Abstract:
A system and method for generating controlled illumination having a color balance that corresponds to prevailing ambient color balance. A multi-spectral light source having a given spectral characteristic is filtered through an active color filter to produce multi-spectral light conforming to the prevailing ambient color balance. Embodiments of the present invention advantageously enable photographic capture of images without introducing color balance inconsistencies seen in prior art illumination solutions.

Description:
BACKGROUND OF THE INVENVTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate generally to photographic lighting systems, and more specifically to systems and methods for controlling color balance for a photographic illuminator. 
         [0003]    2. Description of the Related Art 
         [0004]    Photographic recording conventionally involves projecting a scene image through a lens assembly onto a sampling surface. The scene image represents a section of a scene, as projected and focused by the lens assembly. The sampling surface may be a frame of photographic film or an electronic image sensor configured to sample the scene image for electronic storage. The scene image may be stored as a two-dimensional field of chemical state in the frame of photographic film or as electronic state in a digital memory subsystem. The process of sampling ultimately produces a photographic image representing the scene image. Sampling period (shutter speed), lens aperture, and sampling sensitivity (conventionally referred in terms of an “ISO” index of sensitivity) determine overall image exposure. Proper exposure for the photographic image is based on attempting to emulate natural human visual perception, which is highly adaptive over a large dynamic range. Human visual perception is highly efficient at maximizing perceived tonal balance, and therefore a properly exposed photographic image exhibits good tonal balance. Modern digital cameras can generally achieve good tonal balance and proper exposure in recording photographic images. 
         [0005]    In certain scenarios, ambient lighting within a scene is inadequate to produce a properly exposed photographic image of the scene or certain subject matter within the scene. In certain other scenarios, an additional light source from one or more directions may aesthetically improve or highlight certain aspects of a subject being photographed within the scene. In one example scenario, a photographer may wish to photograph a person (subject) at night in a setting that is inadequately illuminated by incandescent or fluorescent lamps. A photographic strobe may be used to beneficially provide additional light on the subject to achieve a desired exposure, however the color balance (ratios of red, green, and blue light) of the strobe will not match that of the ambient incandescent or fluorescent lighting. 
         [0006]    Human visual perception also dynamically adapts to ambient illumination color to enable proper perception of color despite off-white ambient illumination. For example, a white sheet of paper is commonly perceived as being white regardless of whether the paper is illuminated by inherently white sunlight or inherently orange candlelight. A modern digital camera is typically configured to be able to compensate for ambient illumination color in order to reproduce overall correct colors for the scene. In this way, the camera attempts to emulate human visual perception with respect to white balance. Alternatively, white balance of a digital photograph may be achieved via post processing. Persons skilled in the art will recognize that color balance for a photographic image is conventionally accomplished by modifying channel gain for each one of red, green, and blue color channels over the entire photographic image to compensate for overall scene color. 
         [0007]    One challenge of color photography is that a given scene may have multiple different light sources, each characterized by different color balances. In such a scene, achieving white balance that appears correct can be quite difficult. For example, an incandescent lamp is characterized as emitting significantly more red light than blue light, while a conventional Xenon photographic strobe emits a relatively even mix of red, green, and blue light. If the digital camera uses a color balance based on ambient incandescent or sunset lighting, then objects predominantly illuminated by the ambient lighting will be properly color balanced, while objects that are predominantly illuminated by the strobe will appear overly blue. Alternatively, if the camera assumes a color balance corresponding to the strobe color balance, then objects that are predominantly illuminated by ambient lighting will appear overly red. Because the Xenon photographic strobe produces an inherently different balance of light compared to the ambient light, achieving a realistic white balance is oftentimes impractical while photographing such scenes. 
         [0008]    As the foregoing illustrates, what is needed in the art is a technique for properly illuminating a scene according to existing ambient color balance. 
       SUMMARY OF THE INVENTION 
       [0009]    One embodiment of the present invention sets forth a method for generating color illumination having a target color balance, the method comprising the steps of determining the target color balance, generating a transmission factor for each color channel of a plurality of color channels within an active color filter based on the target color balance and color characteristics of an illumination source, and activating each color channel of a plurality of color channels based on the corresponding transmission factor to transmit illumination having the target color balance. 
         [0010]    Another embodiment of the present invention sets forth a system for generating color illumination having a target color balance, the system comprising an active color filter, configured to selectively transmit different color components of source illumination based on corresponding transmission factors, and a controller. The controller is configured to determine the target color balance, generate a transmission factor for each color channel of a plurality of color channels within the active color filter based on the target color balance and color characteristics of the source illumination, and to activate the active color filter using a control signal representing the transmission factors. 
         [0011]    A further embodiment of the present invention sets forth a portable photographic system, the system comprising a digital camera subsystem configured to sample and store photographic images, a multi-spectral light source, configured to provide the source illumination when triggered by the digital camera subsystem, an active color filter, configured to selectively transmit different color components of the multi-spectral light source based on corresponding transmission factors, and a controller. The controller is configured to determine the target color balance, generate a transmission factor for each color channel of a plurality of color channels within the active color filter based on the target color balance and color characteristics of the multi-spectral light source, and to activate the active color filter using a control signal representing the transmission factors. 
         [0012]    The present invention enables photographers to advantageously illuminate photographic scenes with appropriately color balanced light, resulting in higher quality, more natural looking photographs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0014]      FIG. 1A  illustrates a color compensated flash unit, according to one or more aspects of the present invention; 
           [0015]      FIG. 1B  illustrates a functional diagram of the color compensated flash unit of  FIG. 1A , according to one embodiment of the present invention; 
           [0016]      FIG. 2A  illustrates a color compensation unit configured to attach to a separate flash unit, according to one embodiment of the present invention; 
           [0017]      FIG. 2B  illustrates the color compensation unit attached to the separate flash unit, according to one embodiment of the present invention; 
           [0018]      FIG. 2C  illustrates a filter unit and control unit coupled to the separate flash unit, according to one embodiment of the present invention; 
           [0019]      FIG. 2D  illustrates a functional diagram of the color compensation unit, according to one embodiment of the present invention; 
           [0020]      FIG. 3A  illustrates a digital camera configured to implement one or more aspects of the present invention; 
           [0021]      FIG. 3B  illustrates a side detail of the digital camera, according to one embodiment of the present invention; 
           [0022]      FIG. 3C  illustrates a functional diagram of a color compensated flash module within the digital camera, according to one embodiment of the present invention; 
           [0023]      FIG. 3D  illustrates a front view of a mobile wireless device configured to implement one or more aspects of the present invention; 
           [0024]      FIG. 3E  illustrates a functional diagram of the mobile wireless device, according to one embodiment of the present invention; 
           [0025]      FIG. 4A  illustrates a detailed view of an active color filter, according to one embodiment of the present invention; 
           [0026]      FIG. 4B  depicts a side view of a pixel array, according to one embodiment of the present invention; 
           [0027]      FIG. 4C  depicts a response curve of light transmission as a function of applied voltage for a cell within the pixel array, according to one embodiment of the present invention; 
           [0028]      FIG. 5A  illustrates the pixel array configured to include color filters for red, green, and blue, according to one embodiment of the present invention; 
           [0029]      FIG. 5B  illustrates the pixel array configured to include color filters for red, green, blue, cyan, magenta, and yellow according to one embodiment of the present invention; 
           [0030]      FIG. 5C  illustrates the pixel array configured to include color filters for red, green, blue, cyan, magenta, and yellow according to an alternative embodiment of the present invention; 
           [0031]      FIG. 5D  illustrates the pixel array configured to include color filters for cyan, magenta, yellow, and white according to one embodiment of the present invention 
           [0032]      FIG. 5E  depicts an ideal band pass color filter as a function of wavelength (λ), and centered at λ 0 ; 
           [0033]      FIG. 5F  depicts a typical physical realization of a band pass color filter as a function of wavelength (λ), and centered at λ 0 ; 
           [0034]      FIG. 6  illustrates a technique for controlling multiple levels of transmission within the active color filter, according to one embodiment of the present invention; 
           [0035]      FIG. 7  is a conceptual diagram of a color compensated flash unit comprising functional blocks for measuring ambient color balance and filtering a multi-spectral light signal to generate a controlled illumination signal based on ambient color balance, according to one embodiment of the present invention; 
           [0036]      FIG. 8A  is a flow diagram of method steps for generating controlled illumination based on measured ambient color, according to one embodiment of the present invention; 
           [0037]      FIG. 8B  is a flow diagram of method steps for generating controlled illumination based on a specified color balance, according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1A  illustrates a color compensated flash unit  100 , according to one embodiment of the present invention. The color compensated flash unit  100  comprises an active color filter  120 , an emitter lens  122 , a light source  166 , an ambient sampling lens  138 , and an attachment module  170 . 
