Measurement of Omnidirectional Light Distribution by a Mirrored Ball

摘要

Illuminant estimation in natural scenes includes the problem of estimating a spatial distribution of light sources by omnidirectional observations. The present paper describes a method for estimating an omnidirectional light distribution from the image of a camera aiming at a mirrored ball. The parameters including the camera location in the scene and the focal length of the camera lens are estimated to precisely determine the mapping between coordinates on the ball and light rays in the world. We create two types of image to represent the omnidirectional light distribution in the world. One representation is a polar coordinate system with the origin located at the center of the mirrored ball. The other is a parallel projection on a two-dimensional screen passing the center of the ball. Experiments in a natural scene using a spherical steel ball and a color CCD camera are described. First, we estimate an omnidirectional radiance distribution indoors in a room with fluorescent ceiling lights. Second, we estimate the radiance distribution in the open air. The scene includes strong specular high light by direct reflection of sunlight. We analyze the chromaticity of the omnidirectional light source.