Sensor‐response‐ratio constancy under changes in natural and artificial illuminants

Abstract

We have analyzed the constancy of the response ratio for cones, second-stage mechanisms, and CCD sensors when daylight or an artificial illuminant (A, F2, F7, and F11) is changed to an equal-energy illuminant (E) in scenes containing natural and artificial objects. The response ratios were always found to be roughly constant for all the sensors. For daylight, we have deduced mathematical expressions, which relate the values of these ratios with the correlated color temperature (CCT) and applied these expressions to the synthesis of images of a scene viewed under different daylights corresponding to a variety of CCTs. The results were highly satisfactory in a rural scene for any CCT. In the scene with artificial objects, the results were also good for nonextreme CCTs. We also included in our study artificial illuminants, with which we achieved very good image syntheses for illuminants A and F7.

Recovering of weather degraded images based on RGB response ratio constancy

“…This linear relationship indicates that the ratio of the excitations under illuminant changes for each cone class remains roughly constant for all objects. The same ratio constancy stands for artificial sensors, as we have proved in a previous work [29].…”

“…These relationships occur for a variety of soft daylight spectral power distributions (SPDs) and for artificial illuminants. Romero et al [29] have proved that these relationships are also preserved between the response of artificial receptors (like the ones present in RGB camera channels) under different illuminants. Depicting in the y axis the response of one camera sensor (R, G, or B digital counts of the captured image) for the illuminant present in the scene, and in the x axis the response of the same sensor under a theoretical illuminant, such as an E illuminat (i.e., an equienergetic illuminant), linear relationships appear, with a high correlation coefficient, as we have previously mentioned.…”

“…Several authors [29,30] have shown that there is a linear relationship when the response of a sensor under a certain illuminant is depicted versus the response of the same sensor under another illuminant. These relationships occur for a variety of soft daylight spectral power distributions (SPDs) and for artificial illuminants.…”

Color changes in objects in natural scenes as a function of observation distance and weather conditions

“…When, for any specific cone receptor [33], the excitation values of several objects under a specific illuminant are represented as a function of those obtained under another illuminant, the result is a straight line with a high correlation coefficient [1]. Some color constancy theories used to develop object-recognition algorithms are also based on cone-excitation ratio constancy [9,11]. Figure 7(a) shows an example of cone-excitation ratio constancy.…”

“…The appearance of objects does not change for different days, hours of the day, or atmospheric conditions in spite of the colorimetric changes provoked by the changes in illumination. Working with a wide range of objects, several authors [1,[11][12][13][14] have found a linear relationship with a high correlation coefficient representing the pairs of excitation values for each cone photoreceptor (L, M, or S) determined for each object under daylight illumination at two different color temperatures. Foster and Nascimento [12] explain the color-constancy phenomenon based on these linear relationships.…”