Abstract

In vivo measurements of visual pigments have required intense lights for adequate signal-to-noise ratios. Our retinal densitometer has been redesigned as a photon counter to permit measurements at lower light levels. Counting photons, instead of measuring photomultiplier currents, avoids drifts in sensitivity, reduces the effects of accidental high-energy photons, and reduces thermal noise. The system yields digital data directly for computer processing. More significantly, it permits the data to be stored as collected for later processing, allowing flexibility and avoiding information loss due to on-line filtering. Further, there is no subjective tracking of a meter null point. We retain the important feature of continuous comparison of visual test flashes with infrared control flashes along the same optical path. The shutters are redesigned to eliminate polarizers and their leakage and to avoid bias by using the same sectors successively for test and control flashes. Cooling the photomultiplier tube has reduced the dark count from over 40/s at room temperature to <1 at 0°C. Counts from an artificial eye follow Poisson distributions closely. Deviations, with a real eye, are expected to permit identification and reduction of noise from biological sources, allowing pigment measurements at dim levels of brightness.

© 1985 Optical Society of America