1. The contrast thresholds of a variety of grating patterns have been measured over a wide range of spatial frequencies.2. Contrast thresholds for the detection of gratings whose luminance profiles are sine, square, rectangular or saw-tooth waves can be simply related using Fourier theory.3. Over a wide range of spatial frequencies the contrast threshold of a grating is determined only by the amplitude of the fundamental Fourier component of its wave form.4. Gratings of complex wave form cannot be distinguished from sine-wave gratings until their contrast has been raised to a level at which the higher harmonic components reach their independent threshold.5. These findings can be explained by the existence within the nervous system of linearly operating independent mechanisms selectively sensitive to limited ranges of spatial frequencies.

Application of fourier analysis to the visibility of gratings

Shannon's rate-distortion function provides a potentially useful lower bound against which to compare the rate-versus-distortion performance of practical encoding-transmission systems. However, this bound is not applicable unless one can arrive at a numerically-valued measure of distortion which is in reasonable correspondence with the subjective evaluation of the observer or interpreter. We have attempted to investigate this choice of distortion measure for monochrome still images. This investigation has considered a class of distortion measures for which it is possible to simulate the optimum (in a rate-distortion sense) encoding. Such simulation was performed at a fixed rate for various measures in the class and the results compared subjectively by observers. For several choices of transmission rate and original images, one distortion measure was fairly consistently rated as yielding the most satisfactory appearing encoded images.

The effects of a visual fidelity criterion of the encoding of images

This work presents a perceptual-based no-reference objective image sharpness/blurriness metric by integrating the concept of just noticeable blur into a probability summation model. Unlike existing objective no-reference image sharpness/blurriness metrics, the proposed metric is able to predict the relative amount of blurriness in images with different content. Results are provided to illustrate the performance of the proposed perceptual-based sharpness metric. These results show that the proposed sharpness metric correlates well with the perceived sharpness being able to predict with high accuracy the relative amount of blurriness in images with different content.

A No-Reference Objective Image Sharpness Metric Based on the Notion of Just Noticeable Blur (JNB)

The perception of contrast was measured in humans by a technique of subjective contrast-matching, and was compared with contrast sensitivity as defined by threshold measures. 2. Contrast-matching between different spatial frequencies was performed correctly (especially at frequencies above 5 c/deg) despite the attenuation by optical and neural factors which cause large differences in contrast thresholds. 3. Contrast-matching between single lines of different widths was also veridical, and was not limited by the spatial integration (Ricco's Law) present at threshold. Adaptation to gratings altered the appearance of lines, and this could be best understood in Fourier terms. 4. The generality of these results was shown by matching the contrast of pictures which had been filtered so that each contained a one octave band of spatial frequencies. 5. Within the limits imposed by threshold and resolution, contrast-matching was largely independent of luminance and position on the retina. 6. Six out of eleven astigmatic observers showed considerable suprathreshold compensation for their orientation-specific neural deficit in contrast sensitivity. 7. These results define a new property of vision: contrast constancy. It is argued that spatial frequency channels in the visual cortex are organized to compensate for earlier attenuation. This achieves a dramatic 'deblurring' of the image, and optimizes the clarity of vision.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1975.sp011162

Contrast constancy: deblurring in human vision by spatial frequency channels.

Several authors have measured the detection ability of human observers for objects in correlated (nonwhite) noise. These studies have shown that the human observer has approximately constant efficiency when compared with a nonprewhitening ideal observer. In this paper we add a frequency-selective mechanism to the ideal-observer model, similar to the channel mechanism that has been demonstrated through experiments that measure a subject's ability to detect grating stimuli. For a number of detection and discrimination tasks, the nonprewhitening ideal-observer model and the channelized ideal-observer model yield similar performance predictions. Thus both models seem equally capable of explaining a considerable body of psychophysical data, and it would be difficult to devise an experiment to determine which model is more nearly correct.

Addition of a channel mechanism to the ideal-observer model.

Part I of this series described a method which yielded the sine-wave response of the complete visual system by assuming that the Mach phenomenon is the result of a convolution, in the optical sense, of the object luminance distribution with the effective spread-function of the visual system This second paper is concerned with measuring the response of the visual system to sine-wave and square-wave spatial distributions using the threshold criterion of contrast sensitivity Particular emphasis is placed on the low spatial frequencies, a region which is believed to be critically important in the mechanism of visual contrast phenomena Results strongly imply interaction of two basic mechanisms in the visual system These mechanisms may be characterized individually as a low-pass filter component (optical) and a high-pass filter component (neural, chemical, electrical, etc)

Sine-Wave Response of the Visual System. II. Sine-Wave and Square-Wave Contrast Sensitivity*†

In order to develop a complete specification for the appraisal of the quality of a photographic system by the use of sine-wave response functions, the sine-wave response of the complete visual system must also be known. A method has been worked out for determining the above-mentioned response of the eye. Whenever the eye views a diffuse luminous boundary in which a sudden change of luminance gradient occurs, a bright line appears at the junction of the gradient and the higher luminance, and a dark line at the junction of the gradient and the lower luminance. This phenomenon is a purely subjective one, because physical measurements of the luminances involved show no luminance higher or lower than the two between which the gradient exists.A visual slit photometer has been built with which the subjective intensity distribution may be evaluated directly. By application of Fourier analysis, the objective and subjective intensity distributions have been combined to yield the sine-wave response of the visual system.

Sine-wave response of the visual system. I. The Mach phenomenon.

Automatic gloss meter

The writer believes that the reported reduction in time necessary to dark-adapt the eyes by wearing red goggles for a period of approximately 30 minutes is due to the much reduced brightness of the visual field as viewed through the goggles and not to the wave-length region involved. Data are presented to show that a comparable effect is secured when neutral rather than red filters are used to screen the visual field.

The Effect of Hue on Dark Adaptation

The term “graininess” as applied in this paper refers to the subjective factor which has been well defined by Jones and Deisch as: “the sensation or impression of nonuniformity in a photographic deposit, produced upon the consciousness of the observer when such a deposit is viewed.” The physical aspect of the problem, that is, the size of the ultimate silver grains, the size frequency and the number of grains per given area, has been defined in this laboratory as “granularity” and in this paper the term will be so used. Jones and Deisch used a method of measurement in which the observer determined the distance at which a magnified image of the deposit ceased to appear as nonhomogeneous. This is essentially the same as measuring the necessary magnification when the viewing distance is kept constant. It is the latter method which has been applied in the design of the instrument to be described. A number of methods for the measurement of graininess have been proposed at various times, among which have been the so-called physical methods, such as the measurement of Callier’s “Q” factor or the scanning of the silver deposit and recording its nonhomogeneity by means of the response of a photoelectric cell or other light-sensitive device. These methods measure the granularity of the deposit rather than its graininess. In the last analysis the point of chief interest is the effect upon the consciousness of the observer, and so far as present knowledge is concerned a subjective method is the only one which gives promise of direct interpretation. Following the description of the apparatus and the defining equation some results showing the relation between field brightness and graininess are given for a range of brightnesses from 6 to 100 millilamberts. Data showing the effect of constant field brightness upon the graininess of several different emulsions are presented and a proposal for applying the results to the problem of graininess measurement is made. Results are also included on Q factor determinations as a measurement of graininess, together with a description of the apparatus used for this investigation.

E. M. Lowry

Papers

Sine-Wave Response of the Visual System. II. Sine-Wave and Square-Wave Contrast Sensitivity*†

Sine-wave response of the visual system. I. The Mach phenomenon.

Automatic gloss meter

The Effect of Hue on Dark Adaptation

An Instrument for the Measurement of the Graininess of Photographic Materials