Performance of video-displayed letter charts

A software package was developed for an Apple-II microcomputer to display Snellen letters as test charts for high speed measurement of visual acuity. With direct viewing of a 30 cm (12 inch) monitor, letter sizes can range from 6/100 to 6/6. This range can be extended by using a modified monitor to display reversed letters for indirect viewing, to increase the testing distance. To circumvent patient learning, random presentation order of different charts (recalled rapidly from diskette) can be used. The video charts were compared with a Bailey-Lovie (IogMAR) Snellen chart by duplicating the letter sizes and format. Acuities were on average one line less for the video chart compared to the wall chart.     

Improving the reliability of visual acuity measures in young children.

Whilst the methodology of adult letter acuity measurement has been substantially refined over the last two decades, relatively little development has occurred in methods for quantifying letter acuity in young children. This study compares a recently developed visual acuity test (Glasgow Acuity Cards), which incorporates several key design features used in adult test charts to improve the sensitivity and reliability of visual acuity measurements. The equivalence of acuity measurements made with Glasgow Acuity Cards were compared with the Bailey-Lovie logMAR chart and Snellen chart in adults, and with traditional Single Letter Acuity and a modified Single Letter Acuity test in children. The test-retest reliability of acuity measurements made with Glasgow Acuity Cards and the Single Letter Acuity tests were also assessed in a large group of visually normal children. In addition, the ability of the pre-school letter acuity tests to detect differences in acuity between the two eyes, and to detect amblyopia were examined. Ninety-five percent of vision measurements made with the Bailey-Lovie chart and Glasgow Acuity Cards differ by less than 0.07 log unit. Furthermore, the sensitivity of Glasgow Acuity Cards to detecting changes in acuity longitudinally and inter-ocular differences in acuity is considerably greater as compared with traditional Single Letter Acuity tests. Improvements in paediatric acuity chart design are important for the effective detection and management of children with amblyopia.     

An arabic letter distance visual acuity test chart for young children and illiterate adults

This paper describes the design of a kinetic response Arabic letter distance visual acuity test chart for young children and illiterate adults. An Arabic letter ein which has previously been used in the design of Arabic alphabet VA charts was employed. Four different orientations of the letter were constructed on a 5 x 5 unit format and graded according to log MAR principle of acuity scaling using an Apple computer. Inter-letter space in each row was made equal to the width of each letter in the row, and inter-row space was made equal to the height of letters in the next lower row. The chart has 14 acuity rows ranging from 4/40 to 4/2 (6/60 to 6/3) (20/200 to 20/10) (log MAR 1.0 to -0.3) at 4 metres. To establish the validity of the chart, acuity values obtained using the chart were compared with those from an existing Arabic log MAR distance VA chart and the Bailey and Lovie distance VA chart. Test retest reliability of the chart was also examined statistically. Results show that VA values from the new chart were significantly similar with those from which it was compared, and that values from the chart are reliable. The chart will be useful for evaluating vision, especially for young children and illiterate adults.     

Design of an Arabic near visual acuity chart

This paper describes the design of an Arabic test chart for measurement of visual acuity at near. The chart was designed employing specially selected Arabic letters and was based on the logMAR principle devised by Bailey and Lovie, Ten Arabic letters of nearly equal legibility values (0.92–1.05) (mean = 1.00), (SD = 0.05) were used in the design of the chart. Each row of the chart has 5 letters and row legibility values range from 4.82 to 5.03 with a mean of 4.92 (SD = 0.06). The logMAR method of visual acuity scaling was used, hence the sizes of letters in the rows progress in a uniform step of 0.1 log unit. The inter letter spacing is equal to the width of each letter in the row, while inter–row spacing is equal to the height of letter in the subjacent row. The height of letters ranged from 3.67 to 0.46mm corresponding to visual acuity of 2.4 M to 0.3 M which is equivalent to reduced Snellen 6'36 to 6:4.5 at 0.4 m. The chart is designed for use at 40cm with a recommended luminance level of 160cd/m².     

