Showing papers on "Contrast (vision) published in 1968"

Application of fourier analysis to the visibility of gratings

Abstract: 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.

The human eye as an optical filter

Abstract: The properties of the image-forming elements of the human eye are compared with the properties of the retina-brain system by considering them as two linear optical filters in cascade. The contrast sensitivity of the complete system is measured at a number of spatial frequencies by generating grating patterns of variable contrast and spatial frequency on an oscilloscope. Similar gratings are generated directly on the retina by means of Thomas Young interference, using a neon-helium laser as a coherent light source. In this manner the effect of the eye's dioptrics are bypassed. The quality of the image formed on the retina may then be determined from these two sets of measurements. The effect of pupil size on the quality of the retinal image is described to illustrate an application of the technique.

Visual factors in the design of computer-controlled CRT displays.

Abstract: This paper is concerned with the important visual variables that determine image quality on computer-controlled CRT displays A strategy is developed that leads to general conclusions about each variable even though most of these variables interact For each variable considered, the recommended range of values is determined on the basis of experimental evidence and is compared with the values presently used on displays Where discrepancies between these two exist, alternative solutions are mentioned Conclusions are(i) presently used values of display luminance, chromaticity (color), and resolution are adequate; (ii) several displays flicker; (iii) characters are large enough but may be marginal in terms of number of elements; (iv) luminance contrast is not adequate

Color and brightness factors in simulated and full-scale traffic sign visibility

Abstract: A TRAFFIC SIGN REQUIREMENTS STUDY WAS CONDUCTED ON FACTORS AFFECTING ABILITY AND ATTENTION VALUE. OUTDOOR OBSERVATIONS WERE MADE BY SUBJECTS RIDING OVER A STANDARDIZED COURSE. MATHEMATICAL MODELS BASED ON KNOWN VISUAL AND LOGICAL RELATIONSHIPS WERE TESTED AGAINST EACH SET OF LABORATORY RESULTS. IT WAS DETERMINED THAT AVERAGE VISIBILITY AND ATTENTION VALUE OF HIGHWAY SIGNS CAN BE ESTIMATED FROM CHARACTERISTICS OF LEGEND, SIGN AND ENVIRONMENTAL BACKGROUND. RELATIVE BRIGHTNESS AND CONTRAST OF SIGN-TO- BACKGROUND AND OF LEGEND-TO-SIGN ARE OF PRIMARY IMPORTANCE FOR VISIBILITY AND THE ATTENTION VALUE. IN VISIBILITY EFFECTS FROM COLORS, RELATIVE BRIGHTNESS IS OF MOST IMPORTANCE, BUT HUE CONTRAST ENHANCES THE BRIGHTNESS EFFECTS IN SOME CASES. FOR BEST VISIBILITY, A SIGN SHOULD BE DARKER AGAINST A BRIGHT DAY BACKGROUND BUT BRIGHTER AGAINST A DARK DAY OR NIGHT BACKGROUND. A FORMULA FOR ESTIMATING VISIBILITY EFFECTS IS SUGGESTED ON AN APPROXIMATE BASIS AND WITHIN LIMITS.

Letter and sign contrast, brightness, and size effects on visibility

Abstract: EXPERIMENTAL RESULTS ARE PRESENTED ON SIGN SIZE AND BRIGHTNESS AND LETTER-TO-SIGN BRIGHTNESS RATIO. THIS SERIES OF EXPERIMENTS WAS CONDUCTED TO MEASURE: (1) THE EFFECTS OF SIGN SIZE, BRIGHTNESS, AND LETTER-TO-SIGN BRIGHTNESS RATIO, AND (2) THE EFFECTS OF COMPETING SIGNS ON HIGHWAY SIGN VISIBILITY. A TYPICAL HIGHWAY SCENE FOR DAY OR NIGHT CONDITIONS WAS PROJECTED CONTINUOUSLY ON THE SCREEN IN FRONT OF THE SUBJECT. FROM TIME TO TIME UNDER AUTOMATIC CONTROL, KEYED BY ITS OWN RESPONSES TO AN AUXILIARY TASK, FOUR SIMULATED INTERSTATE GREEN HIGHWAY SIGNS WERE FLASHED ON THE SCREEN SUPERPOSED ON THE PREVIOUS HIGHWAY SCENE. IT WAS DETERMINED THAT THE EXPECTED ADVANTAGE FOR THE LARGEST OF FOUR SIGNS WAS FOUND CONSISTENTLY WHEN THE SIMULATED GREEN SIGNS WERE SEEN AGAINST EITHER DAY-SNOW OR THE NIGHT BACKGROUND. IT WAS DETERMINED THAT AN ADVANTAGE FOR VISIBILITY WAS ENHANCED BY A COMBINATION OF: (1) GREATER SIGN BRIGHTNESS, (2) GREATER RELATIVE SIGN SIZE, (3) CONTRAST OF SIGN AGAINST BACKGROUND, AND (4) LETTER-TO-SIGN BRIGHTNESS. FINDINGS SUGGEST THAT SILHOUETTE SEEING IS OF IMPORTANCE FOR VISIBILITY OR ATTENTION VALUE WHERE SIGNS ARE SEEN AGAINST A BRIGHT BACKGROUND. OPTIMUM COMBINATIONS OF THESE VARIABLES MUST BE FOUND TO OBTAIN BEST ATTENTION VALUE AND BEST LEGIBILITY.

The effect of a horizontally structured field and target brightness on visual search and detection times.

