Approximately 900,000 people in the United States are registered as being legally blind, and a further 2.4 million have low vision.1 However, it is known that far greater numbers of people have reduced vision without being registered,2 and that uncorrected refractive error and normal aging can reduce the vision of people without any identified eye disease.3,4
Contrast sensitivity is one aspect of visual function that declines with age3 as well as with eye disease.5,6 Reduced contrast sensitivity is known to be associated with difficulty in reading,6 – 8 face recognition,9 object recognition,10 mobility,11,12 and communication in people with reduced hearing.13 Poorer contrast sensitivity also increases the risk of being involved in an “at-fault” motor vehicle collision.14
To be read fluently, text must be presented at several times higher contrast than the contrast sensitivity threshold.7 In a key article, Whittaker and Lovie-Kitchin reviewed the literature on reading and presented the “contrast reserve” needed to read text at all, and to read it with fluency, with high fluency, and under optimal conditions (Table 1).15 The contrast reserve is the ratio of the text contrast to the contrast sensitivity.
In this article, we analyze the contrast sensitivity data from a large epidemiological study investigating visual function in a population of older adults living in Maryland (the Salisbury Eye Evaluation [SEE]). The SEE study was a population-based evaluation of adults aged between 65 and 84 years living in the community.16 We use these data to determine what proportion of the population would be able to read text presented in different formats, assuming that the visual acuity criterion is met (i.e., the text is large enough to be comfortably resolved with the current spectacle prescription).
The SEE identified adults aged between 65 and 84 years living in the Salisbury, Maryland, metropolitan area, using the Health Care Financing Administration Medicare eligibility lists. In total, 4624 subjects were identified, of whom 2520 received a comprehensive visual assessment. As part of this assessment, contrast sensitivity was measured using the Pelli–Robson chart.17 This test was performed monocularly on each eye, at a distance of 1 m, in a well-lit room (chart luminance: approximately 100 cd/m2), using the optimal refractive correction. Credit was given for each letter read correctly. Data from the better eye are presented in this article.
Participants were also asked a visual function questionnaire (the Activities of Daily Vision Scale).18 One question on the Activities of Daily Vision Scale specifically addresses reading newsprint: “Would you say that you read the ordinary print in newspapers with: (a) no difficulty at all; (b) little difficulty; (c) moderate difficulty; (d) extreme difficulty; or (e) you have not read a newspaper recently because of your vision?”
Ethical approval was obtained for the study, informed consent was received from all participants, and the study conformed to the Declaration of Helsinki.
Weber contrast values for different types of text were obtained by measuring the luminance of the background (a blank area of white) and of print (a filled area of black ink or print) using a photometer (CS-100, Minolta, Tokyo, Japan). This measurement was performed in a bright office (room luminance: 940 lux [Testo 540, Testo, Lenzkirch, Germany]), although Weber contrast is invariant to changes in background luminance. The contrast sensitivity required to read each text format at all, with fluency, with high fluency, and with optimal performance was calculated using the values published by Whitaker and Lovie-Kitchin.15
Contrast sensitivity was recorded for 2507 subjects. Data were missing for 0.5% of the sample (14 of 2520). Mean age of participants was 71 years (interquartile range: 68–76 years), and the median best corrected distance visual acuity was −0.02 logMAR (20/20+1, 6/6+1, interquartile range: −0.1 logMAR to +0.06 logMAR). Visual acuity and contrast sensitivity were moderately linearly related (r = −0.69).
Median contrast sensitivity was 1.60 log units (interquartile range: 1.50–1.70 log units). Fig. 1 shows the cumulative percentage of contrast sensitivity values. This figure shows the proportion of the population whose contrast sensitivity is at, or is poorer than, the value shown on the horizontal axis.
Table 2 indicates the Weber contrast values for the different formats of text.
Fig. 2 and Table 3 show the proportion of the population unable to read text presented in different formats at different levels of fluency. The poorest performance was found for electronic books (Fig. 2A): only 44.71% of the population was expected to be able to read electronic books or newsprint optimally. One percent of this population was unlikely to be able to read newsprint with any level of fluency. LED computer monitors had the highest contrast text: 99% of the population could read this fluently, yet more than one person in four (29.6%) could not read this with optimal fluency.
One thousand eight hundred thirty-six participants (72.9%) did not report any difficulty in reading newsprint, and 666 (26.4%) reported some level of difficulty or had not read newsprint because of their vision. Data were missing for 18 participants (0.7% of the sample). Mean contrast sensitivity in those reporting no difficulty was 1.6 log units, whereas in those reporting any level of difficulty, it was 1.45 log units. Table 4 shows the mean contrast sensivity values for different levels of self-reported difficulty in reading newsprint.
