,” R. Bala discloses color device characterization according to a device-independent paradigm involving device transformations between device-dependent and calorimetric representations. 
       BRIEF DESCRIPTION 
       [0009]    A method is provided for calibrating and characterizing a color display. A light source is operated in order to emit light from one or more light emitters on the light source. A color capture device, e.g., a digital camera, is calibrated and characterized based on the emitted light. Then, color images are displayed on the color display and captured on the color capture device. The color display is calibrated and characterized based on the captured color images. 
         [0010]    A method is also provided for generating a controlled light source. A light source is provided, including a plurality of light emitters. Light source selections are displayed to a user on a display device, and a user light source selection is received. One or more of the light emitters and associated intensity level are selected such that the selected light emitters produce a light output matching the user light source selection as closely as possible when operated at the associated intensity levels. The selected light emitters are then operated at the associated intensity level. 
         [0011]    A computer program product for calibrating and characterizing a color display is also provided. Computer program instructions are recorded on the computer readable medium, and are executable by a processor, for performing the following calibrating and characterizing steps. A light source is operated in order to emit light from one or more light emitters on the light source. A color capture device, e.g., a digital camera, is calibrated and characterized based on the emitted light. Then, color images are displayed on the color display and captured on the color capture device. The color display is calibrated and characterized based on the captured color images. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a computer system incorporating concepts of the present application; 
           [0013]      FIG. 2  is a flowchart showing a method for characterizing a digital camera according to concepts of the present application; 
           [0014]      FIG. 3  is a flowchart showing a method for characterizing a display according to concepts of the present application; 
           [0015]      FIG. 4  is a flowchart describing using a light source as a calibrated illuminant according to concepts of the present application; 
           [0016]      FIG. 5  is a flowchart describing using a light source as a general light source according to concepts of the present application; and 
           [0017]      FIG. 6  is a block diagram showing a configuration of a system according to concepts of the present application. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present application describes a simple camera-based method for performing a color characterization of a display that does not require measurements from specialized instruments such as colorimeters and/or spectroradiometers. By modulating the relative intensities of the various LEDs on a light source, the light source is made to produce a wide range of color stimuli of known spectral composition and brightness. The system operates by first using the light source serving as a pre-calibrated emissive target to characterize a digital camera and then using the digital camera to characterize the color display. The software controls these characterization operations and computes the color management profiles for the devices. 
         [0019]    With reference to  FIG. 1 , an exemplary apparatus is shown consisting of a color display  10 , a digital camera  12  such as, e.g., a webcam (low resolution video) or a digital still camera, a light source  14  for characterizing the camera  12 , a computer  16  running a software program  17  to control the characterization processes, and holding brackets  18 ,  20  for positioning of the light source  14  and the camera  12 . 
         [0020]    The light source  14  preferably houses multiple, colored, light emitting diodes (LEDs)  22 , the brightness of which can be individually adjusted. Advantages of LEDs with respect to the present application include, but are not limited to: standardized light emitting characteristics, narrowband emission characteristics enabling them to be combined in a controlled fashion to produce a wide variety of selected spectral radiances, and ready availability as inexpensive commodity items. Table 1, for example, shows the capabilities of a small range of commercially available LEDs. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Operating Characteristics of a Small Range of LEDs 
               
               
                 OPT 0603 Super Bright Electro-Optical Characteristics 
               
             
          
           
               
                   
                 V F  (V) 
                 λ (nm) 
                 L v  (mcd) 
               
             
          
           
               
                 Part Number 
                 Lighting Color 
                 Material 
                 Typ 
                 Max 
                 λ D   
                 λ P   
                 Δλ 
                 Typ 
               
               
                   
               
             
          
           
               
                 OPT-0603UC640-140C 
                 Red 
                 AlGaInP 
                 1.90 
                 2.40 
                 631 
                 640 
                 23 
                 75 
               
               
                 OPT-0603UC630-140C 
                 Red 
                 AlGaInP 
                 1.90 
                 2.40 
                 624 
                 635 
                 25 
                 85 
               
               
                 OPT-0603UC620-140C 
                 Orange 
                 AlGaInP 
                 1.90 
                 2.40 
                 615 
                 620 
                 18 
                 100 
               
