Visibility Threshold Research Articles

Peripheral contrast reversal inhibits visually evoked potentials in the fovea

Visual evoked potentials (VEP) to a 0.7° foveal test spot of different luminances projected on a white background were recorded with (1) a stationary peripheral black and white grating and (2) with contrast reversal of the peripheral grating. In the latter case, the near-threshold reduction of visibility of the test spot was accompanied by a reduction of VEP amplitude. To restore threshold visibility or to return to the former amplitude, the same luminance compensation was required. The VEP technique proved to be very sensitive; clear responses were obtained below the visibility threshold and the response curves showed saturation for a Weber ratio of only 1%.

Individual Responses to Lighting Parameters for a Population of 235 Observers of Varying Ages

The authors present the results of a comprehensive study of the effects of observer age upon various relationships between lighting parameters, task visibility, and dynamic visual performance. Included are studies of the effects of age (a) in reducing effective task visibility by increasing visibility threshold, (b) in increasing visibility needs for equal visual performance, (c) in increasing the benefits of increases in the level of illumination due to changes in the shape of the RCS function of luminance, (d) in suggesting use of substantially higher levels of illumination to compensate for losses in the transmittance of the ocular media, and (e) in increasing sensitivity to the disability glare effect, thus emphasizing an increase in the importance of lighting quality for older observers. Of perhaps special interest is the finding that disability glare effect is more than 1.5 times as great a detriment to visual performance in night driving as had been previously supposed.

Population Data for 140 Normal 20–30 Year Olds for use in Assessing Some Effects of Lighting upon Visual Performance

The authors present a cohesive body of experimental data on the accuracy of dynamic visual performance for arrays of five Landolt rings. Correlated data are presented on the visibility thresholds of the same observers for a single Landolt ring, which permit determinations to be made by the direct method of values of the visibility level involved in the visual performance experiments. Data are also presented to define the off-axis sensitivity profiles and the RCS function of luminance of the observers for Landolt ring test objects. Groups of up to 147 observers were used in different phases of the work.

Spatial adaptation of short-wavelength pathways in humans.

Color-selective spatial adaptation of the short-wavelength, or blue-sensitive, pathway was demonstrated. The adaptation was orientation selective and strongly monocular. Adaptation was assessed by measuring visibility thresholds for monochromatic gratings in subjects adapted to high-contrast violet gratings designed to stimulate only blue-sensitive cones. The results showed spatially selective, adaptable channels within the short-wavelength pathway.

Subjective assessment method for television digital codecs

Results of previous experiments to compare the performance of quality and impairment types of subjective grading scales for television pictures have been re-examined with particular reference to performance near the threshold of visibility. No evidence was found to suggest that the quality scale should be superseded for assessing television digital codecs.

Perceptibility of Composite Triple Beat

The findings outlined in this paper are the result of tests undertaken by a joint committee of the Canadian Cable Television Association and the Departmeht of Communications to determine the visibility threshold for composite triple beat under various conditions. The test results, which are discussed in detail, are used to establish a tentative specification and measurement method for carrier-to-triple beat ratio (CBR).

Development of a Compact Gas Turbine Combuster to Give Extended Life and Acceptable Exhaust Emissions

The paper describes the development history of the Olympus gas turbine combustor from the time that it was first applied to an industrial application in the early 1960s. The design improvements made — • to permit a change in fuel (from kerosene to diesel and/or natural gas), • a 60 percent increase in engine performance, and • to reduce emission levels — are detailed, and the in-service problems associated with these changes are also discussed. The emphasis is placed upon improvements in combustor life and capability to produce smoke levels well below the visible threshold, and significant success is shown to have been achieved in these two factors. The final sections of the paper are concerned with the latest on-going development effort, which is primarily to produce a low emission combustor that can be retrofitted into today’s engines.

Is spatial adaptation caused by prolonged inhibition?

