Abstract:
An apparatus for determining a type of an illuminant including a spectrum diffraction unit and a sensor coupled to the spectrum diffraction unit. An illuminant discrimination unit is coupled to the sensor and configured to receive a set of signals representative of a set of spectral components in the illuminant and to determine the type.

Description:
BACKGROUND INFORMATION 
   1. Field of the Invention 
   This invention is related to the field of use of image capture. More particularly, this invention is directed to a method and apparatus for determining the nature of the illuminant during still and video image capture. 
   2. Background 
   Video and still image capture using a digital camera has become very prevalent. Video capture may be used for such applications as video conferencing, video editing, smart vision and distributed video training. Still image capture with a digital camera may be used for such applications as photo albums, photo editing, and compositing. Many components of hardware and software need to work seamlessly to both get the video data (also referred to as a video “stream”) or the still image data from the hardware through the various layers of software on the PC and made visible on a display medium. 
   Digital cameras are typically expected to operate under a variety of scene illuminations. Some common illuminant conditions are fluorescent lighting, day light (also known as D65 lighting), and tungsten illumination. These light sources have different spectral (wavelength) components, as shown in FIG.  1 . For example, the tungsten light source has a stronger red wavelength component (600 nm to 700 nm) compared to the blue wavelength component (400-480 nm), and green wavelength component (480-580 nm). In comparison, D65 lighting has stronger blue and green wavelength components with a relatively weaker red wavelength component. Imaging under these conditions requires careful exposure settings to establish linear region of operation of the digital camera within the photospace in all color channels. Part of the challenge is in obtaining color balance and accurate reproduction of color and tone under these different illuminating conditions. If the type of illuminant is known, then the correct exposure controls and algorithms can be applied to obtain accurate colors in images. 
   Currently, many imaging systems rely on pre-metering information and a set of histograms to identify the relative strengths in the red, green and blue channels to identify the nature of the illuminant. This process consumes time and sets a lower bound on the pre-metering time. 
   SUMMARY 
   What is disclosed is an apparatus for determining a type of an illuminant including a spectrum diffraction unit and a sensor coupled to the spectrum diffraction unit. An illuminant discrimination unit is coupled to the sensor and configured to receive a set of signals representative of a set of spectral components in the illuminant and to determine the type. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is a spectral irradiance diagram of different light sources. 
       FIG. 2  is a block diagram of an imaging system configured in accordance with one embodiment of the present invention. 
       FIG. 3  is a block diagram of an illumination detection system configured in accordance with one embodiment of the present invention. 
       FIG. 4  is a block diagram of a set of pixel sensors used to sample a spectrum. 
       FIG. 5  is a block diagram of an illuminant detection circuit configured in accordance with one embodiment of the present invention. 
       FIG. 6  is a current-mode selection/scaling/negating unit in the illuminant detection circuit of  FIG. 5  configured in accordance with one embodiment of the present invention. 
       FIG. 7  is a voltage-mode selection/scaling/negating unit in the illuminant detection circuit of  FIG. 5  configured in accordance with one embodiment of the present invention. 
       FIG. 8  is an illuminant condition log in the illuminant detection circuit of  FIG. 5  configured in accordance with one embodiment of the present invention. 
       FIG. 9  is a winner-takes-all illuminant condition log in the illuminant detection circuit of  FIG. 5  configured in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention provides a system for discriminating among different illuminants. The system provides an approach to identifying the nature of the illuminant. Previous approaches use a histogram computation during pre-metering to identify the illuminant, which adds to the time required to set the appropriate exposure controls and also requires that there is sufficient signal level to make the estimation. Also, a scene based illumination method introduces some level of ambiguity because the light source spectral content will be convolved with the spectral reflectance from the scene and it is very difficult to deconvolve the two for a variety of imaging situations. 
   The present invention is independent of the information on the active imaging area and the illuminant is identified before the pre-metering is done, providing some time savings in the shutter to shutter time. Another advantage is that the illuminant identification can be enabled all the time and values can be stored in a register. Any change in illuminant condition will automatically flag the change and new register values will be provided to the exposure table, which chooses the appropriate look up tables for image capture. 