         [0039]    The active color filter  120  is configured to selectively pass different wavelengths of visible light, based on a set of one or more electronic control signals. The active color filter  120  is disposed between the light source  166  and the emitter lens  122 . Light emitted by the light source  166  passes through the active color filter  120  and then through the emitter lens  122  to be emitted as controlled illumination  123 . A reflector  168  may be configured to direct light emitted from the light source  166  towards the active color filter  120 . 
         [0040]    The ambient sampling lens  138  is configured to receive and diffuse ambient light to yield an optical signal that is representative of ambient lighting for a particular setting. Ambient light may be represented as ratios among red, green, and blue ambient light intensity. Ambient light may also be represented as having a color temperature, according to the industry standard color temperature Kelvin scale. The optical signal is sampled by a color measurement apparatus, described in greater detail below. 
         [0041]    The attachment module  170  is configured to mechanically couple the color compensated flash unit  100  to a host device, such as a camera, a stand apparatus such as a tripod, or any other device or base. The attachment module  170  is also configured to transmit signals between the color compensated flash unit  100  and the host device. In one embodiment, the transmitted signals comprise electrical signals. In an alternative embodiment, the transmitted signals comprise electromagnetic signals. In another alternative embodiment, the transmitted signals comprise magnetic signals. In yet another alternative embodiment, the transmitted signals comprise mechanical signals. Certain of the signals may be configured to transmit commands, such as a strobe trigger command, a target strobe intensity, or a target strobe color. Persons skilled in the art will recognize that certain commands, such as a strobe trigger, are transmitted from a prior art camera to a prior art flash unit. However, prior art flash systems are not configured to receive target strobe color information. In one embodiment of the present invention, a target strobe color is transmitted to the color compensated flash unit  100 . In an alternative embodiment, a measured ambient color is sampled by the color compensated flash unit  100  and transmitted to the host device. 
         [0042]    In one embodiment, the attachment module  170  comprises a strobe “hot shoe,” the light source  166  comprises a Xenon flash tube, and the active color filter  120  comprises a liquid crystal array manufactured to include a plurality of color cells with individual light transmission characteristics controlled by the one or more electronic control signals. The emitter lens  122  comprises a Fresnel lens configured to control dispersal of the controlled illumination  123 . 
         [0043]      FIG. 1B  is a functional diagram of the color compensated flash unit  100  of  FIG. 1A , according to one embodiment of the present invention. Functional elements of the color compensated flash unit  100  comprise the emitter lens  122  of  FIG. 1A , the active color filter  120 , the light source  166 , a light source driver  164 , a flash controller  160 , a filter driver  114 , and a color controller  110 . In certain embodiments, the color compensated flash unit  100  further comprises the ambient sampling lens  138 , an ambient color sensor  136 , a color receiver  130 , user input/output (I/O) circuitry  140 , a battery  152 , and a power controller  150 . 
         [0044]    The light source  166  is configured to generate multi-spectral visible light, including component wavelengths of red, green and blue light. Persons skilled in the art will understand that visible light is descriptive of a range of light wavelengths spanning approximately 700 nm to 380 nm, with each color of light approximately correlated to human perception of color. Humans perceive light color according to a perception of red, green, and blue components, with a perceptive peak of red at approximately 580-620 nm, a perceptive peak of green at 535-565 nm, and a perceptive peak of blue at approximately 440-460 nm. Some individuals may perceive color slightly differently than others, however there are standard color models in the art and the meaning of red, green, and blue components is commonly accepted. 
         [0045]    The active color filter  120  is configured to selectively pass different component colors of light generated by the light source  166 , based on transmission factors transmitted in a filter control signal  116 . Each transmission factor characterizes a ratio of light passed through an element of the active color filter  120  versus an amount of light made available to the element. Each color component is optically transmitted through the active color filter  120 , according to a corresponding transmission factor. The active color filter  120  includes a plurality of color filter elements, each configured to respond to an associated transmission factor. The color filter elements are described in greater detail below in  FIGS. 4-6 . Each color filter element is configured to pass one or more color components of light. For example, one color filter element may be configured to primarily pass red light, with significant attenuation of green and blue light. A second color filter element may be configured to primarily pass green light, with significant attenuation of red and green light. A third color filter element may be configured to primarily pass yellow light, with significant attenuation of blue light, and so forth. 
         [0046]    The active color filter  120  may employ any technically feasible technique for implementing a transmission factor for associated color components. In one embodiment, a three color (red, green, blue), multi-level liquid crystal array of pixels implements the active color filter  120 . Each red, green, and blue pixel is driven to an appropriate value for a transmission factor to produce an overall light color that is suitable for a particular setting. The overall light color may be selected based on an ambient light color measurement for the setting. 
         [0047]    The emitter lens  122  may be configured to produce a specific dispersal pattern for the controlled illumination  123 . In one embodiment, the active color filter  120  is configured to be an integral component of the color compensated flash unit  100 . In this embodiment, the active color filter  120  is fixed in position relative to the light source  166 . In another embodiment, the active color filter  120  is configured to be movably disposed relative to the light source  166 . In this embodiment, the active color filter  120  may be removed from or inserted into an optical path from the light source  166  to the controlled illumination  123 . For example, the active color filter  120  may be mounted on a movable slide bearing, allowing the active color filter  120  to be positioned either in the optical path or not in the optical path. In yet another embodiment, the active color filter  120  may be detached from and reattached to the body of the color compensated flash unit  100 ; the emitter lens  122  may also be detached and reattached as part of a module comprising the active color filter  120  and the emitter lens  122 . 
         [0048]    The light source driver  164  generates electrical signals used to activate the light source  166 . In one embodiment, the light source  166  is a Xenon flash tube, and the light source driver  164  is configured to generate a drive voltage and a trigger voltage for the Xenon flash tube. The drive voltage is typically over two hundred volts and the trigger voltage is typically in a range of several thousand volts. The drive voltage is typically supplied by a capacitor and is applied at each end of the Xenon flash tube prior to activation. A trigger voltage is applied at an offset from one end, causing the tube to be activated and to generate light. The Xenon flash tube may be extinguished by turning off the drive voltage. Activating and extinguishing the Xenon flash tube each typically take less than a millisecond. In an alternative embodiment, the light source  166  comprises at least one multi-spectral light emitting diode (LED), such as a phosphor-based white LED, and the light source driver  164  is configured to generate a driver current to activate the at least one LED. Removing the drive current extinguishes the at least one LED. 
