Patent Number: 
Section: description

A system that can adjust the color temperature of the auxiliary illuminating device used to help illuminate a scene greatly improves the color balancing of the captured scene. One embodiment of the current invention comprises an array of light emitting diodes (LED). The array is made with three different color LED""s (see FIG. 4). Two of the three colors are blue (402) and green (404). The third color is either red or amber. In another embodiment the array of LED""s contain four colors, red, green, blue, and amber. In another embodiment a broadband light source, for example a halogen bulb, is combined with an array of LED""s of a single color. In another embodiment a broadband light source, for example a halogen bulb, is combined with an array of LED""s of two different colors. The array of LED""s may contain multiple LED""s of one color and the array may contain more of one color than another color. For example the array may contain 10 red LEDs, 10 blue LEDs and 8 green LEDs. All the LEDs of one color make up a set of LEDs. Each set of LEDs can be independently controlled as to how much light the LEDs of that set are producing. When each set of LEDs is producing a predetermined ratio of power compared to the other sets of LED""s, the total light output from the LED array would be white. For the array of LEDs to simulate the color temperature of the ambient light, the type of illumination to be matched must be known. One way is for the user to select the type of lighting from a list of choices. Another way is for the camera or an auxiliary device to measure the current light in the scene and determine the type of illumination. Once the type of illumination to be matched has been determined, the amount of light coming from each set of color LEDs is adjusted such that the total amount of light coming from the LED array is a calorimetric match to the ambient illumination source. Each type of ambient light source would typically have a different ratio of light coming from the sets of color LEDs. FIG. 1 shows the spectral power distribution for a tungsten bulb with a filament temperature of 3250 K. FIG. 5 shows the spectral power distribution of 4 LEDs, a blue LED (502), a green LED (504), an amber LED (506), and a red LED (508). The ratio of power for three of the LED""s from FIG. 5, for example the red, green and blue LED""s, to match an ambient light source can be calculated with the following equations. Using standard calorimetric formulas (well know in the art), the chromaticity of the ambient light source is calculated, for example x0=0.4202 and y0=0.3976 where x0 and y0 are the chromaticity coordinates of the ambient light source. Matching the given chromaticity coordinates can be done by determining the CIE tristimulus values X, Y, Z. The tristimulus values are calculated from the tristimulus functions X(xcex), Y(xcex), Z(xcex) and the total output power from the LED arrays. The power from the LED arrays is represented by the spectral output distribution of the three LED arrays RLED(xcex), GLED(xcex), BLED(xcex) and a multiplier for each array E1, E2, and E3. X=∫X(xcex)(E1Rled(xcex)+E2Gled(xcex)+E3Bled(xcex))dxcexxe2x80x83xe2x80x83Equation 1 Y=∫Y(xcex)(E1Rled(xcex)+E2Gled(xcex)+E3Bled(xcex))dxcexxe2x80x83xe2x80x83Equation 2 Z=∫Z(xcex)(E1Rled(xcex)+E2Gled(xcex)+E3Bled(xcex))dxcexxe2x80x83xe2x80x83Equation 3 Where the integral is evaluated over the visible spectrum, for example 350 nm to 780 nm. From these equations the chromaticity coordinates of the LED arrays can be calculated as:                     y        =                  Y                      X            +            Y            +            Z                                              Equation        ⁢                  xe2x80x83                ⁢        4                                x        =                  X                      X            +            Y            +            Z                                              Equation        ⁢                  xe2x80x83                ⁢        5             Because we are interested in the relative power of each LED set, we can say that: E1+E2+E3=1xe2x80x83xe2x80x83Equation 6 Equations 1, 2 and 3 are then substituted into equation 4 and 5. Therefore it can be shown that the chromaticity coordinates of the LED arrays can be expressed in terms of E1 and E2: x(E1,E2)=x0 y(E1,E2)=y0 Where x0 and y0 are the desired chromitisity coordinates of the ambient light. The Newton-Raphson method (discribed in xe2x80x9cNumerical regresion: the art of scientific computingxe2x80x9d by W. H. Press, B. P. Flannery, S. A. Peukoastky, and W. T. Vetterling, Cambrige University Press 1988) can be generalized in the 2D case as follows:       [                                                      x              n                        -                          x              0                                                                                      y              n                        -                          y              0                                            ]    =            [                                                                  ∂                                  x                  n                                                            ∂                                  E                                      1                    ,                    n                                                                                                                          ∂                                  x                  n                                                            ∂                                  E                                      2                    ,                    n                                                                                                                                          ∂                                  y                  n                                                            ∂                                  E                                      1                    ,                    n                                                                                                                          ∂                                  y                  n                                                            ∂                                  E                                      2                    ,                    n                                                                                          ]        ⁡          [                                                                  E                                  1                  ,                  n                                            -                              E                                  1                  ,                                      n                    +                    1                                                                                                                                          E                                  2                  ,                  n                                            -                              E                                  2                  ,                                      n                    +                    1                                                                                          ]       For the nth itteration the partial derivitive xn and yn with respect to E1,n and E2,n are calculated numericly. This gives new values of E1 and E2 based on a first aproximation