Prolonged viewing of a grating raises the threshold for detecting gratings of similar orientations (Gilinsky, 1968) and spatial frequencies (Mantle and Sekuler, 1968; Blakemore and Campbell, 1969). It has been suggested that adaptation is caused by prolonged inhibition of the detection channels and not by neural fatigue (Blakemore, Carpenter and Georgeson, 1971; Tolhurst, 1972; Blakemore, Muncey and Ridley, 1973; Kulikowski and King-Smith, 1973; Kulikowski, Abadi and King-Smith, 1973; Sharpe, 1974; Dealy and Tolhurst, 1974). Two arguments have been advanced by these authors for this view. The first maintains that the narrow bandwidths obtained with threshold summation of sine-wave gratings (Sachs, Nachmias and Robson, 1971; Kulikowski and King-Smith, 1973; Kulikowski et al., 1973) reflect the range over which channels can be excited. The bandwidths obtained with adaptation (Blakemore and Campbell, 1969) and masking (Campbell and Kulikowski, 1966; Stromeyer and Julesz, 1972). being considerably broader, are claimed to reflect the spread of inhibition between channels. Recent studies on threshold facilitation (Stromeyer and Klein, 1974; Nachmias and Weber, 1975; Barfield and Tolhurst, 1975), the detectability of frequency modulated gratings (Stromeyer and Klein, 1975), and probability summation (King-Smith and Kulikowski, 1975) suggest that gratings may not be detected by narrowband mechanisms. Thus, it is not clear that there is a discrepancy between bandwidths obtained with adaptation and other methods. A second argument that adaptation is caused by inhibition between channels and not by fatigue was presented by Dealy and Tolhurst (1974) who showed that when a 4.0 c/deg adapting grating just reached its visible threshold it started to raise the threshold of a subsequently presented, 6.7 c/deg test grating (3/4 octave higher than the adapting pattern). The authors

Evaluation of retinal thresholds for C.W. laser radiation

Rhesus monkeys were exposed to ocular radiation from the yellow Une, 568.2 nm emitted by a krypton C.W. gas laser to determine the minimal ophthalmoscopic visible damage threshold on the retina. The effective dose 50 per cent probability points were determined for the extramacula sites having an exposure time range of 16–500 msec. Observations of the fundi were made prior to, during, and after each exposure, using a fundus camera. Damage thresholds for krypton were examined and compared to the reported results of helium-neon (632.8 nm), argon (488 nm), and neodymium YAG (1060 nm).

Cavitation in Helium at Low Frequencies

Previous studies showed that cavitation noise and the half-order subharmonic occurred at much lower thresholds than required to produce macroscopic visible bubbles in the superfluid helium II. It might be supposed that this low “audible” threshold is associated with the presence of subvisible resonant bubbles. Accordingly experiments are in progress at lower frequencies in anticipation that any such bubbles will be larger in size with consequently less distinction between audible and visible thresholds. One investigation employed a sandwich-type transducer coupled to the liquid through a velocity transformer operating at 9.93 kHz. Measurements of particle velocity were made by including the transducer as one arm of a Wheatstone bridge and monitoring the off-balance voltage. Pressure thresholds could be deduced from this information assuming progressive wave radiation. In helium II, at a temperature of about 2.0°K, pressure amplitude thresholds were found to be as follows: for subharmonic generation 0.4 mbar; for white-noise generation, 3.4 mbar; and for visible bubble generation about 30 mbar; i.e., there was still a large difference between audible and visible thresholds. A device designed to operate around 100 Hz is presently under construction.

The sonically induced cavitation of liquid helium

The onset of sonically induced cavitation in liquid helium at frequencies between 30 and 40 kHz has been studied. In helium II, two types of cavitation activity were identified: acoustic cavitation whose characteristic noise can be detected, and visible cavitation in which vaporous cavities grow to visible size. The onset of acoustic cavitation is statistical in nature with increasing event rates as the sound pressure amplitude is increased and whose threshold depends on the waiting time at that particular amplitude. The acoustic threshold sound pressure amplitude in helium II between 1.8° K andTλ was found to lie within 0.15 mb of 0.3 mb, the variation of ±0.15 mb occurring from one determination to another, whereas the sound pressure amplitude corresponding to the visible threshold was about a hundred times larger. These two distinct types of sonically induced cavitation appear to be unique to liquid helium. However, aboveTλ the two thresholds were found to coincide at a sound pressure amplitude within 0.4 mb of 0.8 mb. The characteristics of the onset of acoustic cavitation were found to be independent of applied static pressure of up to 1.5 atm above and belowTλ and in helium II they were unaffected by filtering, heat flushing, or rotating the liquid. The results suggest that liquid helium is nucleated by random events initiated by the ambient cosmic radiation or by vortices generated in the liquid, and they imply that at ultrasonic frequencies this liquid cannot withstand a tensile stress and behaves in this respect like water saturated with gas and containing dust motes. Attempts to determine the onset of acoustic cavitation by scattering light off the bubbles or by detecting sonoluminescence were not successful: The upper limit to the size of these bubbles was shown to be about 30 µm and the intensity of any sonoluminescence must have been less than 10−4 of that from cavitating water. The possibilities of exploiting the two types of cavitation activity in liquid helium in the construction of a posttriggerable ultrasonic bubble chamber for visualizing the tracks of ionizing particles are discussed, as are the theoretical background and future development of the work presented in this paper.