   In one embodiment, the sensing elements and circuits needed to implement the discrimination circuit are located on a sensor in the digital camera. In another embodiment, the sensing discrimination circuit is implemented as a separate element in the digital camera. 
   An embodiment of the invention as an imaging system  100  is shown as a logical block diagram in FIG.  2 . Imaging system  100  includes a number of conventional elements, such as an optical system having a lens  104  and aperture  108  that is exposed to the incident light reflected from a scene or object  102 . The optical system properly channels the incident light towards a sensor array  114  that generates sensor signals in response to an image of object  102  being formed on sensor array  114 . The various control signals used in the operation of sensor array  114 , such as a clock signal, a RESET/SAMPLE signal and an ADDRESS signal, are generated by a system controller or sequencer  160 . System controller  160  also receives an ILLUMINANT ID signal that identifies the illuminant from sensor  114 . System controller  160  may include a microcontroller or a processor with input/output (I/O) interfaces that generates the control signals in response to instructions stored in a non-volatile programmable memory. Alternatively, a logic circuit that is tailored to generate the control signals with proper timing can be used. System controller  160  also acts in response to user input via a local user interface  158  (as when a user pushes a button or turns a knob of system  100 ) or a host/PC interface  154  to manage the operation of imaging system  100 . 
   To obtain images, a signal and image processing block  110  is provided in which hardware and software operate according to image processing methodologies to generate captured image data in response to receiving the sensor signals. Optional storage devices (not shown) can be used aboard system  100  for storing the captured image data. Such local storage devices may include a removable memory card. Host/PC interface  154  is included for transferring the captured image data to an image processing and/or viewing system such as a computer separate from imaging system  100 . Imaging system  100  can optionally contain a display means (not shown) for displaying the captured image data. For instance, imaging system  100  may be a portable digital camera having a liquid crystal display or other suitable low power display for showing the captured image data. 
     FIG. 3  is a block diagram of an illuminant detection sensor array  300  and an illuminant discrimination circuit  302  contained in sensor  114  and used to identify illuminants. The system uses an angular spread of the spectrum of an illuminant within a diffraction order to identify the illuminant. The diffracted light with the wavelength spread is incident on illuminant detection sensor array  300 , which in one embodiment is a one dimensional row of pixels. Further, each pixel collects all the light within a narrow wavelength band of 2Δλ. In one embodiment, Δλ has the value of 20 nm. A diffraction unit  304 , which contains a plastic lens, binary or multi-level phase grating and an aperture or baffle to block higher diffraction orders, is used to provide the diffracted light source used in manipulating the light source provided to illuminant detection sensor array  300 . 
   Each pixel would receive different amounts of light depending on the relative strength of the wavelength components. Depending on the resolution or sensitivity needed for the detection circuit, the number of pixels needed and the spacing between pixels can be chosen accordingly. Thus, in other embodiments, additional pixels may be used for illuminant detection sensor array  300  to make the sensor a multi-directional sensor array and/or to add more sensors for the array. The total of captured light energy can also be adjusted by adding/removing pixels and/or changing Δλ to obtain a desired resolution of the spectrum. 
   Illuminant discrimination circuit  302  allows the detection of any illuminants that are present in the field of view of imaging system  100 . Illuminant discrimination circuit  302  receives signals from illuminant detection sensor array  300 . These signals represent the amount of light within an established spectrum of electromagnetic radiation, broken down into discrete bands. 
   Sampling of the spectrum can be done in several ways.  FIG. 4  illustrates one implementation with 8 discrete pixels that are spaced uniformly apart. The center wavelengths sampled within each window are shown sampling the 420 nm, 460 nm, 500 nm, 540 nm, 580 nm, 620 nm, 660 nm, and 700 nm wavelengths. In this implementation, Δλ is equal to 20 nm. Thus, the total spectrum that is covered is 400 nm to 720 nm. 
   Diffracted light falling within each window produces a current or a voltage response from each pixel in illuminant detection sensor array  300 . Assigning the value of the detected response to each of the pixels in illuminant detection sensor array  300  from left to right as A 1  through A 8 , Table 1 contains the parameters used in one embodiment of the invention for identifying each illuminant and the corresponding binary output. The parameters are obtained from Color Science: Concepts and Methods, Ouantitative Data and Formulae, Gunter Wyszecki &amp; W. S. Stiles, pages 18-28, 2 nd  Edition, Wiley-Interscience Publication, 1982. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Illuminant Identification Parameters/Output 
             