               
                 OPT-0603UC610-140C 
                 Amber 
                 AlGaInP 
                 1.90 
                 2.40 
                 605 
                 610 
                 25 
                 75 
               
               
                 OPT-0603UC590-140C 
                 Yellow 
                 AlGaInP 
                 2.00 
                 2.50 
                 588 
                 590 
                 32 
                 85 
               
               
                 OPT-0603AE575-140C 
                 Green 
                 AlGaInP 
                 2.10 
                 2.60 
                 578 
                 575 
                 18 
                 50 
               
               
                 OPT-0603MN525-140C 
                 True Green 
                 InGaN 
                 3.20 
                 3.80 
                 527 
                 525 
                 40 
                 250 
               
               
                 OPT-0603MN505-140C 
                 Turquoise 
                 InGaN 
                 3.30 
                 4.00 
                 509 
                 505 
                 37 
                 250 
               
               
                 OPT-0603CE470-140C 
                 Blue 
                 GaInN 
                 3.40 
                 4.00 
                 465 
                 460 
                 68 
                 80 
               
               
                 OPT-0603CE430-140C 
                 Blue 
                 GaN/SiC 
                 4.00 
                 4.50 
                 465 
                 430 
                 60 
                 8 
               
               
                   
               
             
          
         
       
     
         [0021]    The software program  17  controls the brightness of individual LEDs in order to generate color stimuli of various luminance and chromaticity. The color stimuli are used to characterize the digital camera. However, it is to be appreciated that although the present application describes the light source  14  as using LEDs, alternate light sources can be used with sufficient efficacy such as, e.g., phosphors, liquid crystals with filters, LEDs, or other. 
         [0022]    In one embodiment of the light source  14 , the light from the LEDs  22  is optically integrated or blended via a lens or diffuser system  24  to produce a composite stimulus. Although the lens or diffuser system  24  is shown as a separate component for clarity in the Figure, it is to be understood that the lens system is preferably an included feature of the light source  14 . In a second embodiment, the light from each LED remains distinct from the remaining LEDs, much like an array of differently colored lights. An advantage of the first embodiment is that the light source  14  can be used to produce white light with a wide range of correlated color temperatures. This could be used, e.g., to calibrate the white-balance adjustment of the camera and hence to calibrate the correlated color temperature of the display  10 . An advantage of the second embodiment of the light source  14 , however, is that multiple stimuli can be captured by the camera in parallel, thus expediting the camera characterization process. To enable white point characterization, the second embodiment can be additionally equipped with a number of appropriately selected and filtered “white-light” LEDs as known in the art to allow one or several different white lights to be produced. 
         [0023]    Whereas the light emitting diodes  22  of the light source  14  in embodiments of the present application are readily available commodity LEDs as described above, other known emissive targets described in the art such as, e.g., the emissive chart for imager calibration described by DiCarlo, Jeffrey M., Glen Eric Montgomery and Steven W. Trovinger, utilize specially designed LED-based emissive targets for general-purpose characterization of digital cameras. Further, the present application describes herein a method of using ordinary digital cameras for characterizing a display, i.e., the camera is characterized only to serve as a color measurement instrument. And, still further, the present application describes alternative uses for the light source beyond serving as a camera characterization target. For example, the light source  14  can be used as a keyboard light or a desk light when not in use for calibration purposes. Furthermore, because the light source  14  can provide a range of white light at different correlated color temperatures and with known spectral composition, it can be used for critical viewing tasks where controlled lighting is desirable. 
         [0024]    As shown in the Figure, the display  10 , the light source  14 , and the camera  12  are directly connected to the computer  16  via links  26 ,  28  and  30  connected to appropriate communication ports on the computer  16 , respectively, so that the software program  17  running on the computer  16  can control settings and operation of the attached devices. Suitable communications ports include, but are not limited to, various serial, parallel and wireless ports such as, e.g., USB, FireWire, Bluetooth and 802.11x. Each communication port sends control signals and data to and from the respective device and may also provide operating power to it. However, it is to be appreciated that a direct connection to the computer  16  is preferable, but not required, and any form of operative communication between the devices such as, e.g., wireless connection or network connection, will suffice for embodiments of the present