A novel high-frequency visual acuity chart

A high-frequency eye test chart using letters or figures of alternating black and white stripes (or dots) on a grey background was developed. Any cross-section of the letters has a Fourier transform with a zero frequency component equal to the luminance of the grey background. When the letters are out of focus, their image on the retina fades rapidly into the grey background, rendering them invisible rather than merely blurred as in a standard chart. The chart was calibrated by simulating refractive errors with defocusing lenses applied to a photographic camera and to subjects' eyes. No constant ratio was found between the size of the Snellen letters and the size of the high-frequency letters for equal visibility. The new chart requires letters for 20/200 acuity to be only 3.6 times larger than those for 20/20 vision. Results confirm the arbitrary nature of the Snellen fraction and warn about the accuracy of visual acuity determined by using charts of different letter types, calibrated by Snellen's system.     

Preschool visual acuity tests: A comparison between two prototype charts and two visual acuity charts in current use

The lack of standardised visual acuity charts with established norms for preschool children has resulted in a diverse variety of visual acuity tests in use for the paediatric patient. All available tests appear to have some limitations, including inadequate norms, long test times, poor reliability, and frequently use test parameters and stimuli which are different from those used with adults. This study compares results for four visual acuity test charts. A Bailey-Lovie Illiterate E chart and an abbreviated Sheridan-Gardiner test were compared with two prototype acuity charts; a modified Bailey-Lovie letter chart and an Arrow chart. A novel symbol such as an arrow constructed as a Snellen optotype incorporated into a chart on the Bailey-Lovie principle, was the test of choice. This test was interesting to preschool children and results showed significant correlation with those obtained using charts based on adult standards.     

Visual acuity and its measurement

Despite its critical importance to our daily life, the most common measurement of visual function, visual acuity, is a relatively crude and narrow one testing only a small portion of the broad range of visual functions. Visual acuity is the measurement of the ability to discriminate two stimuli separated in space at high contrast relative to the background. Clinically, this is measured by asking the subject to discriminate letters of known visual angle. The visual acuity is represented as the reciprocal of the minimal angle of resolution (the smallest letters resolved) at a given distance and at high contrast. Other measurements of visual acuity also exist, including Vernier acuity. Newer charts, such as the ETDRS chart, use letters of equal recognition difficulty and use the log of the minimal angle of resolution; these charts have significant advantages over the old Snellen-type charts. This article reviews visual measurements in children and in patients with low vision, and it reviews factors affecting visual acuity, such as pupil size, refractive error, media opacities, and pharmacologic agents.     

The development of a "reduced logMAR" visual acuity chart for use in routine clinical practice

BACKGROUND/AIMS: The advantages of logMAR acuity data over the Snellen fraction are well known, and yet existing logMAR charts have not been adopted into routine ophthalmic clinical use. As this may be due in part to the time required for a logMAR measurement, this study was performed to determine whether an abbreviated logMAR chart design could combine the advantages of existing charts with a clinically acceptable measurement time. METHODS: The test-retest variability, agreement (with the gold standard), and time taken for "single letter" (interpolated) acuity measurements taken using three prototype "reduced logMAR" (RLM) charts and the Snellen chart were compared with those of the ETDRS chart which acted as the gold standard. The Snellen chart was also scored with the more familiar "line assignment" method. The subjects undergoing these measurements were drawn from a typical clinical outpatient population exhibiting a range of acuities. RESULTS: The RLM A prototype chart achieved a test-retest variability of +/-0.24 logMAR compared with +/-0.18 for the ETDRS chart. Test-retest variability for the Snellen chart was +/-0.24 logMAR using clinically prohibitive "single letter" scoring increasing to +/-0.33 with the more usual "line assignment" method. All charts produced acuity data which agreed well with those of the ETDRS chart. "Single letter" acuity measurements using the prototype RLM charts were completed in approximately half the time of those taken using the ETDRS and Snellen charts. The duration of a Snellen "line assignment" measurement was not evaluated. CONCLUSION: The RLM A chart offers an acceptable level of test-retest variability when compared with the gold standard ETDRS chart, while reducing the measurement time by half. Also, by allowing a faster, less variable acuity measurement than the Snellen chart, the RLM A chart can bring the benefits of logMAR acuity to routine clinical practice.     

