1.  Field of the Invention
This invention pertains generally to camera color calibration, and more particularly to performing accurate calibration with small calibration color sets. 
2.  Description of Related Art
Color charts are utilized in a number of color-related fields for performing various types of color characterization, calibration, correction, adjustment, and so forth.  A color chart comprises a physical arrangement of standardized color samples, used for color comparisons and measurements such as in calibrating an image sensor, checking the color reproduction of an imaging system, demosaicking, denoising, white balancing, color correction and so forth.  Standardized color chart targets are produced by a number of companies. 
Color charts are often used in calibrating the response of digital imaging (camera) devices.  Digital imaging devices proliferate our modern world, from cell phones, point-and-shoot cameras, video cameras, digital SLRs, as well as high-end still and video camera equipment.  One of the challenges of manufacturing this equipment is that of providing a correct color calibrations, and similar characterizations and/or response modifications of color imagers and systems. 
It will be recognized that various types of sensing elements are employed in the manufacture of digital imaging devices, which may utilize RGB, RGBE, CMYG sensors, and others.  Each type and model of sensing device has its unique spectral sensing characteristics and presents different performance in color calibration.  In addition, variations exists even for a given sensor model, between and even within a given manufactured lot. 
Providing proper calibration requires determination of a linear adjustment matrix (AM) to match the XYZ or L*a*b* values determined from the actual camera to that of human visual perception.  One typical method of performing color calibration involves the use of a color chart.  For example, the color chart typically used in digital still camera (DSC) color calibration usually consists of multiple color patches as the calibration target.  It would seem that the more color patches used, the better would be the resultant calibration performance.  However, in practice this is not always true because colors are basically energy distributions in a spectral range.  Various colors inevitably have spectral redundancy which can be reduced without affecting the overall calibration. 
Previous color calibration applications by the applicant have separately described the use of a non-negative matrix factorization approach to directly estimate a color chart with a reduced number of colors, and added an orthogonality constraint to optimize the calibration chart in the 2nd order statistical range.  Regarding the orthogonality constraint the optimal color set provided approximately the same calibration performance as rendered by the use of large color sets (e.g., Macbeth Color Chart having 24 patches), while significantly reducing the number of color patches (e.g., down to seven), with calibration being performed in response to multiple illuminants.  Another application described an extended approach which derived an additional constraint for generating an illuminant-independent colored rays set for color calibration. 
However, each of the above methods requires a color chart with spectral reflectance which has similar 2nd order statistical characteristics of a calibration standard such as found in the Macbeth color checker standard.  The above techniques all require significant calibration time to perform even though they provide for creating a useful color calibration chart with a significantly reduced number of color patches. 
Accordingly a need exists for a system and method of performing illuminant independent color calibration in a short calibration cycle.  These needs and others are met within the present invention, which overcomes the deficiencies of previously developed color calibration systems and methods.