           
        
         
             
               Type of 
                 
                 
             
             
               Light Source 
               Spectrum Evaluation 
               Output Code 
             
             
                 
             
             
               D-65 Day 
               A1 &lt; A2 AND 
               10000000x2 
             
             
               Light 
               A2 &gt; A3 &gt; A4 &gt; A5 &gt; A6 &gt; A7 &gt; A8 AND 
             
             
                 
               A1/A8 &gt; 1 
             
             
               Tungsten 
               A1 &lt; A2 &lt; A3 &lt; A4 &lt; A5 &lt; A6 &lt; A7 &lt; A8 
               01000000x2 
             
             
               Carbon Arc 
               A1 &gt; A2 &gt; A3 &gt; A4 &gt; A5 &gt; A6 AND 
               00100000x2 
             
             
                 
               A6/A7 &gt; 0.9 AND A7/A8 &gt; 0.9 
             
             
               Mercury 
               A1 &gt; A2 &gt; A3 AND A4 &gt; A3 AND 
               00010000x2 
             
             
               Vapor 
               A5 &gt; A3 AND A4 &gt; A8 
             
             
               Fluorescent 
               A1 &gt; A3 AND A4 &gt; A3 AND A4/A5 &lt; 
               00001000x2 
             
             
                 
               A4/A3 
             
             
               Xenon filled 
               A1/A2 &gt; 0.8 AND A3/A4 &gt; 0.8 AND 
               00000100x2 
             
             
               Flashtube 
               A1 &gt; A2 &gt; A3 &gt; A4 &gt; A5 &gt; A6 &gt; A7 &gt; A8 
             
             
                 
             
           
        
       
     
   