         [0049]    The flash controller  160  is configured to receive a host flash control signal  173  and to generate a light source control signal  162 . The host flash control signal  173  may include any number of individual electrical or optical signals and may carry any technically feasible flash control protocol without departing the scope and spirit of the present invention. A simple, exemplary hot-shoe flash control protocol includes three electrical wires corresponding to a neutral (ground), a flash trigger, and a flash extinguish signal. When the flash trigger signal is driven by a host camera, the flash controller  160  causes the light source driver  164  to activate the light source  166 . When the flash extinguish signal is driven by the host camera, the flash controller  160  causes the light source driver  164  to extinguish the light source  166 . The flash trigger signal is driven in response to a shutter release event within the camera, and the extinguish signal is driven separately upon accumulation of sufficient light exposure. Conventional flash control protocols presently implement bidirectional communication between a flash unit and a camera, and enable sophisticated flash features beyond simply triggering and extinguishing the flash. In one embodiment, the host flash control signal  173  implements a conventional hot-shoe flash control protocol and the flash controller  160  is configured to communicate with the host device via the flash control protocol. The flash controller  160  may also transmit flash status information via internal flash control signal  113  and receive flash commands from the color controller  110  via internal flash control signal  113 . The status information may include flash readiness. The internal flash control signal  113  may transmit flash commands including flash trigger and flash extinguish commands. 
         [0050]    Ambient light  139  enters the ambient sampling lens  138  and is therein filtered to produce a representative color for the ambient light  139 . The representative color is optically transmitted to the ambient color sensor  136 . The ambient color sensor  136  is configured to generate an electrical ambient color signal  132  corresponding to the representative color. The electrical ambient color signal  132  includes a color component value for each color component sensed by the ambient color sensor  136 . Any technically feasible technique may be used to represent each color component value without departing the scope and spirit of the present invention. For example, an analog voltage or current value may be used to represent a color component value. Furthermore, the analog voltage or current may comprise a linear representation, a logarithmic representation, or any other technically feasible representation. Alternatively, each color component value may be represented by a corresponding digital signal. In one embodiment, the ambient color sensor  136  is configured to sense red, green and blue color components and to generate the electrical ambient color signal  132  comprising independent color component values for red, green, and blue. 
         [0051]    The color receiver  130  is configured to receive the electrical ambient color signal  132  and to generate a corresponding digital ambient color signal  134 . For embodiments implementing an analog representation of the electrical ambient color signal  132 , the color receiver  130  may provide amplification, current to voltage conversion, voltage to current conversion, linear to logarithmic conversion, logarithmic to linear conversion, analog-to-digital (AD) conversion, or any technically feasible combinMion thereof to generate the digital ambient color signal  134 . Furthermore, the color receiver  130  may be configured to implement any non-linearity or mapping function. Any technically feasible technique may be used to implement the digital ambient color signal  134 . 
         [0052]    The color controller  110  is configured to receive the digital ambient color signal  134 , a host control signal  175 , or any combination thereof, and to generate a color control signal  112 . The filter driver  114  receives the color control signal  112  and generates the filter control signal  116 . In one embodiment, the filter driver  114  comprises a voltage translation amplifier for driving liquid crystal cells, and the active color filter  120  comprises a liquid crystal array with a plurality of independently controlled color cells. The color controller  110  computes the color control signal  112  based on the digital ambient color signal  134 . In certain embodiments, the color controller  110  accounts for spectral emission characteristics of the light source  166  and transmission characteristics of the active color filter  120 . For example, the light source  166  may generate a certain magnitude of light for red, green, and blue light, and the active color filter  120  is characterized as having an independent transmission factor for each of red, green, and blue light based on corresponding color control signals  112 . In this example, the color controller  110  can generate color control signals  112  to produce a ratio of red, green, and blue light within the controlled illumination  123  that preserves the ratio of red, green, and blue ambient light sensed by the ambient color sensor  136 . 
         [0053]    In one embodiment, an N-bit binary integer is used to represent each transmission factor, with integer value 0 indicating minimum transmission, and an integer value of 2̂N−1 indicating maximum transmission. For an 8-bit value, N is equal to 8 and 2̂N−1 is equal to 255. Persons skilled in the art will understand that any technique for encoding and transmitting a transmission factor may be implemented without departing the scope and spirit of the present invention. 
         [0054]    In one embodiment, the color controller  110  implements an index table for each of red, green, and blue to map an ambient light level of each color component to a corresponding component of the color control signal  112 . For example, if the digital ambient color signal  134  comprises eight bits per color component (red, green, blue), and the color control signal  112  comprises eight bits per color component, then the color controller  110  would implement three index tables that each map eight bits to eight bits. The three index tables can be configured to account for arbitrary nonlinearities within the ambient sampling lens  138 , ambient color sensor  136 , color receiver  130 , filter driver  114 , active color filter  120 , as well as color balance variations in the light source  166 . In this way, the entire system may be calibrated based on three tables. The tables may additionally account for temperature effects, which may impact system components in varying amounts. Furthermore, the tables may additionally account for component colors being transmitted through other color filters. For example, a table configured to determine a red transmission factor for the active color filter  120  may be primarily based on relative red ambient intensity, but the table may also account for red transmission through green and blue filters by including at least a portion of the ambient components for green and blue as index values for the table. 
         [0055]    In certain embodiments, the color receiver  130  implements sensor calibration tables or calibration parameters to map the analog representation of the electrical ambient color signal  132  to a standard set of intensity values represented in the digital ambient color signal  134 . The sensor calibration tables (or parameters) are configured to map the analog representation of the electrical ambient color signal  132  to an appropriately calibrated value for the digital ambient color signal  134 . The sensor calibration tables (or parameters) account for nonlinearities within the ambient sampling lens  138 , ambient color sensor  136 , and analog portions of the color receiver  130 . The color controller  110  implements a separate set of emission calibration tables for generating the color control signal  112 , comprising a color channel for each color component. In such embodiments, each color channel is processed via two tables, and the digital ambient color signal  134  represents a standard color representation that relates sensed ambient color balance to target color balance for the controlled illumination  123 . Any self-consistent standard color representation may be used without departing the scope and spirit of the present invention. In certain embodiments or modes of operation, the host control signal  175  transmits a target color balance for the controlled illumination  123 , in accordance with the standard color representation associated with the digital ambient color signal  134 . In such embodiments, a host device, such as a digital camera, performs an ambient color balance measurement and transmits results of the ambient color balance measurement to the color controller  110  via the host control signal  175 . Alternatively, the host control signal  175  is configured to transmit an ambient color balance measurement performed by the color compensated flash unit  100  to the host device. The host device may then use or record the ambient color balance measurement. 
         [0056]    In one embodiment, user I/O circuitry  140  provides user input devices such as buttons and user output devices such as light-emitting diode (LED) indicators, an LCD display, and the like. In particular, the user I/O circuitry  140  may provide an on/off control, means for setting color temperature on the standard Kelvin scale, means for setting ratios of red to green to blue component intensities, or any combination thereof to produce a corresponding controlled illumination  123 . Furthermore, the user I/O circuitry  140  may be configured to display measured ambient color temperature, measured ambient intensities or ratios of red, green, and blue color components, or any combination thereof. 
         [0057]    The battery  152  may comprise a replaceable rechargeable battery, a fixed rechargeable battery, or replaceable primary battery. Any battery chemistry may be implemented without departing the scope of the present invention. Alternatively, a super capacitor may implemented in place of or in combination with the battery  152 . Power controller  150  provides charging circuitry, as well as power management and voltage conversion and regulation functions, according to specific implementation requirements. Persons skilled in the art will understand that the power controller  150  may implement various power management techniques, according to requirements for a specific embodiment. Power signals  171  may be used to transmit power from the power controller  150  to an external device, or receive power from an external device. Power signals  171  may also convey power status between devices. 
         [0058]      FIG. 2A  illustrates a color compensation unit  200  configured to attach to a separate flash unit, according to one embodiment of the present invention. The color compensation unit  200  includes active color filter  120  of  FIG. 1B , a mechanical opening  210 , and a coupling  212 . The color compensation unit  200  may also include emitter lens  122 . The mechanical opening  210  is configured to encompass an optical output port of the separate flash unit. The coupling  212  may comprise a mechanical latch, a friction fitting, a magnetic latch, a magnetic signal connector, an optical signal connector, an electrical signal connector, a mechanical signal connector, or any combination thereof. 
