The current color constancy methods are based on an image processing of the sensor's RGB data to estimate the color of illumination. Unlike previous methods, whitebalPR measures the illuminant by separating diffuse and specular components in a scene by taking advantage of the polarizing effect occurring to light reflection. Polarization difference imaging (PDI) detects the polarization degree of the neutrally reflected (specular) parts and eliminates the remitted (diffuse) non-polarized colored parts.
Different experiments explore the signal level within the polarization difference image in relation to multicolored objects, different object surfaces and to the arrangement of light source, camera and object. The results exhibit a high accuracy of measuring the color of illumination for glossy and matte surfaces. As these setups work best for achromatic objects, this new approach for data analysis combines the ideas of the dichromatic reflection model (DRM) and whitebalPR and delivers reliable results for mainly colored objects. Unlike the DRM needs to segment the image referring to the objects in the scene, the new proposal (polarization difference line imaging, PDLI) is independent from any knowledge of the image content. A further arbitrarily segmentation of the image into macro-pixels of any size reduces the computational effort and diminishes the impact of noise on the PDI signal. An according experiment visualizes the coherency between the size of the macro-pixels, the angle of incidence and the accuracy of the process. To sum up, by means of the segmentation the PDLI process gains further stabilization in detecting the color of the illuminant while the computational effort decreases.
This new color constancy method is based on the polarization degree of that light which is reflected at the surface of an object. The subtraction of at least two images taken under different polarization directions detects the polarization degree of the neutrally reflected portions and eliminates the remitted non-polarized colored portions.
Two experiments have been designed to clarify the performance of the procedure, one to multicolored objects and another to objects of different surface characteristics. The results show that the mechanism of eliminating the remitted, non-polarized colored portions of light works very fine. Independent from its color, different color pigments seem to be suitable for measuring the color of the illumination.
The intensity and also the polarization degree of the reflected light depend on the surface properties significantly. The results exhibit a high accuracy of measuring the color of the illumination for glossy and matt surfaces. Only strongly scattering surfaces account for a weak signal level of the difference image and a reduced accuracy.
An embodiment is proposed to integrate the new method into digital cameras.