Method for identifying amblyopes whose reduced line acuity is caused by defective selection and/or control of gaze

Three visual tests were administered to a group of 15 amblyopic children, 15 adult amblyopes and two age-matched control groups, each of 20 subjects. Test results comprised visual acuity for recognizing high contrast letters presented in line (i.e. Snellen) format, isolated-letter format and repeat-letter format. The classical Snellen format confounds the effects of gaze control defects with the effects of adjacent contours on a patient's ability to recognize a foveated letter. We designed a repeat-letter format intended to unconfound these effects. The repeat letter format is much less sensitive to gaze control defects, and somewhat more sensitive to adjacent contour interactions than is the Snellen format. We report that amblyopic eyes can be subdivided empirically into three repeat-letter categories: repeat-letter acuity significantly better than Snellen acuity; repeat letter acuity not significantly different from Snellen acuity; and repeat letter acuity significantly worse than Snellen acuity. We report that this subdivision cuts across the clinical subclassification of amblyopia and also across the crowding/no crowding subclassification. We suggest that, rather than abnormal lateral interactions, defective selection and/or control of gaze is an important factor in depressed visual acuity in amblyopic eyes of the first repeat-letter category but not for the third type, in which abnormal lateral interactions may be important. To test the hypothesis that the response to patching and refractive therapy may be less satisfactory in our first category of amblyopic eyes, we are carrying out a prospective study.(ABSTRACT TRUNCATED AT 250 WORDS)     

Crowding depends on contrast

Previous studies of visual "crowding" in children have been restricted to the use of high contrast optotypes. In this study, visual acuity was measured in a group of 30 normally sighted children using high (96%), medium (11%), and low (4%) contrast Snellen charts and isolated letter cards. The crowding effect was found to be less for low contrast letters than for high contrast letters.     

Control of chart memory for retesting acuity

Background: Enhancement of visual acuity has been demonstrated when retesting takes place within 24 hours and appears to be due to chart memory. Significant chart letter subset memory has been found after a single acuity measurement and can still be present 10 days after an examination involving several acuity measurements. Control of chart memory may increase the precision of acuity measurement. Method: A single acuity measurement was used to establish chart letter subset memory. Retroactive interference methods for the inhibition of chart letter memory have been compared using a letter recognition memory test. The experimental methods used to promote forgetting included a spelling backward exercise and near acuity assessment using a contra‐chart. The contra‐chart is constructed from the 16 letters not involved in the distance chart that was used to establish chart memory. Results: Assessment of near vision using a contra‐chart was found to inhibit chart letter memory. Conclusions: The contra‐chart appears to have research and clinical applications that require control of measurement imprecision due to chart memory. Contra‐charts obviate the need to use equivalent charts, which have the disadvantages of being difficult to use and control only for memory of letter sequences. The contra‐chart can be included easily in an examination protocol for research or in a clinical examination sequence for which it will have the additional purpose of providing a guide to near vision performance.     

Comparison of visual acuity levels in pediatric patients with amblyopia using Wright figures, Allen optotypes, and Snellen letters.

