Selig Hecht

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.

The Colors of Natural Objects and Terrains, and Their Relation to Visual Color Deficiency*

The colors of many natural objects and terrains were determined by visual matching with samples of known spectral reflectance (Munsell’s A Color Notation). There are only three important groups of colors. Foliage lies in a yellow-green region of dominant wave-length 557–574 mμ. Earths range between yellow and orange-red, 576–589 mμ. Water, sky, and distant objects are blue, 459–486 mμ. Colors of autumn foliage spread into the yellows and reds to the end of the spectrum. The outstanding characteristic of these terrain colors is their lack of saturation, most having an excitation purity of 40 percent or less. Furthermore, objects quickly lose saturation with increasing distance from the observer, so that even in the clearest weather encountered no object over 312 miles away had an excitation purity of over 11 percent. To the color-deficient observer these colors appear even less saturated than to the normal. In addition, the yellow-to-green region of the spectrum, where the majority of discriminations must be made, is one in which color blinds show poor hue discrimination. Thus, in both hue and saturation the color deficient is at a decided disadvantage in discriminating terrains.

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.

Brightness, visual acuity and colour blindness.

Summary1.Measurements have been made of the dark-adapted foveal threshold of normal and colour blind persons in five parts of the spectrum using a 1° circular test field.2.Compared to normals, protanopes (red-blinds) show an elevation of the threshold which increases slowly from blue to yellow and rises rapidly thereafter until in the red the threshold is more than ten times as high as normal. Deuteranopes (greenblinds) do not show so high an elevation, their maximum in the green being only about 70% above normal.3.These threshold elevations correspond to luminosity losses in the spectrum. For the protanope the total loss in the spectrum is nearly one-half of the normal luminosity; for the deuteranope it is nearly two-fifths of normal.4.Measurements were made of the illumination necessary for equal visual performance in normal and colour blind subjects. It was found that protanopes require 119% and deuteranopes 55% more light to achieve a visual acuity of 0.29 reciprocal minutes. These values agree closely with those found in the luminosity measurements.5.Such losses support the idea that colour blindness corresponds to the loss of one of the three receptor systems usually postulated to account for normal colour vision. However, the colour sensations reported by colour blind persons, especially monocular colour blinds, do not support the idea of a lost or inactivated receptor system. A fresh explanation for colour blindness is needed to reconcile these conflicting kinds of evidence.

The Visibility of Lines and Squares at High Brightnesses1

Field experiments show that a wire may be seen against a sky of high brightness when its diameter subtends only half a second of visual angle. For this the wire must be at least one degree long. Shorter wires must be thicker. Silhouetted squares may be seen at angles of only 18 seconds. Considering length and width, squares are more efficient visual targets than fine lines by about a factor of 3.

Eine Grundlage für die Beziehung zwischen Sehschärfe und Beleuchtung

I. Die F~higkeit des Auges, Einzelheiten zu un*erscheiden, ~ndert sich mit der 13eleuchtung. S i e kanil gemessen werden als Sehschg~rfe, als reziproker Wert des Winkelabstandes zwischen zwei Konturen, die eben noch getrennt erkannt werden k6nnen. Diese Funk*ion des Auges ist seit nahezu zwei Jahrhunderten bekannt und yon zahlreiehen Forscheril uiltersucht worden, am genauesten vielleicht von K6~Ia (I). K6~Ia land vor 3 o Jahren, dab die SehschXrfe nahezu mit dem Logarithmus der Beleuchtungsst~rke w~chs*. Seine Ergebnisse sind in Fig. 1 wiedergegeben.

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.

Photochemical Nature of the Photosensory Process

On page 243, Vol. ii, No. 3, January 20, 1920, line 12 (counting each equation as a line) for a photochemical reaction read a reversible photochemical reaction.