If a purpose of photostasis ‐ absorption of a constant number of photons by the retina, regardless of incident light levels ‐ is to maintain rods at saturation during the light period, then in retinal regions where light intensity is low, rhodopsin concentration should be high, and vice versa. Our ocular transmission photometric measurements revealed that the distribution of light intensity across the rat retina was not as simple as had been thought and, furthermore, that the local concentration of rhodopsin had a high negative correlation with the light intensity. The reciprocity between these two parameters leads to nearly uniform rates of photon absorption in rods across the retina.

We describe a photometer that measures light transmitted through excised eyes. The instrument, an ocular transmission photometer, employs sensitive single photon-counting techniques, and its usefulness has been tested by the study of the absorbance of rhodopsin in retinal rod cells in situ. We find that absorbances of rat rods agree well with those predicted by microspectrophotometry without making corrections for cellular mosaics. Additional tests of the ocular transmission photometer show that (a) the instrument is sensitive to subtle differences in rhodopsin absorbance, known to exist in specific locations in the rat retina, and (b) using the rate of rhodopsin bleaching as the measure of intensity, we can determine the intensity distribution at several locations across the rat retina.

An investigation into the distribution of light intensity across the rat retina was carried out on excised, intact rat eyes exposed to Ganzfeld illumination from a helium‐neon laser (543 nm). Some of the light entering the eyes exits through the sclera where its intensity can be monitored with an optical ‘pick‐up’ that samples the intensity coming from a small region of external sclera and underlying retina. The spatial resolution of the pick‐up is such that it samples light that has passed through ca 2 % of the rods in the rat eye. Some of the laser light is absorbed by the rod pigment, rhodopsin, which gradually bleaches. Bleaching in the retina, in turn, causes an exponential increase in intensity emanating from the sclera. By monitoring this intensity increase, we are able to measure two important parameters in a single bleaching run: the local rhodopsin concentration and the local intensity falling on the rods. With an ocular transmission photometer, we have measured both the local intensity and the local rhodopsin concentration across wide regions of rat retina. Both pigmented and albino rats were studied. The distributions of rhodopsin and intensity were both nearly uniform; consequently, the product, (rhodopsin concentration) × (intensity), was similarly nearly equal across the retina. This means that the initial rate of photon absorption is about the same at all retinal locations. Interpreted in terms of photostasis (the regulation of daily photon catch), this means that the rate of photon absorption is about the same in each rod, viz. 14 400 photons absorbed per rod per second. Since this rate of absorption is sufficient to saturate the rod, one possible purpose of photostasis is to maintain the rod system in a saturated state during daylight hours.

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