         [0059]    The separate flash unit (not shown) emits strobe illumination  211 , which passes through the mechanical opening  210 . The strobe illumination  211  is subsequently filtered by the active color filter  120  to yield controlled illumination  123 . In one embodiment, the controlled illumination  123  is passed through emitter lens  122 . 
         [0060]      FIG. 2B  illustrates the color compensation unit  200  of  FIG. 2A  attached to separate flash unit  202 , according to one embodiment of the present invention. The mechanical opening  210  is configured to receive strobe illumination  211  generated by the flash unit  202  and transmit the strobe illumination  211  to the active color filter  120 . The active color filter  120  filters the strobe illumination  211  to generate controlled illumination  123 . Coupling  212  (shown in  FIG. 2A ) is configured to attach the color compensation unit  200  to the flash unit  202 . The coupling  212  may attach using friction, magnetic attraction, a mechanical structure such as a latch, or any other technically feasible attachment means. The color compensation unit  200  may also include emitter lens  122 . 
         [0061]    The flash unit  202  may include an attachment module  170 , configured to mechanically couple the flash  202  unit to a host device, such as a camera, a stand apparatus such as a tripod, or any other appropriately configured device or base. The attachment module  170  may be configured to transmit signals to the flash unit  202 , for example, to trigger the flash unit  202  to generate the strobe illumination  211 . The attachment module  170  may also be configured to transmit signals to the color compensation unit  200  via the flash unit  202 . For example, a camera, coupled to the attachment module  170 , may transmit an activation signal to the color compensation unit  200  via the attachment module  170  and flash unit  202 . Any technically feasible signal may be used as an activation signal, including signals comprising electrical, electromagnetic, magnetic, or mechanical energy. 
         [0062]    In one embodiment, the color compensation unit  200  is configured to measure ambient color and transmit results of the ambient color measurement via the coupling  212  to the flash unit  202 , which further transmits the results via the attachment module  170 . An attached camera or related device may receive the ambient color measurement. The ambient color measurement includes red, green, and blue color components. The ambient color measurement may be taken through ambient sampling lens  138  (not shown), using ambient color circuitry described in  FIG. 1B  and comprising ambient color sensor  136  and color receiver  130 . 
         [0063]    In one embodiment, color compensation unit  200  includes an electrical power source, such as a battery. In an alternative embodiment, the color compensation unit  200  draws electrical power from the flash unit  202 , for example via an electrical connector within the coupling  212 . 
         [0064]      FIG. 2C  illustrates a filter unit  203  and control unit  204  coupled to the separate flash unit  202 , according to one embodiment of the present invention. The filter unit  203  and control unit  204  collectively implement color compensation unit  200  of  FIG. 2B . Specifically, the filter unit  203  includes active color filter  120  and mechanical opening  210 , and control unit  204  implements control and measurement functions of color compensation unit  200 . 
         [0065]    In one embodiment, control signals are transmitted from the control unit  204  to the filter unit  203  via control cable  205 . Control cable  205  may include at least one connector (not shown) used to couple the control cable  205  to the control unit  204 , the filter unit  203 , or both units. In one alternative embodiment, control signals are transmitted from the control unit  204  to the filter unit  203  via a signal path within the flash unit  202 . In another alternative embodiment, control signals are transmitted from the control unit  204  to the filter unit  203  via electromagnetic signaling, such as a radio-frequency signal or an optical signal path. Control unit  204  may include ambient sampling lens  138  and an ambient color measurement circuit comprising ambient color sensor  136  and color receiver  130 , as described previously in  FIG. 1B . 
         [0066]    In one embodiment, the ambient color measurement circuit transmits ambient color measurement results via attachment module  170  to an attached host device, such as a camera. In alternative embodiments, the ambient color measurement circuit transmits results to a host device via a radio signal, infra-red signal, or any other technically feasible data carrying signal. The ambient color measurement circuit may also transmit ambient color measurement results to control units  204  coupled to other flash units  202 . 
         [0067]      FIG. 2D  is a functional diagram of the color compensation unit  200 , according to one embodiment of the present invention. As shown, the color compensation unit  200  comprises a filter unit  203  and a control unit  204 . The filter unit  203  comprises active color filter  120  and mechanical opening  210 . The filter unit  203  may include filter driver  114 , input lens  124 , and emitter lens  122 . As shown, strobe illumination  211  enters the mechanical opening  210  and is filtered by the active color filter  120  to generate controlled illumination  123 . Strobe illumination  211  may also pass through input lens  124 . Controlled illumination  123  may also pass through emitter lens  122 . 
         [0068]    In one embodiment, color control signal  112  is transmitted via control cable  205  to the filter driver  114 . The filter driver  114  generates filter control signal  116 , which activates active color filter  120  to filter strobe illumination  211  into controlled illumination  123 . In an alternative embodiment, filter driver  114  is disposed within the control unit  204 . 
         [0069]    Control unit  204  includes color controller  110 , which is configured to generate color control signal  112  based on a target ambient color balance. Control unit  204  may also include ambient color sensor  136 , color receiver  130 , user I/O circuitry  140 , described previously. In one embodiment, control unit  204  also includes battery  152  and power controller  150 , configured to receive electrical energy from battery  152  and to generate one or more voltage supplies for the control unit  204 . The power controller  150  may also generate voltage supplies for the filter unit  203 . In one embodiment, power controller  150  is configured to report battery charge level associated with battery  152  to a host device via attachment module  170 . 
         [0070]    In one embodiment, color controller  110  transmits ambient color information, such as color balance measured for ambient light  139  via ambient sampling lens  138  and ambient color sensor  136 , to the host device via attachment module  170 . Battery charge level may be transmitted via a battery charge signal  271 , while ambient color information may be transmitted via a color signal  277 . Similarly, the host device may transmit color control information to the color controller  110  via color signal  277 . Persons skilled in the art will understand that different techniques may be used to transmit color information from the color controller  110  to the host device, and from the host device to the color controller  110 . Furthermore, the host device may be a digital camera, or any other device configured to be coupled to attachment module  170 . 
         [0071]      FIG. 3A  illustrates a digital camera  300  configured to implement one or more aspects of the present invention. The digital camera  300  includes an image lens  366 , a shutter release button  364 , an image sensor (not shown) disposed behind the image lens  366 , a light source (not shown), and image processing and storage circuitry (not shown). The digital camera  300  includes active color filter  120  of  FIG. 1B  disposed in front of the light source, as illustrated in greater detail below in  FIG. 3B . The digital camera  300  may also include emitter lens  122 . In one embodiment, the digital camera  300  includes an image lens cover  368  configured to cover and protect the image lens  366 , for example when the digital camera  300  is turned off. The image lens cover  368  is also configured to uncover the image lens  366  when a user wishes to take a photograph with the digital camera  300 . 
         [0072]      FIG. 3B  illustrates a side detail of the digital camera  300 , according to one embodiment of the present invention. Light source  166  of  FIG. 1B  generates strobe illumination, which is filtered by active color filter  120  to generate controlled illumination  123 . The controlled illumination  123  may be used to illuminate a photographic subject or enhance illumination for the photographic subject. Reflector  168  directs light from the light source  166  to the active color filter  120 . 
         [0073]    In one embodiment, the active color filter  120  is mounted in a fixed position between the light source  166  and a photographic subject (via emitter lens  122 ). In an alternative embodiment, the active color filter  120  is movably mounted so that it may be substantially removed from optical paths leading from the light source  166  to the photographic subject. For example, the active color filter  120  may be slid out of the optical paths, allowing unfiltered light from the light source  166  to be used to illuminate the photographic subject. The emitter lens  122  may remain in the optical paths between the light source  166  and the photographic subject. 
         [0074]    In one embodiment, the digital camera  300  is configured to generate a photograph and to display the photograph on a display module  350 , such as a liquid crystal display (LCD) screen. The camera may also present user interface objects on the display module  350 . Importantly, the digital camera  300  generates a strobe of controlled illumination  123  that is filtered via active color filter  120  to conform to a color balance of ambient scene illumination for the photograph. In this way, the photograph is sampled with consistently colored illumination for a realistic, consistent appearance. 
