Are Letters better than Gratings?

Optometry and Vision Science, 2014

Purpose. An iPad-based letter contrast sensitivity test was developed (ridgevue.com) consisting of two letters on each page of an iBook. The contrast decreases from 80% (logCS = 0.1) to 0.5% (logCS = 2.3) by 0.1 log units per page. The test was compared to the Pelli-Robson Test and the Freiburg Acuity and Contrast Test. Methods. Twenty normally sighted subjects and 20 low-vision subjects were tested monocularly at 1 m using each test wearing their habitual correction. After a 5-minute break, subjects were retested with each test in reverse order. Two different letter charts were used for both the Pelli-Robson and iPad tests, and the order of testing was varied systematically. For the Freiburg test, the target was a variable contrast Landolt C presented at eight possible orientations and used a 30-trial Best PEST procedure. Repeatability and agreement were assessed by determining the 95% limits of agreement (LoA) T1.96 SD of the differences between administrations or tests. Results. All three tests showed good repeatability in terms of the 95% LoA: iPad = T0.19, Pelli-Robson = T0.19, and Freiburg = T0.15. The iPad test showed good agreement with the Freiburg test with similar mean (TSD) logCS (iPad = 1.98 T 0.11, Freiburg = 1.96 T 0.06) and with narrow 95% LoA (T0.24), but the Pelli-Robson test gave significantly lower values (1.65 T 0.04). Low-vision subjects had slightly poorer repeatability (iPad = T0.24, Pelli-Robson = T0.23, Freiburg = T0.21). Agreement between the iPad and Freiburg tests was good (iPad = 1.45 T 0.40, Freiburg = 1.54 T 0.37), but the Pelli-Robson test gave significantly lower values (1.30 T 0.30). Conclusions. The iPad test showed similar repeatability and may be a rapid and convenient alternative to some existing measures. The Pelli-Robson test gave lower values than the other tests.

Journal of the Optical Society of America A, 1999

Earlier studies have reported that grating resolution is sampling-limited in peripheral vision but that letter acuity is generally poorer than grating acuity. These results suggest that peripheral resolution of objects with rich Fourier spectra may be limited by some factor other than neural sampling. To examine this suggestion we formulated and tested the hypothesis that letter acuity in the periphery is sampling-limited, just as it is for extended and truncated gratings. We tested this hypothesis with improved methodology to avoid the confounding factors of target similarity, alphabet size, individual variation, peripheral refractive error, and stimulus size. Acuity was measured for an orientation-discrimination task (horizontal versus vertical) for a threebar resolution target and for a block-E letter in which all strokes have the same length. We confirmed previous reports in the literature that acuity for these targets is worse than for extended sinusoidal gratings. To account for these results quantitatively, we used difference-spectrum analysis to identify those frequency components of the targets that might form a basis for performing the visual discrimination task. We find that discrimination performance for the three-bar targets and the block-E letters can be accounted for by a sampling-limited model, provided that the limited number of cycles that are present in the characteristic frequency of the stimulus is taken into account. Quantitative differences in acuity for discriminating other letter pairs (e.g., right versus left letters E or characters with short central strokes) could not be attributed to undersampling of either the characteristic frequency or the frequency of maximum energy in the difference spectrum. These results suggest additional tests of the sampling theory of visual resolution, which are the subject of a companion paper [

Clinical vision sciences, 1988

I. A consideration of methods for assessing contrast sensitivity leads to the conclusion that, for a clinical test, letters are more suitable than gratings. 2. A letter chart is described in which letters decrease in contrast but not in size. The letters are arranged in groups of three; successive groups decrease in contrast by a factor of IfJ2 from a very high contrast down to a contrast below the threshold of normal observers. A subject's threshold is taken to be the lowest contrast for which at least two letters in a group are correctly reported. 3. A mathematical model of the observer and the chart-testing procedure has been used to predict how the accuracy and repeatability of the test score depend on the parameters of the chart and observer. This reveals that even a low probability of misreporting supra threshold letters will seriously bias the test score if the passing criterion is strict, requiring correct report of all letters in each group, but will have little effect if the passing criterion is less strict. This effect of the passing criterion may explain Rubin's [Clin. Vision Sci. 2, No. I (1987)] finding that the new test, which uses a lenient criterion, has excellent test-retest reliability, much higher than the Ginsburg [Am. J. Optom. Physiol. Opt. 61,403-407 (1984)] chart with its strict criterion.

Clinical and Experimental Optometry, 2010

The explosive growth of automatic examination of the eye, in particular for determining refractive corrections, invites an analysis of the strengths and limitations of these devices and their role in clinical decisions. Subjective refraction procedures are based on a patient's visual responses and these embrace many levels of additional optical and neural processing and factors involving the higher-order nervous system and perception. Because the ultimate mission is the maintenance and improvement of a patient's visual experiences, the optometric examination necessarily extends beyond the employment of automatic devices and therefore, should include engaging the patients in tests of their visual functions.