of E1 and E2. Inverting the matrix gives the next value of E1 and E2       [                                                      E                              1                ,                n                                      -                          E                              1                ,                                  n                  +                  1                                                                                                                    E                              2                ,                n                                      -                          E                              2                ,                                  n                  +                  1                                                                          ]    =                    1                                                            ∂                                  x                  n                                                            ∂                                  E                                      1                    ,                    n                                                                        *                                          ∂                                  y                  n                                                            ∂                                  E                                      2                    ,                    n                                                                                -                                                    ∂                                  x                  n                                                            ∂                                  E                                      2                    ,                    n                                                                        *                                          ∂                                  y                  n                                                            ∂                                  E                                      1                    ,                    n                                                                                          ⁡              [                                                                              ∂                                      y                    n                                                                    ∂                                      E                                          2                      ,                      n                                                                                                                          -                                                      ∂                                          y                      n                                                                            ∂                                          E                                              1                        ,                        n                                                                                                                                                                    -                                                      ∂                                          x                      n                                                                            ∂                                          E                                              2                        ,                        n                                                                                                                                                                  ∂                                      x                    n                                                                    ∂                                      E                                          1                      ,                      n                                                                                                          ]              ⁡          [                                                                  x                n                            -                              x                0                                                                                                        y                n                            -                              y                0                                                        ]       Which is iterated until the total change in E1 and E2 is less than a predetermined error amount, for example 0.0001. The ratio of power for the LED arrays calculated using the above method gives a visual (or calorimetric) match between the LEDs"" light and the ambient light. In most cases this would be adequate for use as the strobe setting for a camera. Further improvement could be achieved by tailoring the calculations and resulting LED power ratio""s to the specific spectral sensitivity of the camera. In camera design it is a goal to have the spectral sensitivities be a linear transform of the color matching functions (X(xcex), Y(xcex), Z(xcex)) but due to signal-to-noise and design constraints it is never precisely reached. It is desirable then to have the LED illumination match the signal received by a camera from the ambient light. This will give a color match as seen by the camera that will differ slightly from the match designed for a human observer (i.e. a colorimetric match). For a match as seen by the camera the analysis is repeated as above except the color matching functions (X(xcex), Y(xcex), Z(xcex)) are replaced with the camera specific spectral sensitivity functions. Using the camera spectral sensitivity functions will result in the correct power ratios for the LEDs to match the color from the ambient light that the camera detects. The power ratio""s created using the visual (or calorimetric) match calculated with the CIE color matching functions (X(xcex), Y(xcex), Z(xcex)) results in a generic flash. The generic flash can be used interchangeably between cameras that have different spectral sensitivities. The difference in spectral sensitivity between cameras can be caused by different CCD designs and/or different color filter pass bands. The power ratio""s created using the camera specific spectral sensitivity functions would work best with the camera they were designed for. The method used above could also be used for determining the power ratio of two sources, for example a red and a blue LED. The method would also work with a broad band light source and a narrow band light source, for example an LED and a halogen light source. With only two light sources the light may not be able to match exactly the ambient source. The two sources could be chosen to maximize the number of ambient light sources or the two sources could be chosen such that a very close match exist for a specific ambient light source. The form of the equation for a broad band light source B and a narrow band light source N would be as follows: X=∫X(xcex)(E1 B(xcex)+E2N(xcex))dxcex Where B(xcex) is the spectral power of the broadband light source and N(xcex) is the spectral power of the narrowband light source. For an adjustable light source with 4 light source components the power ratio between the 4 light sources can be determined using well known numerical methods. The auxiliary illuminating device would contain a table or list of the correct power ratios for a number of ambient sources. The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.