         [0075]      FIG. 3C  illustrates a functional diagram of a color compensated flash module  302  within the digital camera  300 , according to one embodiment of the present invention. The digital camera  300  comprises the color compensated flash module  302  and a digital image module  304 . 
         [0076]    The color compensated flash module  302  comprises system elements, including active color filter  120  of  FIG. 1B , light source  166 , light source driver  164 , flash controller  160 , color controller  110 , and filter driver  114 , each described previously in  FIG. 1B . The color compensated flash module  302  may further comprise reflector  168  and emitter lens  122 . As described previously in  FIG. 1B , the system elements are configured to generate controlled illumination  123 , characterized as having red, green, and blue color components having a color balance determined by a target color balance. The target color balance may be determined by measuring ambient color balance. 
         [0077]    In one embodiment, the target color (white) balance is determined by digital image module  304  using any technically feasible technique and transmitted to the color controller  110  via host control signal  175 . The color controller  110  generates color control signal  112  from the target color balance. In an alternative embodiment, the ambient sampling lens  138 , ambient color sensor  136 , and color receiver  130  of  FIG. 1B  are also included within the digital camera  300  and are configured to sample ambient color balance for a scene being photographed by the digital camera  300 . The target color balance for the color compensated flash module  302  is computed from the sampled ambient color balance by color controller  110 , which may also transmit the target color balance to the digital image module  304 . 
         [0078]    The digital image module  304  includes an electro-optical module  330 , a processing unit  320 , data storage unit  340 , a display module  350 , and input devices  344 . The electro-optical module  330  comprises image lens  366 , and image sensor  332 . The electro-optical module  330  may also comprise focusing apparatus for focusing the image lens  366  with respect to the image sensor  332 . The electro-optical module  330  may also include an iris mechanism for controlling an optical aperture for the image lens  366 . The image lens cover  368  is configured to cover and protect the image lens  366  in one mechanical position, and uncover and expose the image lens cover  368  in a second mechanical position. 
         [0079]    Optical scene information  331  is focused by the image lens  366  onto image sensor  332 , which converts focused optical scene information comprising a focused image into an electrical representation of the focused image. The image sensor  332  is configured via image sensor interconnect  334 . The focused image is sampled according to certain parameters, such as sensor gain and sample timing. The electrical representation of the focused image is transmitted via image sensor interconnect  334  to the processing unit  320 , which formats the electrical representation of the focused image into a digital photograph for storage within data storage unit  340 . Persons skilled in the art will recognize that different digital image storage formats may be used for storing the digital photograph. An electro-optical control interconnect  338  is configured to control mechanical focus and aperture actuators within the electro-optical module  330 . 
         [0080]    The processing unit  320  is configured to process and store image data from the image sensor  332 . The processing unit  320  is configured to transmit image data for a given digital photograph to the data storage unit  340  via storage interconnect  342 . The processing unit  320  is also configured to retrieve image data from the data storage unit  340  via the storage interconnect  342  for display on the display module  350 . The processing unit  320  is configured to transmit display data to the display module  350  via display interconnect  352 . The processing unit  320  is also configured to receive user commands from input devices  344  via input device signals  346 . The commands may include, for example, user interface inputs, shutter release button events, and the like. 
         [0081]    The digital camera  300  includes a power management unit  356  and a battery  354 . The battery may be a fixed battery or a replaceable battery. The replaceable battery may be a primary battery or a rechargeable battery. In one embodiment, the battery  354  comprises a set of replaceable industry standard “AA” primary or rechargeable cells. The power management unit  356  is configured to receive electrical energy from the battery  354  and to generate voltage supplies for use by the digital image module  304  and the color compensated flash module  302 . 
         [0082]    In one embodiment, the target color balance is computed from the electrical representation of the focused image. The optical scene information  331  is focused by the image lens  366  onto image sensor  332 , which generates the focused image having a particular red, green, and blue white balance. The focused image represents a specific region of a corresponding scene being photographed, and does not necessarily represent overall color balance for the scene. 
         [0083]    In another embodiment, the image lens cover  368  is manufactured to be optically neutral and translucent. The image lens cover  368  is configured to transmit at least one sixteenth of all incident light comprising the optical scene information  331 . The image lens cover  368  is configured to receive and diffuse ambient light to yield an optical signal that is representative of ambient color balance for a particular setting. The optical signal is sampled by the image sensor  332  as an unfocused substantially even two-dimensional signal that is representative of the overall color balance for the scene. In this embodiment, the image lens cover  368  and image lens  366  collectively function as ambient sampling lens  138  of  FIG. 1B . The image sensor  332  functions as ambient color sensor  136  and color receiver  130 . To sample color balance in a given scene, the digital camera  300  closes the image lens cover  368  so that only a diffuse, single color representation of the optical scene information  331  is transmitted to the image sensor  332 . The image sensor  332  then samples the single color representation to determine an overall color balance for a corresponding scene. This overall color balance corresponds to the target color balance transmitted to the color compensated flash module  302 . After the image sensor  332  samples the color balance of the scene, the image lens cover  368  is opened by the digital camera  300  to allow the optical scene information  331  to be focused on the image sensor  332 . 
         [0084]    Several techniques have been described herein to sample an ambient color balance, however any technically feasible technique may be used to determine and represent the ambient color balance of a scene. Importantly, the ambient color balance determines the target color balance. As described previously, the color compensated flash module  302  is configured to generate controlled illumination  123  based on the target color balance. 
         [0085]      FIG. 3D  illustrates a front view of a mobile wireless device  370  configured to implement one or more aspects of the present invention. The mobile wireless device  370  includes a digital image module, such as digital image module  304  of  FIG. 3C  having image lens  366 . The mobile wireless device  370  also includes a color compensated flash module such as color compensated flash module  302 , configured to include light source  166  and active color filter  120 . The color compensated flash module  302  generates controlled illumination  123 . The mobile wireless device  370  may include an emitter lens  122 , configured to direct controlled illumination  123  to a photographic subject. The mobile wireless device  370  may comprise a cellular phone, an application platform, a music player, or any other computational or communications functionality. 
         [0086]      FIG. 3E  is a functional diagram of the mobile wireless device  370 , according to one embodiment of the present invention. As shown, the mobile wireless device  370  includes a wireless communications subsystem  372 , an application subsystem  374 , a digital image module  304 , a color compensated flash module  302 , a battery  376 , and a power management unit  378 . 
         [0087]    As described previously in  FIG. 3C , the digital image module  304  is configured to receive, focus, and sample optical information and to generate a digital photograph from the optical information. Color compensated flash module  302  is configured to generate controlled illumination  123 . The battery  376  may be a fixed battery or a replaceable battery. The replaceable battery may be a primary battery or a rechargeable battery. The power management unit  378  is configured to receive electrical energy from the battery  376  and to generate voltage supplies for use by the mobile wireless device  370 . 
         [0088]    In one embodiment, the wireless communications subsystem  372  comprises a digital cellular telephone subsystem. The application subsystem  374  comprises a central processing unit, data storage, an operating system configured to facilitate execution of applications, and one or more applications configured to execute on the operating system. 
         [0089]    During normal operation of the mobile wireless device  370 , a user may choose to take a photograph using mobile wireless device  370 . The user activates a camera application to execute on the application subsystem  374 . The camera application directs the digital image module  304  to take a photograph. The color compensated flash module  302  is triggered to generate a strobe comprising controlled illumination  123 . The photograph may be stored within the mobile wireless device  370 . The photograph may also be transmitted via the wireless communications subsystem  372  to an upstream server (not shown) or other users (not shown). 
