Color brightness model and its imaging applications

We have developed an empirical umbrella model for predicting the brightness of colors. It is based on a modified concept of the radiance factor. Instead of a single reference, i.e., white, for all colors, each color has its own reference color. Our model shows that, for near neutral colors, the boundary of the object-color solid accounts for most of the hue angles, except in the cyan-blue region, where the boundary has to be adjusted by experiments on color charts. We use an exponential function to extrapolate the umbrella to the colors of higher purity. The model is used to adjust the luminance of the Munsell colors to make them equally bright by compensating for the Helmholtz-Kohlrausch effect. It is also applied to real consumer images to reduce their brightness after boosting their color saturation. For both applications, the comparison images show that the model is quite effective.