The essence of all glare sensitivity testing is to attempt to quantify the impact that the introduction of a glare source has on vision. In its simplest form, glare sensitivity can be assessed by introduc- ing a glare source, such as a pen torch or light from an anglepoise lamp, into the visual fi eld close to the line of fi xation while the subject under investigation undertakes a visual task.54 Quantifi ca- tion can be achieved if an acuity chart or low contrast letter chart
is used as the principal target. Correlating the actual reduction in performance with the patient’s subjective comments can prove useful.
The most well known instrument designed specifi cally to assist with the quantifi cation of disability and discomfort glare is the brightness acuity tester (BAT) (Fig. 7.13). With this instrument, the subject views an acuity or low contrast chart through a 12-mm aperture hole in a 60-mm white hemispherical dome, which is held in close proximity to the eye. Acuity measurements can be recorded in the standard way when the rheostat is adjusted to one of three positions (12-, 100- and 400-foot Lamberts). The three settings cor- respond to bright overhead fl uorescent lighting, indirect sunlight on a cloudy day, and direct overhead sunlight.55 Other tests that have also been evaluated as glare disability tests include the Vistech MCI 8000 and the Miller–Nadler glare tester.56
7.8.3 Management of glare
The principal approach to managing glare is to do everything possible to remove the source. If, for example, it is caused by light from a luminare adjacent to, or refl ecting off, the working plane, simply increase the angle of substence between the light source and the eye. Alternatively, one may try to reduce the brightness of the offending source by reducing the bulb strength or using an appropriate diffusing fi lter. If this is impractical or impossible, consideration should be given to fi lter lenses such as the NoIR sun shields, or to peaked caps and typoscope-type devices.
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7.9 Summary
In this chapter, we have sought to review methods used to quan- tify the main visual functions, a defi cit of which may impact on levels of visual impairment. Specifi c attention has been paid to measurements of visual acuity and the variety of charts now avail- able to those interested in the measurement of visual resolution. Attention has also been paid to the assessment of near acuity and the importance of assessing reading speed, ability and fl uency. Third, the importance of evaluating performance when undertak- ing low contrast tasks has been determined. Finally, issues sur- rounding the assessment of visual fi elds, colour vision and glare sensitivity in visually impaired patients have been covered.
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In Chapter 7 the emphasis was on the assessment of visual function and the impact that damage to the visual system may have on specifi c aspects of visual function. The individual with low vision is, however, unlikely to have much interest in the nature of the biomedical indicators (visual acuity, contrast sensitivity, etc.) that we, as practitioners, use to describe their level of vision. They are interested in whether planned interventions, surgical (in the form of a cataract extraction or photodynamic therapy), optical (in the form of a new spectacle prescription or a low vision aid [LVA]) or rehabilitative (in the form of training or advice), will have an impact on their ability to undertake the tasks of their everyday life. Any evaluation of a healthcare intervention must involve a systematic analysis of outcome, and this must include the patient’s perspec- tive. This process of quality assurance is inherent in clinical audit, which is at the core of current National Health Service reform.1
Whereas the majority of early publications on low vision inter- vention attempted to quantify the success of LVA provision in terms of the improved acuities that could be recorded through the device,2 experienced practitioners routinely incorporated ques- tions on task analysis into their case histories and clinical routines. Not only were the near acuities achievable through a LVA mea- sured, patients were also asked about the duration and regularity of LVA usage, the nature of tasks attempted with LVAs, and han- dling experiences and device limitations.3 The process of analys- ing the nature of the task that the visually impaired person wishes
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functional vision
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Ch008-H1815.indd 167 9/15/2006 12:12:22 PMto undertake, and of evaluating performance with and without the assistance of low vision appliances, is inextricably linked to the successful use of ‘functional vision’. It is improvements in func- tional vision that, in turn, enable the patient to achieve improve- ments in their quality of life.