         [0090]      FIG. 4A  illustrates a detailed view of an active color filter  400 , according to one embodiment of the present invention. In one embodiment, at least one instance of active color filter  400  implements active color filter  120  of  FIG. 1B . Active color filter  400  includes a pixel array  402 , and pixel driver circuits, such as column drivers  404  and row drivers  406 . The column drivers  404  are controlled according to column data  414  and the row drivers  406  are controlled according to row data  416 . The column data  414  comprises intensity data for driving individual pixels P along a specified row of the pixel array  402 . The row data  416  comprises row selection information to specify a particular row of the pixel array  402 . The active color filter  400  is configured to accept power via VC  412  and GND  410  ports. 
         [0091]    In one embodiment, pixels P include color filters. For example, each row of pixels P may comprise a repeating color filter pattern of red, green, and blue. In this example, pixels P(a,d), P(b,d), and P(c,d) would respectively include color filters of red, green, and blue. 
         [0092]      FIG. 4B  depicts a side view of a pixel array  402 , according to one embodiment of the present invention. As shown, the pixel array  402  comprises different structural layers, including a back polarizer  448 , a back substrate  446 , a layer of liquid crystal material  478 , column electrodes  470 - 474 , one or more row electrodes  444 , a front substrate  442 , a front polarizer  440 , and a filter layer  456 . Protective front and back layers (not shown) may also be incorporated to protect the filter layer  456  and back polarizer  448 , respectively. 
         [0093]    In normal operation, randomly polarized light  460  from light source  166  of  FIG. 1B  passes through back polarizer  448  to yield polarized back light  462 . The polarized back light  462  passes through column electrodes  470 - 474 , the layer of liquid crystal material  478 , the one or more row electrodes  444 , and into the front substrate  442  to yield polarity modulated light  464 . At each intersection of one of the column electrodes  470 - 474  and one of the one or more row electrodes  444 , the layer of liquid crystal material  478  is able to rotate the polarity of traversing light. An electric potential applied between the column electrodes  470 - 474  and the one or more row electrodes  444  causes localized changes in polarization of the traversing light. The polarity modulated light  464 , therefore, comprises a two-dimensional region of light having a polarity corresponding to electric potentials between the column electrodes  470 - 474  and the one or more row electrodes  444 . The front polarizer  440  converts polarity modulated light  464  into intensity modulated light that passes through a set of color filters  450  within the filter layer  456 . The color filters  450  emit intensity modulated color light  466 . A group of color filters  450 -A through  450 -C collectively yield intensity modulated color light  468 , having individually modulated color components  466 -R,  466 -G,  466 -B. The intensity modulated color light  468  includes individually controlled red, green, and blue color intensity. In one embodiment, the intensity modulated color light  468  corresponds to controlled illumination  123 . 
         [0094]      FIG. 4C  depicts a response curve  484  of light transmission T  480  as a function of applied voltage Va  482  for a cell within the pixel array  402  of  FIG. 4A , according to one embodiment of the present invention. The applied voltage Va  482  corresponds to the electric potential applied between one of the column electrodes  470 - 474  of  FIG. 4B  and one of the one or more row electrodes  444 . Light transmission T  480  refers to a total amount of light energy passing through a region of the pixel array  402  corresponding to an area of one pixel (intersection of one of the column electrodes  470 - 474  and one of the one or more row electrodes  444 ). As shown, approximately maximum light transmission occurs within a “dead band”  486 , centered about a zero applied voltage Va  482 . Increasing the applied voltage Va  482  decreases light transmission according to response curve  484 , which is typically non-linear. Conventional liquid crystal materials tend to degrade when the applied voltage Va  482  is maintained in consistent polarity. Therefore, polarity of the applied voltage Va  482  should be alternated, to produce positive and negative values of Va  482 . 
         [0095]    Persons skilled in the art will recognize that any controlled transmission technology may be used to implement the pixel array  402  without departing the scope of the present invention. For example, bi-stable materials that can alternate between an opaque and clear state may be employed. Applied voltage Va  482  is then used to set a state that for a given pixel that generally persists until being set to a different state. 
         [0096]    Different techniques may be used to generate different color components within the controlled illumination  123 . For example, applied voltage Va  482  may be used to select an overall light transmission factor for each color filter within the pixel array  402 . Alternatively, each pixel within pixel array  402  may be turned completely on or off independently to generate patterns for red, green, and blue pixels that, in aggregate, represent a target light transmission factor for each respective color. This concept is illustrated in greater detail below in  FIG. 6 . 
         [0097]    In an alternative embodiment, intensity modulation is implemented using selective reflection rather than polarization modulation converted to intensity modulation by a polarizer. For example, a micro-machine reflector array is used to selectively direct light through plural color filters to generate the controlled illumination  123 . 
         [0098]      FIG. 5A  illustrates the pixel array  510  configured to include color filters for red, green, and blue, according to one embodiment of the present invention. A pixel group  512  comprises one red cell (RED 3,1), one green cell (GREEN 4,1), and one blue cell (BLUE 5,1). In one embodiment, pixel array  510  corresponds to pixel array  402  of  FIG. 4A , and each cell corresponds to one intersection of one of the column electrodes  470 - 474  of  FIG. 4B , and one of the one or more row electrodes  444 . 
         [0099]      FIG. 5B  illustrates the pixel array  520  configured to include color filters for red, green, blue, cyan, magenta, and yellow according to one embodiment of the present invention. A pixel group  522  comprises one red cell (RED 0,1), one green cell (GREEN 1,1), one blue cell (BLUE 2,1), one cyan cell (CYAN 3,1), one magenta cell (MAGENTA 4,1), and one yellow cell (YELLOW 5,1). In one embodiment, pixel array  520  corresponds to pixel array  402  of  FIG. 4A , and each cell corresponds to one intersection of one of the column electrodes  470 - 474  of  FIG. 4B , and one of the one or more row electrodes  444 . 
         [0100]      FIG. 5C  illustrates the pixel array  530  configured to include color filters for red, green, blue, cyan, magenta, and yellow according to an alternative embodiment of the present invention. A pixel group  532  comprises one red cell (RED 0,0), one green cell (GREEN 1,0), one blue cell (BLUE 2,0), one cyan cell (CYAN 0,1), one magenta cell (MAGENTA 1,1), and one yellow cell (YELLOW 2,1). In one embodiment, pixel array  530  corresponds to pixel array  402  of  FIG. 4A , and each cell corresponds to one intersection of one of the column electrodes  470 - 474  of  FIG. 4B , and one of the one or more row electrodes  444 . 
         [0101]      FIG. 5D  illustrates the pixel array  540  configured to include color filters for cyan, magenta, yellow, and white according to one embodiment of the present invention. A pixel group  542  comprises one cyan cell (CYAN 0,0), one magenta cell (MAGENTA 1,0), one yellow cell (YELLOW 2,0), and one white cell (WHITE 3,0). In one embodiment, pixel array  540  corresponds to pixel array  402  of  FIG. 4A , and each cell corresponds to one intersection of one of the column electrodes  470 - 474  of  FIG. 4B , and one of the one or more row electrodes  444 . 
         [0102]      FIGS. 5A-5D  illustrate different techniques for organizing different color filters, with a goal of filtering multi-spectral light, such as from light source  166  of  FIG. 1B  and to generate controlled illumination  123 , having a specific color balance.  FIGS. 5A-5D  illustrate four examples of organizing color filters, however, persons skilled in the art will recognize that any organization of color filters structured to implement active color filter  120  is within the scope and spirit of the present invention. 
         [0103]      FIG. 5E  depicts an ideal band pass color filter as a function of wavelength (λ)  552 , and centered at λ 0 . As shown, light transmission  550  within window λ w  is 1.0 (full transmission), while light transmission  550  outside the window λ w , is 0.0 (no transmission). In practice, however, a color filter exhibits non-ideal characteristics, as illustrated below in  FIG. 5F . 
         [0104]      FIG. 5F  depicts a typical physical realization of a band pass color filter as a function of wavelength (λ)  552 , and centered at λ 0 . As shown, light transmission  550  within window λ w  is greater than light transmission  550  outside the window λ w . For example, a physical implementation of a “red” color filter will actually have imperfect transmission of red light and have non-zero transmission for green and blue light. 