Abstract: An experiment was performed in which detection, times were compared for free and systematic search for a small visual target. Three different levels of target brightness were employed. The systematic search pattern consisted of a horizontal zig-zag between pairs of lines drawn on the visual field. The major findings were (1) that the systematic search and structured visual field only reduced the detection times of low contrast targets, and (2) that target contrast was an important variable independent of the condition of the visual field and the search instructions used. Subsequent analysis of the data revealed that the systematic search results could not be explained in terms of an increased uniformity of coverage of the visual field.

The Human Eye as an Optical Filter

Abstract: l'be properties of the image-forming elemen8 of the human eye are compared with the properties of the retina-brain system by comider- ing them as two linear optical filters in cascade. The contrast sensitivity of the complete system is m-ed at a number of spatial frequencies by generating grating patterns of variable amtrast and spatial frequency 011 an oscilloscope. Similar grating are genemted directly 011 the retina by meam of Thomas Yo- interference, using a nem-khn laser as a coherent ligbt source. In this manner the &ect of the eye's dioptrics are bypafed. The quality of the image formed 011 the retina may then be determiued from these two sets of measurements l'be effect of pupil size on the quality of the retinal image is described to illmtrate an application of the technique.

Color film evaluation system

Abstract: This specification discloses a system for evaluating the color balance density and contrast of color film by means of projecting the color film onto a screen and visually comparing it with a standard reference image on the screen, and to apparatus for such system.

Signal-to-Noise Ratios for Hologram Images of Subjects in Strong Incoherent Light

Abstract: When normal laser holographic technique is applied to subjects in strong incoherent illumination, an additional darkening of the photographic plate due to the incoherent light is introduced. Effects of the darkening on the signal-to-noise ratio of the reconstructed image are analyzed using a simplified model for photographic plate characteristics. Optimum values of the fraction of laser light used as reference, plate exposure, and S/N are obtained. Reconstructed images of reasonable contrast and pictorial quality have been obtained for 3-D subjects illuminated with substantial incoherent light intensity in addition to the laser light.

Resolution of the Hα double-limb controversy

Abstract: The discussion of the Hα double limb had reached the point where the question of its existence as a real solar phenomenon could not be resolved without new observations made with the Lockheed filter and the Mount Wilson spectroheliograph. A study of the instrumental profiles had indicated that there was sufficient off-band light to produce the observed inner limb step in the Mount Wilson instrument, but this analysis was not completely satisfactory because of limitations inherent in the measurement of instrument functions with a Hg-198 source. The instrumental profile work did indicate, however, that the spectral purity of the instruments in question could be substantially improved by the use of narrow-band interference filters. An experimental program was thus launched to determine the effect of such a blocking filter on the appearance of the Hα limb. The results of these observations with three Halle filter systems and the Mount Wilson spectroheliograph are that the inner limb completely disappears at the center of Hα when a blocking filter is used to reduce unwanted light, which originates at wavelengths beyond ±0.8 A. In addition, the contrast and visibility of the chromospheric fine structure is increased by eliminating the off-band light. Thus the experiment conclusively demonstrates that the apparent inner limb is not a solar feature but is due entirely to instrumental parasitic light.

The effects of tinted ophthalmic media on the recognition of red traffic signals

Abstract: At present (1968) there appear to be no controls on the coloration per se in sunglasses, motor vehicle windshields, etc. This paper shows that spectrally neutral grey lenses and windshields should have no effect on the perception and recognition of red traffic signal lights, in contrast to the detrimental effects expected for the use of certain coloured lenses and windshields. Allowable coloration limits are derived quantitatively, and a recommendation is made that the sale and use of coloured lenses and windshields should be regulated by legislation (A).

Heat treatment process to control contrast in nonsilver light sensitive systems

Abstract: THE CONTROL OF CONTRAST AND/OR GAMMA OF A NONSILVER PHOTOSENSITIVE FILM BY CONTROLLING THE TEMPERATURE OF THE FILM DURING OPTICAL DEVELOPMENT OF THE SAME.

Transfer functions in optical and visual testing. 1. Contrast transfer.

Abstract: ABSTRACT Summary, with a minimum of mathematics, of the concept of transfer functions and of their practical use. Transfer functions combine an evaluation of resolution and contrast and are most valuable and helpful in the testing of mirrors, lenses, and whole optical and electro‐optical systems, as well as in the evaluation of the performance of the eye.

Visual Acuity and Eye Movements

Abstract: Several longstanding theories and some recently published experimental evidence support the hypothesis that eye movements serve to improve acuity. By measuring eye movements during a simple acuity task, and during a control non-acuity task, we have shown that certain patterns of eye movement are characteristic of acuity tasks. Similarly, specific patterns of eye movement are generated during spatial localization tasks. These observations provide circumstantial evidence for the existence of mechanisms by which eye movements mediate acuity and spatial localization information. Through a comparison of acuity for stabilized retinal images with acuity for normal retinal images we have found that eye movements improve acuity very slightly at most, and that even this small improvement may be adequately accounted for by the residual fade ­ out effects commonly observed during prolonged viewing of stabilized images. Measurement of distance and angle estimation ability in both normal and stabilized vision reveals much the same result. Stabilization diminishes the accuracy of these estimates only slightly, as might be expected from the persistent fade effects observed during the stabilized trials. Residual retinal image movement in the stabilized trials was less than approximately 3 min arc. If such acuity improving mechanisms exist, they either operate on very small retinal image movements (less than 3 min arc), or they improve acuity only slightly (e.g., by less than 0.1 log unit in sine wave grating contrast sensitivity). Thus eye movements serve to sustain all sensory visual inflow by countering the slow process of fading of a stabilized image. They do not, however, play a vital role in the much more rapid processes which determine visual acuity as well as distance and angle estimation ability.