     FIG. 5  illustrates one embodiment of illuminant discrimination circuit  302  coupled to illuminant detection sensor array  300 . Illuminant discrimination circuit  302  contains a set of signal duplication/scaling/negating units  304  with output coupled to a comparison unit  306  over a comparison bus  308 . The output from comparison unit  306  is fed to a set of illuminant condition logs  310 , which holds the intermediate values based on comparisons and then computes the final output code and transfers the detected illuminant code to system controller  160 . 
   The decision circuit implements a series of comparisons using comparison unit  306  among the discrete wavelength values captured by illuminant detection sensor array  300 . The results of these comparisons are stored within a set of illuminant condition logs and logic for detecting when appropriate conditions are met, which in one embodiment is an array of shift registers. As described below, there is one shift register for each potential illuminant. The shift register for each illuminant is the same length (e.g., contains enough bits) as the number of specific conditions required to discriminate that particular illuminant. 
   As described below, comparison unit  306  may be implemented to work with continuous-time current-mode processing or sampled voltage-mode processing implementations. Each pixel (e.g., wavelength detector) in illuminant detection sensor array  300  creates a signal, which is selectively passed as a positive or negative value depending on the comparison, and may be scaled to perform some of the conditional tests. The output of the comparison for each selected inputs represents the existence of a condition, which is passed into the appropriate set of shift registers in the set of illuminant condition logs  310 . Once all conditions have been tested and the results loaded into the set of illuminant condition logs  310 , the number of detected conditions for each illuminant is analyzed to discriminate among the possible sensed illuminants. 
     FIG. 6  contains a current-mode selection/scaling/negating unit  600  with a photodiode  602  and a transistor  608  connected to an operational amplifier  606 . A transistor  612  and a transistor  610  are used for signal reproduction with optional signal amplification. Current-mode selection/scaling/negating unit  600  also contains a comparison pre-processor unit  604  with a first mirroring transistor  614  coupled to a transistor  624  and a second mirroring transistor  616 . A positive selection transistor  618  receives a Sel +  signal, while a negative selection transistor  620  receives a Sel −  signal. An amplified negative selection transistor  622  is used to provide a scaled negative input from the Sel −a  signal. Negative selection transistor  620  and amplified negative selection transistor  622  are coupled to a transistor  626  and a transistor  628 , respectively. This combination of transistors allows the flexibility of selecting how this value is being compared to other values in the array. 
   The selection transistors are used to receive an input from the pixel and provide it as a positive or a negative value. The scaling transistors are used to scale the positive or negative value if needed for comparison purposes. For example, in the detection of Carbon Arc light sources, there is a need to compare A 6  and A 7  to determine if A 6 /A 7 &gt;0.9. Thus, the system needs to determine whether A 6 &gt;(0.9)A 7 . In this case, either the signal for A 6  may be amplified or the signal for A 7  may be attenuated by 10% such that the comparison may be made. 
     FIG. 7  contains a voltage-mode selection/scaling/negating unit  700  with a photodiode  702  with a reset transistor  704 . A sample switch  706  is used to capture the signal received from photodiode  702  in a capacitor  708 . The captured signal is fed to a comparison pre-processor unit  718  to amplify, scale, and negate the input signals for comparison purposes. 
   Comparison pre-processor unit  718  includes a positive selection transistor  710  and a negative selection transistor  712  connected to a positive selection current bias source  714  and a negative selection current bias source  716 , respectively. The comparison may then be performed at comparison unit  720 , which is a part of comparison unit  306 . 
   A control unit  312 , which may be a state machine, is used to control the sequencing of selected inputs from set of signal duplication/scaling/negating units  304  for the comparison operation over comparison bus  308  and the shifting of the comparison outputs into the appropriate shift registers in set of illuminant condition logs  310 . Once all conditions have been tested through the comparison of the detected values at each wavelength, the stored condition logs in set of illuminant condition logs  310  is used to determine which illuminant conditions have been met, thus creating an output code that discriminates among the different illuminant possibilities. 
   In one embodiment, the output code can represent an all-or-none decision for each illuminant, whereby the output is asserted if ALL conditions exist, and is not asserted when any ONE of the conditions fails.  FIG. 8  illustrates an illuminant condition log  800 , which includes a set of registers  802  and an associated set of output transistors  804 , which create a NAND function. Illuminant condition log  800  also contains a precharge or weak pull-down transistor  806 . Illuminant condition log  800  also contains a logical NOT gate  808  to invert the output of set of output transistors  804 . 
   Another implementation of the system is to determine which illuminant has the highest number of its respective conditions asserted, and determine the most probable illuminant in a discriminant computation.  FIG. 9  illustrates a winner-takes-all (WTA) type of illuminant condition log unit  900 . The unit contains a set of registers  902  and an associated set of output transistors  904 , sending output to a gate  906 . These transistors are not used as part of the NAND gate, but instead supply currents which are summed and are input to the WTA computation unit. An additional bias value could also be used to set the current level for each transistor. A logical NOT gate  914  is coupled to the input of set of registers  902 . Other illuminant condition logs are connected to illuminant condition log  900  through a transistor  910  and a transistor  912 . These transistors make up one element of the WTA computation. There are as many elements as there are condition logs and thus as many as the number of possible illuminants. In addition, a WTA bias circuit  908  is used to bias the system as needed. 
   In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.