PURPOSE: To compare and correlate the clinical performance of Wright figures in visual acuity assessments of pediatric patients with amblyopia to those obtained through Allen cards and Snellen letters. SUBJECTS AND METHODS: Best-corrected visual acuity of 26 amblyopic children were measured with the Wright figures(c), Snellen letters, and isolated Allen optotypes, respectively. Amblyopia was defined as two lines of visual acuity difference or a visual acuity level of 20/30 or lower as determined by Snellen chart. The results were evaluated for statistical intergroup differences using the Wilks' Lambda multivariate analysis of variance and for correlation using the Pearson correlation coefficient test. RESULTS: The mean age of the subjects was 8.27 +/- 2.46 years (range: 5 to 15 years). The mean logMAR values for the Wright figures(c), Snellen letters, and Allen optotypes were 0.40 +/- 0.20, 0.47 +/- 0.23, and 0.29 +/- 0.28, respectively. When compared with Snellen letters, the Wright figures correlated to a higher degree ( r = 0.46, P < 0.001) than Allen optotypes ( r = 0.67, P < 0.001). With a visual acuity of 20/40 or worse on Snellen letter testing, the sensitivity of Wright figures(c) and Allen cards in diagnosing amblyopic eyes was 87.0 and 56.5%, respectively ( P = 0.016). CONCLUSIONS: Wright figures, designed primarily to evaluate the vision in the preliterate pediatric population, correlate more closely to Snellen letters and have a higher rate of correctly identifying amblyopia than isolated Allen optotypes in pediatric patients.     

Arabic visual acuity chart for low vision examination

An Arabic visual acuity chart has been designed for low vision examination, using ten Arabic letters constructed on a 5 × 5 framework. The chart contains seven pages with acuity values ranging from 20/600 to 20/80, corresponding to letter sizes of 26.5–3.5 cm. Interletter spaces were made one-fifth the size of each letter in a row while inter-row spaces were made equal to the height of the letters in the row below. The chart will be useful to eye care practitioners who may wish to use Arabic charts in low vision evaluation.     

Letter legibility and chart equivalence.

INTRODUCTION: We tested the appropriateness of the assumption that charts, which only vary in having different sequences of five letters per line, chosen from the same 10 letter set, can be regarded as equivalent for the purpose of making valid comparisons of visual acuity. METHOD: Visual acuity findings from samples of 400 patients, for each of two nominally equivalent Bailey-Lovie charts, have been used to determine the relative legibility of individual letters and lines, using the percentage incorrect method of analysis. RESULTS: The chance of error for the hardest letter is approximately 13 times greater than for the easiest letter on each chart. Significant between-chart differences in error rates for the hardest letters ('F' and 'H') were found. Some letters that have adjacent ranks on the legibility scale were found to have significantly different legibility. Significant differences in difficulty can occur for the same nominal line on apparently equivalent charts because of chance combinations of easier or harder letters in that line. Uneven line-to-line scale intervals have been confirmed for the charts examined, by showing corresponding differences in the distributions of lines of threshold acuity. CONCLUSIONS: The use of varying examination distances may be inappropriate for Bailey-Lovie (or similar) charts that depart from their nominal interval scaling. When different versions of these charts are assumed to be equivalent, the discrepancy between repeated measurements may be significantly increased. There is the possibility of increasing measurement precision using charts having different sequences of the same 10 letters in each line to achieve equal scaling of line intervals. Equivalent charts can be validly constructed using different sequences of the same 10 letters in each line.     

Differences in the legibility of letters at contrast threshold using the Pelli-Robson chart

One disadvantage of using high-contrast letters as test objects when measuring visual acuity is the fact that they are not of equal legibility. A number of charts are now commercially available that assess contrast sensitivity using letter targets. This study attempted to assess the legibility of letters at contrast threshold on the Pelli-Robson letter contrast sensitivity chart by determining the percentage of correct responses for each of the ten Sloan letters at contrast threshold. Results of 493 contrast sensitivity measurements taken in optometric practice indicated that there is a definite difference in legibility between letters at contrast threshold as for letters at acuity threshold. The data suggest that the probability of correctly identifying two out of a group of three letters at threshold on the Pelli-Robson chart varies between 67% and 97% due to letter type alone. Because of the very regular and pronounced miscalling of the letter C as an O, we suggest that this should be accepted as a correct call during threshold measurements on the Pelli-Robson chart. This helps to balance the legibility of different groups of letters.     