application. In fact, with reference to the light source  14  and the camera  12 , embodiments of the present application do not require any operative connection to the computer  16 . For example, the software program  17  may instruct an operator via the display  10 , or other output device, to manually make settings or perform specific operations of the camera  12  and/or the light source  14 . The operator may then indicate completion of the instruction by means of an input device  32  such as a keyboard, or pointing device  34 . Alternatively, if the display  10  is a “touch screen” type of device, the operator can indicate completion of an instruction via buttons or active areas shown on the display  10  in a menu or dialog box. Further, in alternate embodiments, the software program  17  may communicate instructions to the operator verbally or by other audible signals via an output sound device  36  such as, e.g., a speaker system. This feature would essentially eliminate the need for displaying menus and/or instructions on the display  10  during calibration and free it for other calibration purposes without limitation. 
         [0025]    The software program  17  of the present application provides a number of functions including: providing control signals to the light source to produce different color stimuli, providing control signals to the camera to adjust camera settings, controlling the device characterization process for both the camera and the display device, and construction of color management profiles for the camera and the display device and installation of them into the operating system. As previously described, the control signals can be provided either automatically or manually by the operator as requested by the software component. It is to be appreciated that the software program  17  can be provided as a separate object on, e.g., a CD or DVD or other computer readable media which is then loaded into the computer  16 , or it can be provided to the computer  16  by means of a network connection, running either remotely on a server or locally on the computer  16 . It is to be further appreciated that the software program  17  can be programmed directly on the computer  16  by one or more programmers utilizing the input device  32  or a network connection to the computer  16 . 
         [0026]    As previously mentioned, the camera  12  can be, e.g., a video camera or a digital still camera and may even be supplied as part of a system including selected components as shown in the Figure. The camera should preferably, however, have the capability to operate with fixed exposure settings such as ISO speed, exposure compensation, shutter speed, aperture, and lens focal length. It is also preferable for the camera to be capable of being remotely controlled by the software program  17 . The function of the camera in this system is essentially to act as a sensor for calibrating the computer display. A typical use case scenario involves using the light source  14  to calibrate and characterize the camera  12  and then using the camera  12  to calibrate and characterize the display  10 . An alternative use case involves performing a simple calorimetric camera calibration for the purpose of subsequently calibrating the display. Both use cases are described below, as are use cases for simply using the light source as a source of illumination. 
         [0027]    With reference now to  FIG. 2  and continuing reference to  FIG. 1 , a flowchart is shown describing a method for characterizing the digital camera  12 . At step  40  a user of the computer  16  installs or accesses the software program  17  for the purpose of characterizing the digital camera  12 . At step  42 , the camera  12  is aligned with the light source  14  at a fixed distance from the camera  12 , preferably centered on the camera&#39;s optical axis as shown by numeral  41  in  FIG. 1 . In the embodiment shown, the camera  12  is placed in the bracket  20  which aligns it to the light source  14  which is also fixed in position by a second bracket  18 . However, any means of fixing the distance between the camera  12  and the light source  14  can be utilized. The software program  17  sets camera settings at step  44  either automatically or by requesting that the user manually perform the camera settings. The camera settings preferably include ISO speed, shutter speed and aperture so that relatively strong but not saturated camera signals are generated when each of the LEDs  22  in the light source  14  are illuminated. 
         [0028]    The software  17  then determines at step  46  which of the LEDs in the light source  14  can be used to generate the best “single-channel signals” for the R, G and B channels of the camera. The light source  14  includes in the LEDs  22  multiple LEDs of each color. Each LED is operated individually to find the best “single-channel signal” for each channel of the camera  12 . Commodity cameras usually have a single-type sensor with a color filter over it. The object here is to determine which LED produces the largest signal in one channel and, correspondingly, the smallest in the remaining channels. For example, a suitable measure is the ratio such as R/G. Three LEDs are selected, one for each of the R, G and B channels. The software program  17  then, at step  48 , generates an intensity step series from zero power to full power for each selected LED and captures a camera image for each step. At step  50 , camera  12  RGB data from the captured images and the known LED luminances for the selected LEDs are used to compute transforms relating “intensity-proportional” R′G′B′ values to camera RGB channel counts, for each channel using methods known in the art. While any suitable transform known in the art may be used, the camera RGB counts are normally nonlinear as are the computed transforms. 
