Colour Variations Within Light Fields: Interreflections and Colour Effects

Abstract

The human visual system incorporates knowledge about local chromatic and lightness effects of interreflections [1]. Here we study basic principles behind chromatic effects of interreflections using computational modelling and photometric measurements. The colour of interreflections varies as a function of the number of bounces they went through. Using a computational model we found that those colour variations can show brightness, saturation and even hue shifts. Using a chromatic Mach Card, a concave folded card with both sides made of the same colour, we demonstrated those three types of colour effects empirically. Finally, we tested the effects of such coloured interreflections on light fields in 3D spaces. Via cubic spectral illuminance measurements in both computer simulations and full mock up room settings under different furnishing scenarios we measure the chromatic variations of first order properties of light fields. The types of chromatic variations were found to depend systematically on furnishing colour, lighting and geometry, as predicted, and also vary systematically within the light field, and thus throughout the space. This study forms a theoretical and empirical basis for extending our light field framework to the chromatic domain: we will next compare how the basic effects found will work out for natural scenes and do extended spatial measurements of chromatic light fields (over grids of locations in the scenes, altogether forming a unique database). On the basis of this study we expect differential chromatic effects for the different components (ambient, focus, brilliance light) of our framework. Additionally, we expect differential effects of combinations of chromatically different sources (f.i. diffuse lighting by a blue sky plus directed lighting by a yellowish sun) and the scene's material's spectral reflectances. We aim to compare those physical light fields with measurements of visual light fields (including their chromatic properties).