Reduced contrast sensitivity creates difficulty in reading text. Although the proportion of the population who are unlikely to be able read text with fluency because of low contrast sensitivity is relatively small, this translates to large numbers of people.
For example, 1% of our population was unlikely to read newsprint with any fluency because of reduced contrast sensitivity. If this population is representative, this indicates that 395,000 older people in the United States and 768,000 older people in the European Union are unable to read newsprint fluently because of its text contrast.
Laser print is relatively easy to read, although 0.8% of the population (316,000 older Americans, 614,000 Europeans aged >65 years) is unable to read laser-printed text even with the slowest “spot reading” fluency. If laser-printed documents were instead sent electronically and viewed on a computer monitor, an additional 0.08% of the older population would be able to read them. This number may sound small, but it equates to 31,600 more people in the Unites States and 61,400 more people in the European Union being able to read them. This is of clear importance for banks, utility companies, and government agencies that send communication by post. Of course, presenting text on a computer monitor is no guarantee that it will be easily readable by the end user: color contrast, the boldness of text, and the ability to manipulate the format and spacing of text are also important.
Use of self-reported visual function data indicated that declining contrast sensitivity was related to an increased probability of reporting difficulty in reading newsprint. Approximately one-quarter of our population reported difficulty in reading newsprint. This is greater than the proportion we predicted to read the newspaper with less than high fluency based on our contrast sensitivity model. Of course, many other visual and nonvisual properties will affect newspaper reading performance, including visual acuity, the presence of visual field loss, and cognitive and neurological factors. However, the relationship between self-reported reading performance and contrast sensitivity confirms the importance of contrast sensitivity on reading ability.
Our Weber contrast values are higher than those previously reported by, for example, Brilliant.19 We suggest that this is because of advances in printing technology, which has enabled print to be darker and paper to be brighter. It may be that British newspapers and paperback books (which we have assessed in our study) have higher contrast than the North American publications described in many low vision textbooks.
The population of the SEE may not be truly representative. This population is older (minimum age: 65 years; median: 71 years) and includes subjects of predominantly white and African American origin. Furthermore, these data are from a population with Medicare insurance and access to health care. As contrast sensitivity is significantly reduced in cataract, lower values may be expected in a population with poorer access to health care and without health insurance. These data are now several years old, but we have no reason to doubt their applicability at present. The population is aging, and the mean age of a random sample of older adults would be expected to be older, increasing the prevalence of age-related eye disease. However, this aging will be mitigated by recent advances in ophthalmology, such as anti–vascular endothelial growth factor treatment for age-related macular disease.
The data presented in this article examine only the effect of text contrast on reading performance, and have not considered visual acuity. As there is a moderate relationship between visual acuity and contrast sensitivity, it would be expected that many of the people unable to read print because of poorer contrast sensitivity would also struggle with text size. There will be other individuals who have sufficient contrast sensitivity to access text but who will not be able to resolve text because of its size. These people are likely to respond well to magnification. Reduced contrast sensitivity can only be ameliorated by electronic means, such as the use of an electronic magnifier or by manipulating text presented on a computer monitor.
We have only considered the eye with better contrast sensitivity in this investigation. As there is some binocular summation of contrast in people with good vision,20,21 one might expect true binocular contrast sensitivity to be better than the values presented here. However, as binocular contrast sensitivity was not measured in the present study, and contrast summation is limited or reduced in some people with eye disease,22 we felt more comfortable using a measured response than an inferred binocular contrast sensitivity value.
A final limitation of our work is that we have compared contrast sensitivity values measured at 1 m using large characters to a reading task, typically performed with smaller characters viewed from a distance of about 40 cm. This approach has been used before, for example, by Mohammed and Dickinson.23 By extending the viewing distance of a Pelli–Robson chart, these authors found that contrast sensitivity did not vary significantly with angular size of the letters on the chart.
Reduced text contrast restricts accessibility to the written word for a large number of people in the developed world. Making text available in a format that can be presented on an LED computer monitor will increase access to written documents.
UCL Institute of Ophthalmology
11–43 Bath Street
London EC1V 9EL
The authors wish to thank GF Mueden for providing the motivation for this analysis. MDC is supported by the National Institute for Health Research grant PDF/01/2008/011. This report describes independent research arising from a grant supported by the National Institute for Health Research. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health.
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