         [0029]    A series of images from the camera is collected by the software program  17  at step  52 , each image corresponding to one of the narrow band LEDs  22  in the light source  14  being illuminated. The image statistics are computed at step  54  for each image, and the camera RGB values are converted to linearized R′G′B′ values using the transforms computed above in step  50 . The camera spectral responsivities are then computed at step  56  from the known spectral power distributions of the LEDs  22  and the R′G′B′ signals obtained from step  54 . After this, at step  58 , the spectral responsivities from step  56  and the nonlinear transforms from step  50  are used by the software program  17  to compute color management profiles for the camera  12 . Colors are classified according to the way humans perceive them with their visual system. Devices like cameras are designed to capture colors in a similar manner but are not exactly the same. Sometimes there are colors that a human can perceive as being different, but the camera records as being identical, and vice versa. The spectral response of the camera sensor is not normally identical to (or even a linear combination of) the spectral response of the human visual system. If the human visual system and the camera were linearly related, then they would have exactly the same ability to differentiate between colors at a certain level of intensity. Thus, the transform between the camera  12  RGB and tri-stimulus XYZ which are part of the camera  12  profile will normally be a nonlinear transform. The signals obtained from the camera  12  won&#39;t predict the colors seen by a human exactly, but since the object here is to characterize a display which is itself constrained by a limited number of light emitting sources, the camera  12  is not being used to capture the entire range of colors that can exist in the real world. It is being used eventually to capture a constrained set of spectral stimuli that come from the display  10 . Thus, it is possible to characterize a priori the ensemble statistics of a class of spectra expected to emanate from a given display type, and to use this in conjunction with the camera  12  spectral sensitivities and color matching functions to generate a camera profile suitable for capturing the subject class of spectra. 
         [0030]    As a final step  60 , various white point balancing options of the camera  12  can be characterized if desired, by determining the scale factors applied to R, G and B. 
         [0031]    Once the digital camera  12  has been characterized spectrally, or a characterized digital camera has been otherwise obtained, the digital camera  12  can be used to characterize the display  10 . With reference now to  FIG. 3 , and continuing reference to  FIG. 1 , a flowchart is shown describing a method for characterizing the display  10 . At step  70 , if not previously done, the user of the computer  16  installs or accesses the software program  17  for the purpose of characterizing the display  10 . The user then accesses the light source  14  and digital camera  12  at step  72  using appropriate communications ports on the computer  16  as previously described with reference to  FIG. 1 . The user configures the software program  17  at step  74  by providing details such as, e.g., the camera type and model and the display type to be characterized. In some embodiments, this occurs automatically for plug and play devices. The software program  17  preferably disables or requests disabling of any currently running color management application and then guides the user to adjust the manual control settings on the display  10  such as brightness, contrast, white point, etc. 
         [0032]    The software program  17  next generates an alignment target on the display  10  at step  78  and instructs the user to position the camera  12  in front of the display  10 , with the alignment target centered in the camera image, using a bracket provided for the purpose. An exemplary alignment target  79  is shown in  FIG. 1 , however, any alignment target providing a means of centering the display area of the display  10  in the camera image may be used. At step  80 , the software program  17  displays a patch at the maximum white value of the display and sets the camera to an automatic white balance mode, or requests the user to set the camera  12  to automatic white balance mode. The camera shutter speed and aperture are then adjusted at step  82  to obtain RGB values slightly less than the saturation level of the camera  12 , i.e., slightly less than 255 for an 8 bit RGB encoding such as, e.g., 250-254. A series of color patches are then displayed on the display  10  at step  84 , and the software program  17  records the camera RGB values for each of the patches. The patches displayed preferably represent an effective sampling of the color gamut of the display  10 . There should be sufficient sampling of the display gamut boundary colors to enable an accurate description of the gamut boundary to be determined. There should also be a sampling of the display  10  neutral axis (R=G=B) as well as ramps along each of the display  10  primaries (R,G,B) and their complements (C,M,Y) from both black to full color and white to full color. Color sampling inside the gamut should be comprehensive but may be biased toward certain specific memory colors. 
