IMAGING OF RETROREFLECTIVE OBJECTS UNDER HIGHLY NONUNIFORM ILLUMINATION

The answers to two common questions are attempted. The first is the ma thematical representation of the Visibility characteristics of a retro reflector. The second question concerns the construction of the necess ary viewing criterion under highly nonuniform illumination. The proble m of viewing a retroreflective object is discussed from the general vi ewpoint of the optical reciprocity theorem, Nonuniform illumination le ads to peculiarities in calculating the contrast because the absence o f any reference image region that usually serves as a background. For this reason, an optical sensor focuses its attention on the maximal br ightness gradient of an image to compare the radiances of two adjacent resolution elements. In such a case, the valid signal is shown to be determined only by the radiance of direct light from a dummy source ha ving an angular pattern of radiation that is the same as the angular p attern of the real receiver sensitivity, whereas optical interferences are obtained by taking multiply scattered light into account, The cal culations are made under the small-angle diffusion approximation with the direct light isolated. One interesting example of observing retror eflective and Lambertian objects is presented. In particular, the case of viewing a retroreflector on a Lambertian background with the same albedos is considered, Here, the observation is possible only due to t he different angular patterns of reflection. (C) 1996 Society of Photo -Optical Instrumentation Engineers.