Low-contrast letter charts as a test of visual function

Visual pathway disorders can cause visual loss that is not detected by the Snellen test: visual sensitivity to coarse detail may be depressed, even when visual sensitivity to fine detail is unaffected. Sinewave grating test targets can detect such hidden visual loss. However, electronic apparatus for generating sinewave gratings is expensive, while the inexpensive Arden plates provide no check on the patient's accuracy. We have tested 10 patients and 10 control subjects with a set of five letter charts (including the standard Snellen chart). These letter charts were of different contrasts, namely 10%, 22%, 31%, 64% and 93%, but otherwise were substantially alike. Subjects were also tested with sinewave gratings. We found good agreement between sinewave grating and letter chart findings. In particular, the charts picked up visual loss that was not detected by the standard Snellen chart: they detected visual pathway dysfunction in all seven patients whose sinewave data were abnormal. Our findings suggest that even one low-contrast letter chart could provide a valuable supplement to the standard Snellen chart. Compared with other available devices, these charts have the advantages of cheapness, simplicity and of providing the ophthalmologist with an immediate check on patients' accuracy.     

Arabic near test chart for partially sighted patients

This paper describes the design of an Arabic alphabet near acuity chart for examination of partially sighted patients. The alphabets used have similar legibility values (0.92 to 1.05) (Mean = 1.00) (SD = 0.05) and each row of the chart has similar legibility values as the others. The LogMAR method of acuity scaling was used, therefore each row is larger in size than the preceding row by a constant value of 1.26 (0.1 log unit). The inter-letter space in a row is equal to the breadth of each letter in the row and the inter-row spacing is equal the height of letters in the subjacent row. The chart has 10 rows and visual acuity values range from 9.6 to 1.2 M (LogMAR 1.4 to 0.5) at 40 cm, corresponding to Snellen 6/144 to 6/18, 20/480 to 20/60. The chart will be useful for examining Arabic-speaking partially sighted subjects.     

Low-contrast letter charts in early diabetic retinopathy, ocular hypertension, glaucoma, and Parkinson''s disease.

Diabetes can cause visual loss that is not detected by standard reading tests such as the Snellen test but can be detected by low-contrast letter charts. This visual loss is quite different from loss caused by refractive error. These low-contrast charts are diagnostically at least as sensitive as the sinewave grating contrast sensitivity test. They are inexpensive, and the test is brief and simple. Preliminary evidence is that patients with diabetes who have abnormal low-contrast chart results give abnormal intravenous fluorescein (IVF) test results, even though visual acuity is normal. Low-contrast charts also detect visual loss in patients with ocular hypertension, glaucoma, and Parkinson's disease, including patients with normal visual acuity.     

Measurement of visual acuity with two different charts; a comparison of results and repeatability in patients with cataract

Purpose: To compare two different optotypes to measure visual acuity. Methods: Experiment 1: Fifty patients with moderate cataracts were asked to read a chart consisting of letters of the alphabet (Sloan letters) first and a chart comprising Landolt's broken rings afterwards. Experiment 2: Half of patients were instructed to repeat the reading with a second letter chart, the other half was instructed to read the chart with the broken rings again. Results and conclusions: Experiment 1: It was found that with the letter chart more optotypes (two to four) were recognized than with the broken ring chart. The different result of visual acuity measurement with the two optotypes is irrespective of the visual acuity. Experiment 2: The re-read instruction revealed that the measurements were reproduced equally for both charts.     

Changing from Snellen to LogMAR: debate or delay?

Visual acuity is the most commonly used test to assess visual function. The Snellen chart is the universally accepted tool for testing visual acuity despite its poor reliability and reproducibility. Newer logMAR charts are now available that have negated the disadvantages of the Snellen chart. However, these charts are not being used regularly in daily practice. This article discusses the reasons for the delayed acceptance of the logMAR chart.