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Models of the human pupil response have been widely investigated. Yet, besides the well known model for the pupil diameter as a function of light intensity and some studies on the influence of wavelength, a model for the dependence of the pupil diameter on the geometry of an observed image is still missing. This paper describes an experimental setup to study the influence of the geometry of a light distribution stimulating the human eye and its pupil. Moreover, experimental results of the human pupil response as a function of the time and of geometric parameters of the stimulating field are presented. To measure the behavior of the pupil, a test person is positioned in front of a back-side illuminated diffusing pane. This pane approximates a Lambert-radiator. The illuminated area is adjustable to different circular aperture sizes. While varying both size and intensity, the pupil response of a test person is observed by an infrared CCD- camera. Electronic image processing is used to determine the diameter of the captured pupil in the image. The first step is to convert the gray-scale image into a binary image with an optimized threshold. Then, the two dimensional image is reduced to a one-dimensional column-image by collecting and adding all pixels of each row (parallel projection). This step enables a very effective frequency-domain analysis (Fourier transform) leading to the detection of the pupil diameter. The actual setup is able to process ten images per second with a resolution of 0.2 mm. Short-time effects of the pupil diameter can therefore be observed as well as long-time effects. It was found that previous results regarding the pupil diameter as function of the light intensity were confirmed. This indicates that the automatic system works well and the psycho-physical experiments are conducted reasonably. However, as a new result, it was found that the pupil response seems to be dependent on the aperture size of the stimulating field.
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