Simon Shlaer

An Adaptometer for Measuring Human Dark Adaptation

An instrument is described with which one can measure the course of human visual dark adaptation under specified but flexible conditions useful for physiological and clinical investigation. The preadapting light adaptation is controlled in brightness, retinal location, and duration, and the subsequent dark adaptation of a specific retinal area is measured with light flashes whose spectral composition and duration are controlled.

Size, Shape, and Contrast in Detection of Targets by Daylight Vision. I. Data and Analytical Description1

The influence of size and symmetry has been studied on the contrast required for the recognition of rectangular targets against background brightnesses of 2950 and 17.5 footlamberts. Targets less than 2 minutes in diameter require the addition of a constant total light flux to the background. Larger targets require less contrast but more total flux as the area increases, until beyond 200 square minutes when the required contrast becomes independent of area. For areas below 100 square minutes, square targets are most efficient for their area; the greater the ratio of length to width the greater the contrast required. All the measurements can be unified on the supposition that the visually critical region of a target is a ribbon just inside its perimeter and about 1 minute wide. Evidently, contrast is not judged over the area of a target, but across its boundary.

Size, Shape, and Contrast in Detection of Targets by Daylight Vision. II. Frequency of Seeing and the Quantum Theory of Cone Vision*

Frequency-of-seeing curves have been obtained for targets having various image perimeters at background brightnesses of 2950 and 17.5 foot-lamberts, respectively. A description of the data has been obtained on the basis of the assumption that the absorption of a light quantum by a foveal cone is a random event which is subject to the laws of chance. On this basis the data indicate that the detection of a target takes place across the image boundary; that in order to detect the target, at least one of the cones along the boundary must absorb at least 4 quanta, and that this critical number of quanta is the same for each of the two background brightnesses investigated. At the higher brightnesses, this critical number of quanta absorbed from the target is about equal to the random fluctuation to be expected in the number absorbed from the background during the critical time of one exposure.

A Photoelectric Transmission Spectrophotometer for the Measurement of Photosensitive Solutions

This spectrophotometer was designed specifically to measure in the visible region the absorption spectrum of visual purple, the photosensitive substance of the retinal rods. It is independent of both the photoelectric cell and amplifier characteristics, relying only on the law of crossed polarizing prisms. The sensitivity of the detecting system is high enough so that the measuring light produces no detectable effect upon the measured solution. The entire system can measure a transmission of 1 percent to a precision of about 1 percent.

Contrast Discrimination Charts for Demonstrating the Effect of Anoxia on Vision

A test of visual contrast discrimination has been developed for demonstrating visual impairment due to anoxia at altitudes of 15,000 feet or above. It is made of photographically printed Landolt rings only slightly darker than their white background, and varying in contrast so as to cover the range just barely discriminated by different individuals at sea level and at altitude. Since moderate anoxia has little effect on vision in daylight, but causes a deterioration in contrast discrimination at twilight and night, the tests are designed to be viewed at a twilight brightness of about 0.1 millilambert. Trials with aviation cadets and other untrained persons have shown that the tests fulfill their purpose of demonstrating the effect of anoxia on vision. They also show the existence of large individual differences in the score at sea level and in the effect of altitude. The tests have been used in researches with skilled subjects on the effects of various procedures for producing anoxia, and have successfully given a quantitative measure of the resulting visual impairment.