         [0033]    The colorimetric specifications of each of the displayed color patches are computed by the software program  17  at step  86  using the characterization model (profile) of the camera  12 . At step  88 , the software program  17  uses the camera&#39;s determination of the display  10  white point from step  80  and calorimetric data computed in step  86  to compute a model relating display RGB to CIE XYZ using methods known in the art. This model is then used at step  90  to create a color management profile for the display  10 . The above-described display characterization process offers an inexpensive and convenient process to characterize display devices which beneficially enables reliable softproofing and color editing capabilities. 
         [0034]    The procedure described with reference to  FIGS. 2 and 3  provide methods for characterizing a digital camera and a color display respectively. However, the light source  14  described therein can be used for other purposes as well. For example, using the light source  14  as a calibrated illuminant offers more reliable proofing when viewing hardcopy proofs. With reference now to  FIG. 4 , and continuing reference to  FIG. 1 , a procedure is described for using the light source  14  as a calibrated illuminant. If not previously done, the user installs the software program  17  in the computer  16  at step  100 . The software program  17  then displays an interface screen at step  102  for the user to select a correlated color temperature of the light source  14  or to simulate any of a number of standard reference illuminants. The interface screen can permit selection of the color temperature or reference illuminant by methods known in the art. For example, an array or matrix of color patches can be displayed for the user to select; the user may provide a color temperature or intensity; or the user may enter specific color values via the keyboard or pointing device. The software program  17  then, at step  104 , modulates the intensity of the various LEDs in the light source to implement the user&#39;s selection. One benefit of using the light source  14  as a calibrated illuminant is that it offers more reliable proofing when viewing hardcopy proofs. 
         [0035]    In addition to using the light source  14  as a calibrated illuminant, with reference now to  FIG. 5 , and continuing reference to  FIG. 1 , a procedure is described for using the light source  14  as a general light source such as, e.g., a keyboard light or a desk light. Assuming the software program  17  has been previously installed, at step  110 , the light source  14  is mounted to a bracket that allows it to be supported from the computer display  10  or supported on a desk surface. The bracket may be the bracket  18  previously discussed or may be a bracket specifically adapted to this purpose. The light source  14  is then controlled at step  112  by the software program  17  in one of several modes that might include, e.g., matching the light source  14  to the correlated color temperature of the display  10  to minimize visual adaptation differences for the user; setting the light source  14  to produce its maximum illumination intensity regardless of color temperature; setting the intensity and/or color temperature of the light source  14  to a preference of the user as selected through a user interface of the software program  17 ; or automatically switching the light on or off by detecting user activity such as, e.g., mouse or keyboard activity, or in synchronization with a computer screen saver or security application. 
         [0036]    In one embodiment, the light source  14  and software program  17  are provided as a package to be used with an existing camera  12  owned by the user or purchaser. In alternate embodiments, a more complete package is provided, including the camera  12 , for consumers who don&#39;t already own or have access to a digital camera. 
         [0037]    With reference now to  FIG. 6 , a block diagram showing a configuration of a system according to an embodiment of the present application is provided. A central processing unit (CPU)  120  controls various elements of the system, particularly those attached to a system bus  122 . A memory  124  is provided which preferably includes random access memory (RAM) and read-only memory (ROM). An I/O controller  126  performs the function of controlling the input and output of data to and from the memory  124  for devices attached to the I/O controller  126  such as, an auxiliary display  128 , a network interface  130 , a user interface  132 , an external storage device  134 , and an I/O device  136 . The I/O controller  126  further sends control signals to and from the attached devices. 
         [0038]    The user interface  132  preferably includes a user display  138  for the system to display, e.g., commands, software menus, and colorimetric information to the user. Although the system shown includes both an auxiliary display  128  and a user display  138 , it is to be understood that the present application can be utilized for any display operatively connected to the system, and is not restricted to any particular display or number of displays. In fact, the system itself does not necessarily include any displays because, as known in the art, displays can be operatively connected to the system via the network interface  130 . The software program  17  is shown as a separate object in the Figure for the reason that it is typically provided on media such as CD or DVD, or via the network interface  130  via a separate server system. It is to be understood that the software program  17  can be entered into the system via the I/O device  136 , the network interface  130 , by user programming via the user interface  132 , or by any other means known in the art. 
         [0039]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.