Keith F. Stultz

Variation of Visual Acuity with Various Test-Object Orientations and Viewing Conditions*

Visual acuity was measured in terms of the reciprocal of the angle in minutes subtended by the individual lines in a parallel-line test object at the limit of perception of the lines when the observer was allowed to see the target for less than one millisecond. Visual acuity, measured with the lines passing either vertically or horizontally through the visual field, was found to be approximately 20 percent higher than when measured with the lines passing diagonally through the visual field at angles of 45 and 135 degrees with the horizontal. These results indicate that the variation in the visibility f a parallel-line test object as a function of its orientation is not produced by preferential directions for eye movements. The variation of visual acuity with test-object orientation was also measured as a function of pupil diameter and fixation point, but these data do not give any conclusive evidence as to the factors producing the variations.

Visual Acuity as Measured with Various Orientations of a Parallel-Line Test Object*

Visual acuity has been measured in terms of the reciprocal of the angle in minutes subtended by the individual lines in a parallel-line test object at the limit of perception of the lines. Both clear and opaque lines in this test object were of equal width. It was found that the limit of perception of the lines in such a test object depends upon the orientation of the lines. Visual acuity, measured with the lines passing diagonally through the visual field at an angle of 45 degrees to the horizontal, is between 10 and 20 percent lower than that measured with the parallel lines passing vertically or horizontally through the visual field.

Frequency and Amplitude of Ocular Tremor

Data are presented on the magnitude and frequency of the involuntary eye motions (rapid motions superimposed on slow drifts) that take place during attempted fixation on a target. A record of these motions was obtained by directly photographing a blood vessel of the eye. Vertical and horizontal motions were recorded for both monocular and binocular fixation. The rapid motions were found to have an average frequency of 50 cps and an average amplitude of 1.2 minutes of arc. These data are in approximate agreement with previous data of Adler and Fliegelman and of Ratliff and Riggs. Simultaneous tracks of head and eye motions show that the rapid eye motions are independent of head motions.

Experimental Study of rms Granularity as a Function of Scanning-Spot Size*

Experimental data on the constancy of Selwyn granularity, S=σ(D)·(2a)12, with variations in scanning-aperture diameter are presented. Selwyn granularity S is found to be essentially constant for a series of aperture diameters from 7.5 to 384 μ if the sample is clean and has no macroscopic variations in density. The effect of sample imperfections, such as density wedging, streaks, scratches, and dirt is discussed.

Relation between Graininess and Granularity for Black-and-White Samples with Nonuniform Granularity Spectra*

In order to determine the relation between the visual impression of graininess and the objectively determined granularity of a wide range of granularity types, a series of samples was prepared containing uniformly exposed black-and-white materials and prints (transparencies) containing varying degrees of mottle. These samples were judged by the method of paired comparisons at different magnifications and scanned to obtain the standard deviation σ(D) for a wide range of scanning apertures. The psychophysical relationship between the two functions, (1) graininess versus magnification and (2) granularity σ(D) versus the square root of the scanning area, depends on the character of the scanning operation performed by the eye. An estimate of the size of the effective scanning spot of the eye can be obtained from these data.

Photographic Granularity and Graininess. VIII.* A Method of Measuring Granularity in Terms of the Scanning Area Giving a Threshold Luminance Gradient†

It is shown that, when a photographic image of uniform density is viewed under such conditions that graininess is just perceptible, the average spatial luminance gradient on the cones of the eye is a function of the density of the sample alone if the illuminance on the sample is held constant. This function, which bears a logarithmic relation to net density and is independent of the nature of the photographic image, is herein termed the threshold gradient sensitivity function of the eye for graininess. Granularity is defined in terms of the diameter of the scanning aperture that will produce this threshold gradient on the cones of the eye for the density of the sample in question. It is shown that granularity as thus expressed can be multiplied by a constant factor to give the same numerical value of threshold graininess that would be obtained by measuring the sample visually under standard conditions.

Microdensitometer for Photographic Research

A recording microdensitometer is described that has a resolving power of 800 lines/mm when the scanning aperture is 1 μ wide and 200 μ long. The response is linear in density to 3.0 for the same aperture. By narrowing the aperture to 0.1 μ, the resolving power can be increased to 1800 lines/mm. The record is made either on chart paper with rectangular coordinates, on punched cards, or on both.

Role of Chromaticity Difference in Color Graininess Judgments

It is known that, for black-and-white materials, the threshold “graininess” of a random pattern of dots on a background of a different luminance varies in a uniform manner with the difference between the log luminance of the dot and that of the background. For similar samples prepared with dots of one color on a background of a different color, it was found that this relationship does not hold. Data are presented showing that a correlation exists between ΔE and graininess, where ΔE=[(ΔD)2+K(ΔC)2]12. Here, ΔD is the log luminance difference and ΔC the chromaticity difference (as measured on a uniform chromaticity scale) between the dots and the background. As a preliminary to the main experiment, the visual brightness of the 48 samples was measured for a 2-degree field, and the corresponding luminance values were computed. All the differences were erratic and were within the limits of experimental error.

Roles of Sharpness and Graininess in Photographic Quality and Definition

Sensitometrically matched photographic prints were judged by a number of observers for sharpness, graininess, and two over-all attributes that were termed in the instructions “picture quality” and “definition.” The results indicate that the two latter terms had definite meanings for the observers, but the term “picture quality” led the observers to weight sharpness and graininess about equally while the term “definition” led to a high correlation with sharpness and a low correlation with graininess. Both the method of ranking and the method of paired comparisons were used, and they were found to be about equally capable of disclosing inconsistencies.

Photographic Granularity and Graininess. IX. Techniques and Equipment for the Objective Measurement of Graininess

A photoelectric scanning instrument is described which gives granularity values in terms that can be converted into the graininess values that would be obtained by an observer viewing a magnified image of the sample under standard conditions. The procedure is to determine the critical scanning aperture that results in a signal corresponding to the threshold-gradient sensitivity of the eye for the density of the sample. The techniques for making two types of routine graininess measurements are described in detail. Class I measurements give the graininess of samples at a density of 0.8. Class II measurements give granularity numbers that indicate the relative graininess to be expected of matched prints made from the negatives.