         [0105]    While practical color filters do not exhibit ideal band pass characteristics, persons skilled in the art will recognize that such filters can implement satisfactory color filtering characteristics for the purpose of implementing active color filter  120  of  FIG. 1B . 
         [0106]      FIG. 6  illustrates a technique for controlling multiple levels of transmission within an active color filter, according to one embodiment of the present invention. A pixel set  610  includes a plurality of individual pixels having substantially identical color filters. Shaded pixels represent low light transmission, while light pixels represent high light transmission. By selecting which pixels are turned on or off, an aggregate light transmission can be achieved. This technique is advantageously not sensitive to particulars of non-linear response curves, such as response curve  484  of  FIG. 4C . As shown, a binary code of [00000] yields minimal light transmission, while binary code [111111] yields maximum light transmission. 
         [0107]    The pixel set  610  may be interleaved with similar pixel sets of other colors. For example, red, green, and blue pixels may be adjacently disposed, with the red pixels belonging to a red pixel set, the green pixels belonging to a green pixel set, and so forth. The pixel set  610  may also be contiguously disposed (as shown) to create a region of the same color having controlled intensity. A plurality of such regions may comprise active color filter  120 . 
         [0108]      FIG. 7  is a conceptual diagram of a color compensated flash unit  700  comprising functional blocks for measuring ambient color balance and filtering a multi-spectral light signal  732  (D) to generate a controlled illumination signal  742  (E) based on ambient color balance, according to one embodiment of the present invention. The color compensated flash unit  700  comprises an ambient color measurement circuit  710 , a filter controller  720 , a light source  730 , and an active color filter  740 . 
         [0109]    The ambient color measurement circuit  710  is configured to measure ambient light color balance from ambient light signal  712  (A) and to generate a digital ambient color signal  714  (B) based on the ambient light signal  712  (A) and a mapping function M AB . The ambient light signal  712  (A) represents an optical signal (A) comprising different color components, including red, green, and blue color components (A={A red , A green , A blue }). The digital ambient color signal  714  (B) is an electrical representation of the ambient light signal  712  (A). The digital ambient color signal  714  (B) may represent color components of the ambient light signal  712  (A) using any technically feasible technique. For example, one binary integer may be used to represent an intensity value for each color component, with a total of three binary integers used to represent red, green, and blue color components for the ambient light signal  712  (A). This may be expressed as B={B red , B green , B blue }, where each component B red , B green , B blue  comprises one binary integer. In one embodiment, the components of B are normalized against a maximum component value (MAX{B red , B green , B blue }). 
         [0110]    The mapping function M AB  represents an abstraction of the operation of the ambient color measurement circuit  710 . The mapping function M AB  may implement any technically feasible linear or nonlinear mapping from optical intensity to binary representation for the digital ambient color signal  714  (B). Persons skilled in the art will understand that any technique for measuring and representing ambient color may be implemented without departing the scope and spirit of the present invention. 
         [0111]    In one embodiment, the ambient color measurement circuit  710  comprises ambient sampling lens  138  of  FIG. 1B , ambient color sensor  136 , and color receiver  130 . Furthermore, ambient light signal  712  (A) corresponds to ambient light  139 , digital ambient color signal  714  (B) corresponds to digital ambient color signal  134 . 
         [0112]    The filter controller  720  is configured to receive the digital ambient color signal  714  (B) and to generate filter control signal  722  (C). Any technically feasible mapping from the digital ambient color signal  714 (B) to the filter control signal  722  (C) may be implemented without departing the scope of the present invention. In one embodiment, the filter control signal  722  (C) comprises electrical signals, such as voltage or current signals, configured to control transmission of optical color components through the active color filter  740 . For example, the filter control signal  722  (C) may comprise voltage signals for driving a liquid crystal array, which selectively transmits red, green, and blue color components based on the filter control signal  722  (C). The filter controller  720  performs a mapping function M BC  from digital ambient color signal  714  (B) to filter control signal  722  (C). The mapping function M BC  is an abstraction of the operation of the filter controller  720 . The mapping function M BC  may be configured to compensate for non-linear transmission characteristics associated with the active color filter  740 . 
         [0113]    In one embodiment, the filter controller  720  corresponds to a combination of the color controller  110  and the filter driver  114 , and the filter control signal  722  (C) corresponds to filter control signal  116 . 
         [0114]    The light source  730  may implement any technically feasible technology for generating multi-spectral light. For example, the light source  730  may comprise a gas discharge chamber, such as a Xenon flash tube. Alternatively, the light source may comprise a white light emitting diode (LED), such as a white phosphor LED. In one embodiment, the light source corresponds to light source  166 . The multi-spectral light is characterized by multi-spectral light signal  732  (D), comprising plural color components. In one embodiment, the multi-spectral optical signal  732  (D) is characterized as having red, green, and blue color components (D={D red , D green , D blue }). 
         [0115]    The active color filter  740  filters multi-spectral light signal  732  (D) to generate controlled illumination signal  742  (E) in response to filter control signal  722  (C). The active color filter  740  may implement selective transmission filters, such as commonly associated with a liquid crystal display (LCD). The active color filter  740  may also implement selective reflection filters, such as commonly associated with micro-electro-mechanical system (MEMS) based arrays used for projection displays. In one embodiment, active color filter  740  corresponds to active color filter  120 , and color components of the controlled illumination signal  742  (E) comprise red, green, and blue colors (E={E red , E green , E blue }). 
         [0116]    The active color filter  740  may implement a set of individual color filters, based on any technically feasible set of individual colors and purity of color for each filter. A narrow (high purity) color filter has a relatively narrow wavelength transmission window λ w , as illustrated in  FIG. 5F . A narrow color filter will generally pass less overall light, and will appear to have a very distinct, saturated color. A wide (low purity) color filter has a relatively wide wavelength transmission window λ w . A wide color filter may pass a relatively large portion of other color components. For example, a wide color filter for green predominantly passes green light, but may also pass red and blue light. A very wide color filter will generally pass more overall light, and may appear to be unsaturated or tinted rendering of a principle color. 
         [0117]    The active color filter  740  is characterized as an optical transmission function (T DE ), comprising plural, independent transmission components. In one embodiment, the transmission components comprise red, green, and blue components T DE ={T red , T green , T blue }). In alternative embodiments, different transmission components (e.g., cyan, magenta, and yellow) are implemented to generate a net transmission for red, green, and blue. While different variations of the active color filter  740  have been disclosed herein, persons skilled in the art will recognize that any active optical filter technology may be implemented without departing the scope and spirit of the present invention. 
         [0118]    The color compensated flash unit  700  measures ambient light signal  712  (A) and generates controlled illumination signal  742  (E), where E≈A * k. Controlled illumination signal  742  (E) is related to ambient light signal  712  (A) by scalar coefficient k, meaning that ratios among color components are preserved between controlled illumination signal  742  (E) and ambient light signal  712  (A), although corresponding component values may be different by a constant factor of k. This overall operation is described by Equation 1, below: 
         [0000]        E≈D·T   DE ( M   BC ( M   AB ( A )))* k   (Equation 1)
 
         [0119]    Persons skilled in the art will recognize that the active color filter  740  will typically transmit a given color component as a sum of net transmission of the color component through all color component filters. For example, if the active color filter  740  includes red, green and blue components and associated color component filters, then net transmission for the green component is actually a sum of the green transmission associated with the physically implemented red filter, the green transmission associated with the physically implemented green filter, and the green transmission associated with the physically implemented blue filter. This is expressed below in Equation 2: 
         [0000]        T   green   =t   gr ( C   red )+ t   gg ( C   green )+ t   gb ( C   blue )  (Equation 2)
 
         [0120]    In Equation 2, T green  represents net transmission of green light through the active color filter  740 , where t gr  represents the net transmission of green light through the physically implemented red filter as a function of the red component of the filter control signal  722  (C red ), t gg  represents the net transmission of green light through the physically implemented green filter as a function of  Cgreen,  and t gb  represents net transmission of green light through the physically implemented blue filter as a function of C blue . An active color filter  740  that implements perfect, independent color filtering of each color component would have the corresponding coefficient values for green transmission: t gr =0, t gg =1, and t gb =0. However, practical values of t gr , t gg , and t gb  should generally range from greater than zero to less than one. 
