A new portable test chart unit

The Optimed Vision Analysis System

Bill Harvey tries out the latest vision testing unit to come onto the UK market

Figure 1. The laptop screen angle needs to be adjusted for maximum contrast setting

The use of an electronic test chart is increasingly popular in practice and also in domiciliary settings. The limitations of a static chart are well known. They tend to change in contrast over many years and there is little flexibility in terms of choice of target, making guessing of letters a constant source of error. As most are a standard Snellen based chart, there is a limit to the letters available to assess reduced vision.

As far as portability is concerned, any need for back illumination or variation of chart (such as a duochrome target, a fixation disparity target and so on) usually requires a suitcase-worth of equipment and targets for a full assessment to be carried out in the domestic or work environment.

Many recent developments have seen increasing use of computer-based systems. The City 2000 test chart

Figure 2. The unit is both portable and easily mounted on a wall

is familiar to many and the ability to change the charts in a variety of ways has been well reported. These systems rely on a computer, whether a desktop in the consulting room or a laptop in the domiciliary setting. This serves to enhance the flexibility of the system but does mean that the system is reliant upon the characteristics of the computer hardware, particularly the screen quality.

This is particularly important when using such a system on a laptop. Most standard laptops have a screen that varies in contrast depending upon the angle at which it is viewed. When I use such a system, I have to position myself at eye level next to the patient and adjust the screen angle to a maximum contrast setting (Figure 1). This is not a major problem but does use up time. The letter size adjustment is straightforward when working distance needs to be adjusted, but for shorter distances this sometimes affects the smaller letters.  Also, the systems need to be recalibrated on a regular basis to maintain the correct luminance contrast on screen.

I was interested, therefore, to see a new electronic chart system which is not based on a software programme for loading on to a computer, but is an integrated unit with a liquid crystal TFT screen of 38.5cm by 28 cm. This allows it to be clearly seen at a variety of angles. Also, very usefully, there is never any need to recalibrate the screen. The unit contains the infra-red activated display circuitry, but the whole unit is just 8cm deep, allowing it to be both portable and easily mounted on a wall (Figure 2). The unit comes in a carrying case which also includes the remote handset, red/green and polarising goggles as well as a polarising screen which may be mounted over the main screen itself. Weighing in at 3.8kg, the unit is heavier than a typical laptop but obviously much more portable than a desktop based system.

40 DIFFERENT TESTS

Figure 3. The ETDRS chart

As would be expected from such a system, there is a variety of acuity charts. The standard Snellen chart with which most practitioners are familiar may be pre-set to reflect the letters on a practice's old chart. This may be useful for those who prefer their current lettering with which they are most familiar. Once this has been set, the Snellen chart is non-randomisable.

There is, however, the option to use a variety of other charts. The SLOAN chart was originally introduced into practice in 1959 by Louise SLOAN and used a non-serif letter improvement upon previous Snellen types. More recently, the need (driven particularly by practitioners working in low vision assessment) for greater numbers of letters for assessing lower levels of vision and for there to be a regular size change between lines and spacings between letters, has resulted in a variety of logMAR-based charts such as the ETDRS, now considered the standard for the profession, certainly in parts of the US (Figure 3). The ETDRS developed out of the Early Treatment Diabetic Retinopathy Study. Child-friendly charts are widespread, a common one being an HTOV chart which may be used as a matching test. The SLOAN, ETDRS and HTOV are all represented on the Optimed system and, importantly, may all be randomised to prevent a patient from memorising letters between presentations. All acuity charts may be pre-set for working distances from 0.5m up to 8m and there is the option for a reverse chart if the screen is to be viewed through a mirror. Chinese and Hebrew letters are also available.

Another idea I liked was the way the remote used a number of 'hot keys' allowing the practitioner to jump from larger to smaller targets without the need to scroll down. These may be pre-set to the testing patterns of up to three different practitioners which is useful in a busy practice.

CONTRAST
Many eye diseases, particularly those affecting the media, may have little influence upon high contrast acuity targets (as with a standard Snellen chart) but significantly reduce one's ability to see in the variable contrast environment within which we all live.

Figure 4. Contrast sensitivity function test

Increasingly, practitioners are assessing acuity at lower contrast levels. With many computerised systems, it is possible to designate a specific contrast value and so assess contrast at 100 per cent, then, for example, 50 per cent, to see any noticeable difference which might indicate the need for contrast enhancement. Such systems are useful, but to assess a full contrast sensitivity function (CSF), it would be necessary to measure acuity at a whole range of contrasts and to also look at the impact of reduced contrast upon larger spatial frequencies (standard acuity is a measure of high contrast high spatial frequency).

The Optimed system is interesting in that it incorporates a Vistech contrast sensitivity

Figure 7. Animations for the children

function test. The patient looks at a sequence of circular targets, each in a series of a set spatial frequency with diminishing contrast levels and is asked to report the orientation of the lines seen (Figure 4). The point at which the lines are no longer detected for each spatial frequency is stored on the system and this may then be shown on the screen as a CSF curve, indicating contrast sensitivity at a variety of spatial frequencies (Figure 5).

For many years, researchers have argued the usefulness of CSF assessment in general practice, but to my mind this represents one of the easiest and quickest methods of such assessment I have seen and would be very easy to incorporate into a routine examination.

BV AND COLOUR TESTING
Another interesting feature is the incorporation of an Ishihara pseudoisochromatic plate

Figure 6. Ishihara pseudoisochromatic plate

series, making it useful for screening congenital colour defects without the problem of sticky hands on plates (Figure 6). Another nice feature is a number of animations which should help maintain the co-operation of the younger child (Figure 7).

There is a need to use a polarising screen (which may be orientated vertically or horizontally) for fixation disparity assessment. What I found interesting about this test was that, when a disparity is found, instead of using prisms in a trial frame to realign the cursors, the lines are actually manipulated on screen to the correct alignment and the prismatic equivalent to achieve this is shown on screen (Figure 8). This is useful for

Figure 8

reducing the equipment needed, for example in home visits, but also removing the need to keep changing lenses in a trial frame, which takes time.

For further information contact Optimed on 02476 444118.