Visibility for road-signs

This study investigated legibility and readability for road signs under static and dynamic conditions. They were measured by varying the distance between the road sign and subjects. The results showed that (a) there are some ranges of maximum distances with respect to percent contrast or luminance ratio of a randolt ring to its background; (b) visible distance threshold increases with the increase of luminance of white field except percent contrast of 100 percent; (c) dynamic acuity decreases with the increase of target speed under all conditions of luminance of target. (d) in night driving, readability for the road sign decreases with the increase of car speed at relative small luminance difference between the road sign and its surroudding field.

Retinal profiles. A psychophysical test of rod and cone sensitivity.

Modifications of the Goldmann-Weekers adaptometer are described which permit sensitive measurements of visibility thresholds to deep red and deep blue light. The test spots subtend six minutes of arc and fixation is controlled by a movable luminous orange point of light. Consistent measurements, usually made in the vertical meridian, suggest that data obtained with the red test spot represent cone function and data with the blue test spot represent rod function (retinal profiles). Curves are shown for eyes with normal function, color blindness, night blindness, and central defects. It appears that early damage to the macular area (as in chloroquine phosphate retinopathy) can be assessed by this method before it is observed ophthalmoscopically and before it produces notable losses in visual fields or acuity.

Laser-Induced Prebreakdown and Breakdown Phenomena Observed in Cloud Chamber

Direct observation of initiating electrons below the threshold for visible breakdown discharge is reported. With the cloud-chamber techniques used, minimum detectable ionization is estimated at 5000 ion pairs/${\mathrm{cm}}^{3}$. Production of initial electrons is ascribed to photoionization, in a manner which is consistent with Phelp's model. Above the visible breakdown threshold, thermal-gradient effects overcome all other sources of droplet formation in the cloud chamber.

Visible Cavitation in Liquid Helium

Visible cavitation has been obtained readily in liquid helium using a barium titanate cylinder resonant at about 46 kc/sec. The nature of the cavitation changes in various temperature régimes, and these changes are reflected by changes in the threshold, as measured by the driving voltage of the transducer. Below about 2.13°K, the threshold for visible cavitation is an order of magnitude higher than the threshold for noise; but between 2.13°K and the λ point the visibility threshold drops and becomes comparable to the noise threshold. Static pressure has a much greater effect on the visibility threshold than on the noise threshold. The results are discussed and the conclusion is reached that the information obtained is more pertinent to the dynamics of the cavities than their nucleation.

On Visibility Threshold of Colored Light Sources (II)

On Visibility Threshold of Colored Light Sources (II)

Studies on vaccinia virus plaque formation and its inhibition by interferon: I. Dynamics of plaque formation by vaccinia virus

Plaques of vaccinia virus in chick embryo cell monolayers without agar emerged into visibility according to a normal sigmoid curve. The distribution of plaque diameters could be approximated by a normal Gaussian curve, with a variable proportion of plaques hidden below the threshold of visibility (infraplaques). Between 32 and 48 hours, and possibly up to 57 hours after inoculation, all plaque diameters increased linearly with time at the same rate. The total plaque area increased between 32 and 48 hours at an almost exponential rate. Strains of vaccinia virus differ in the rate of appearance of primary and secondary plaques.

Scotopic Visibility Thresholds with Age

Scotopic Visibility Thresholds with Age

Changes in scotopic visibility thresholds with age.

Although it has been shown in a number of studies that scotopic vision decreases with age,1it is not clear to what proportion different factors contribute to this aging process. Changes in absorption, scatter, and reflectivity of the dioptric media, as well as changes in the pupillary aperture and more central visual pathways may all be involved to varying degrees. Pupillary dilatation in darkness does diminish with age2and has detectable effects on scotopic vision.1Senescent changes in the transmissivity of the human lens in vivo have been shown to occur3and must have effects on scotopic visibility independent of pupil size. It was considered of interest to study the effects of lens senescence on dark-adapted thresholds in a manner that was relatively independent of other aging factors. This was achieved by comparing the scotopic thresholds of normal and aphakic observers of various ages to different

Visible sodium airglow on the night of January 30-31, 1960

An enhanced region of the sky was observed visually and photometrically during routine airglow observations. Photometric observations identified as sodium airglow and determined that the threshold of visibility against sky background was 1.3× 0−11 watts/cm2 ster. A correlation between the variability of the sodium and O I (5577 A) airglows was observed. The data indicate that the observed motions are apparent and show a striking similarity to other ionospheric phenomena. It is suggested that the dynamics of the upper atmosphere connect the various observed phenomena.