         [0121]    Larger values for the coefficients t gr  and t gb  indicate a wider window λ w  shown in  FIG. 5F . In certain implementations, higher coefficient values for t gr  and t gb  may be desirable because higher coefficient values indicate greater net transmission, which implies greater overall system efficiency. 
         [0122]    Mapping functions M AB  and M BC  may comprise any technically feasible linear or non-linear transform or transforms that collectively solve Equation 1 for E≈A * k. In one embodiment, mapping function M BC  comprises an iterative function for solving Equation 1. For example, the mapping function M BC  may comprise a method that first assigns a dominant color component, based on MAX{B red , B green , B blue }, for one component of components T red , T green , T blue . In one implementation, the dominant color component may be set equal to a maximum value (e.g., 1.0 on a scale from 0.0 to 1.0). The mapping function M BC  may then assign a second to dominant color component while preserving a ratio between the dominant and second to dominant color components for both B and T. The mapping function M BC  may then assign a least dominant color component while preserving ratios between the dominant color component, second to dominant component, and least dominant color components for B and T. At each stage, the mapping function may iterate to maintain proper ratios for components in T. 
         [0123]    An example mapping function M BC  may receive B={B red , B green , B blue }, where MAX{B red , B green , B blue } is B green , followed by B red  and B blue . If components in C and T are normalized to a range of 0.0 to 1.0, then C green  is set to 1.0, resulting in a green transmission value T green  of 1.0. Next, B red  is set to a value that results in a value of T red  that preserves the ratio B green /B red green =T green /T red . And so forth. Importantly, the red filter associated with T red  may contribute to net green light transmission. The transmission contributions for each color filter to each color may be known in advance, allowing the mapping function M BC  to generate an appropriate filter control signal  722  (C). In one embodiment, a direct lookup table implements mapping function M BC , with components for B comprising table inputs, and components for filter control signal  722  (C) comprising table outputs. Persons skilled in the art will recognize that different techniques for solving Equation 1 may be implemented without departing the scope of the present invention. 
         [0124]    In an alternative embodiment, digital ambient color signal  714  (B) is processed to yield a single scalar value that is descriptive of ambient color balance. A color temperature is one common scalar description of color balance. Hue is another scalar description of color. While a scalar description of ambient color balance represents a narrow gamut of possible color, this approach is broadly useful and accepted in the art. Generating the filter control signal  722  (C) from a scalar value for color balance comprises directly mapping the scalar value to each component of the filter control signal  722  (C) via a set of lookup tables or an appropriate mapping function. Such a direct mapping can account for net transmission of each component for each filter for each value of the scalar value color value. 
         [0125]      FIG. 8A  is a flow diagram of method steps  800  for generating controlled illumination based on measured ambient color, according to one embodiment of the present invention. Although the method steps are described in conjunction with system  FIGS. 1  B- 4 B,  5 A- 5 D, and  7 , persons skilled in the art will understand that any system configured to perform the method steps, in any order, is within the scope of the invention. 
         [0126]    The method begins in step  810 , where the color compensated flash unit  100  of  FIG. 1B  receives a start signal. The start signal indicates that the color compensated flash unit  100  should prepare for a strobe trigger by appropriately driving the active color filter  120  to match ambient color balance. If, in step  812 , the color compensated flash unit  100  is directed to measure ambient color balance, then the method proceeds to step  814 . The color compensated flash unit  100  may be directed to measure an ambient color balance via a user input setting from a physical control such as a button or switch, via a software user interface, or via any technically feasible input means. In step  814 , the color compensated flash unit  100  measures ambient color balance to generate a measured ambient color signal. 
         [0127]    In step  820 , the color compensated flash unit  100  computes filter control values comprising a filter control signal based on the measured ambient color signal. In one embodiment, the color compensated flash unit  100  solves Equation 1 of  FIG. 7  for E≈A * k . In an alternative embodiment, the color compensated flash unit  100  computes filter control values to match an ambient color temperature (scalar) value derived from the measured ambient color signal. 
         [0128]    In step  822 , the color compensated flash unit  100  drives an active color filter, such as active color filter  120 , using the filter control values. In step  824 , the color compensated flash unit  100  triggers a light source, such as light source  166 , in response to receiving a trigger signal from a host device, such as a camera. 
         [0129]    If, in step  826 , ambient color balance was measured by the color compensated flash unit  100 , then the method proceeds to step  830 , where the color compensated flash unit  100  transmits a measured ambient color signal back to the host device. The method terminates in step  832 . 
         [0130]    Returning to step  826 , if the ambient color balance was not measured by the color compensated flash unit  100 , then the method terminates in step  832 . 
         [0131]    Returning to step  812 , if the color compensated flash unit  100  is not directed to measure ambient color balance, then the method proceeds to step  816 , where the color compensated flash unit  100  receives a color balance. The color balance may comprise color components (such as red, green, and blue), a color temperature value, or any other technically feasible color description. The color balance may be received from a host device, such as a camera, a physical input device, such as a button, a software user interface, or any other technically feasible means for conveying a color balance. 
         [0132]    Although the method steps  800  are described with respect to color compensated flash unit  100 , any other color filtering system having an active color filter, such as color compensation flash module  302  of  FIGS. 3C and 3E , is within the scope of the present invention. 
         [0133]      FIG. 8B  is a flow diagram of method steps  802  for generating controlled illumination based on a specified color balance, according to one embodiment of the present invention. Although the method steps are described in conjunction with system  FIGS. 1B-4B ,  5 A- 5 D, and  7 , persons skilled in the art will understand that any system configured to perform the method steps, in any order, is within the scope of the invention. 
         [0134]    The method begins in step  850 , where the color compensation unit  200  of  FIG. 2A  receives a start signal. The start signal indicates that the color compensation unit  200  should begin appropriately driving the active color filter  120  to match ambient color balance. If, in step  852 , the color compensation unit  200  is directed to measure ambient color balance, then the method proceeds to step  854 . The color compensation unit  200  may be directed to measure an ambient color balance via a user input setting from a physical control such as a button or switch, via a software user interface, or via any other technically feasible input means. In step  854 , the color compensation unit  200  measures ambient color balance to generate a measured ambient color signal. 
         [0135]    In step  860 , the color compensation unit  200  computes filter control values comprising a filter control signal based on the measured ambient color signal. In one embodiment, the color compensation unit  200  solves Equation 1 of  FIG. 7  for E≈A * k . In an alternative embodiment, the color compensation unit  200  computes the filter control values to match an ambient color temperature (scalar) value derived from the measured ambient color signal. 
         [0136]    In step  862 , the color compensation unit  200  drives an active color filter, such as active color filter  120 , using the filter control values. If, in step  866 , ambient color balance was measured by the color compensation unit  200 , then the method proceeds to step  870 , where color compensation unit  200  transmits a measured ambient color signal back to a host device, such as a camera. The method terminates in step  872 . 
         [0137]    Returning to step  866 , if the ambient color balance was not measured by the color compensation unit  200 , then the method terminates in step  872 . 
         [0138]    Returning to step  852 , if the color compensation unit  200  is not directed to measure ambient color balance, then the method proceeds to step  856 , where the color compensation unit  200  receives a color balance. The color balance may comprise color components (such as red, green, and blue), a color temperature value, or any other technically feasible color description. The color balance may be received from a host device, such as a camera, a physical input device, such as a button, a software user interface, or any other technically feasible means for conveying a color balance. 
         [0139]    Although the method steps  800  are described with respect to color compensation unit  200 , any other color filtering system having an active color filter is within the scope of the present invention. 
         [0140]    While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.