International Journal of Creative Research Thoughts, 2021

Vision is the most dominant of the five senses and plays an important role in every second of our lives. It is integral to interpersonal and social interactions in face-to-face communication where information is conveyed through non-verbal speech. Visual acuity (VA) is a measure of central vision, indicates how clearly an individual can see an object. It may range from normal to no light perception. It is the most commonly used and universally understood measure of visual function. It is important to measure the visual acuity because it provides a simultaneous measurement of central corneal clarity, central lens clarity, central macular function, and optic nerve conduction. It has to be tested in both eyes for both distance and near objects. If the patient is using spectacles, need to take reading with and without the spectacles, and is measured in each eye separately. Usually, the distance visual acuity is tested by using 'Snellen's chart' for educated and literate adults; 'Tubmling E chart' for illiterates; and 'Lea symbols' for the small children and infants. There are things to follow naturally to boost the eye health and potentially enhance the vision and visual acuity. The present literary review was undertaken to integrate the knowledge of visual acuity and its importance. Details and facts on visual acuity were gathered from the authentic books, published journals, PubMed, google scholar, research gate by using words like visual acuity, vision, eye health, central vision, eye examination etc., then analyzed and summarized the data. Therefore, it is concluded that the measurement of visual acuity is a most sensitive test of the integrity of the visual system and accomplish all standard criteria of a good screening test with minimal cost or risk to the patient.

A simple reading test that can be undertaken by disabled readers is described. The test is suitable for use in the optometric clinic, taking less than 2 minutes to:cdn?:zter per passage. Test-retest scores are acceptably reliable. The test reveals increases in the speed of reading when coloured overlays are used both (1) in children who have made long-term use of coloured overlays for reading, and (2) in children who will subsequently do so.

Investigative Ophthalmology Amp Visual Science, 1997

Purpose. To design and evaluate a new vision test that combines low contrast and reduced illumination to stress the visual system and be sensitive to subtle alterations in function. Methods. A simple new clinical test, the Smith-Kettlewell Institute Low Luminance (SKILL) Card, is designed to measure spatial vision under conditions of reduced contrast and luminance using normal office lighting. The SKILL Card consists of two near acuity charts mounted back to back. One side has a chart with black letters on a dark gray background designed to simulate reduced contrast and luminance conditions. The other side has a high-contrast, black-on-white letter chart. The SKILL score is the acuity loss (number of letters) between the light and dark sides. Results. Age norms for a large normal population have been established and show that test scores increase with age, particularly after age 50. Repeatability is as good as that of standard Snellen acuity. The SKILL score is affected minimally by blur, but it is affected by large variations in light level. SKILL scores are sensitive to the presence of visual disease such as "recovered" optic neuritis. Conclusions. The SKILL card allows quick, reliable measurement of the effect of reduced luminance and contrast on acuity. SKILL scores are not correlated with other vision measures in patients with optic neuritis, which shows that the SKILL card measures a different dimension of vision function than existing clinical tests. Invest Ophthalmol Vis Sci. 1997;38:207-218. J. he Smith-Kettlewell Institute Low Luminance (SKILL) Card was developed to provide a simple, rapid, and inexpensive method for measuring vision function at reduced contrast and luminance. The combination of low contrast and reduced luminance is expected to increase sensitivity to vision changes caused by age and disease compared to standard highand low-contrast acuity measures and simulates conditions under which people most often report real-world task performance problems. The familiar black-on-white letter chart, originally

Ophthalmic and Physiological Optics, 2006

Determining causes of poor reading ability is an important step in trying to ameliorate reading performance in low-vision patients. One important parameter is word acuity. The principal aim of the current study is to develop a method to reliably measure acuities for isolated lowercase letters and words of differing length that can be used to test low-vision patients. Using isolated stimuli means that testing is relatively free of potential crowding and/or distracting attentional effects from surrounding words, it is unambiguous which stimulus subjects are trying to read and response times can be recorded for each stimulus. Across a series of experiments, subjects with normal vision were asked to read isolated lowercase single letters and lowercase words of 4, 7 and 10 letters, in separate tests. Acuities for uppercase Sloan letters were also measured to provide a reference, as they are commonly used to measure visual acuity. Each test was based upon the design principles and scoring procedures used in the Bailey-Lovie and ETDRS charts. Acuities for uppercase Sloan letters were found to be equivalent whether measured using ETDRS charts or the computer-based method. Measurement of acuities for lowercase single letters and lowercase words of 4, 7 and 10 letters had a reliability that was no worse than acuities for uppercase Sloan letters. Lowercase word acuities were essentially independent of word length. Acuities for single lowercase letters and lowercase words were slightly better than uppercase Sloan letters acuity. Optimal processing of lowercase single letters and 4-, 7-and 10-letter words occurred at character sizes that were at least 0.2-0.40 log MAR above acuity threshold, i.e. between 1.5 and 3 times threshold acuity for that particular stimulus. In general, critical character sizes appear similar across word lengths as progressive increases or decreases in these values were not observed as a function of the number of letters in the stimulus. We conclude that a computer-based method of stimulus presentation can be used to obtain highly repeatable measures of acuity for lowercase single letters and lowercase words in normal vision.

Optometry and Vision Science, 1994

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