Technological advances in test charts

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Both projection charts and computer displays offer a much wider variety of optotypes than encountered on conventional backlit charts. Using projection charts it is possible to isolate individual lines and letters but they suffer from the disadvantage of a lack of letters, numbers or pictures on an individual line due to the limitation of the projected image area which can lead to memory influencing patient responses. Computer driven charts can offer an almost infinite number of customisable charts and advantageously offer chart types which are not normally encountered including LogMAR and contrast sensitivity charts.

The duochrome or bi-chromatic chart can be made more versatile with projection charts since it is possible to superimpose a red-green masking filter over an Optotype chart and thus the practitioner gains a greater choice of acuity levels over which the test can be applied. The same feature is possible with a computer driven chart which has a further advantage of being able to calibrate the red-green colours which is not possible with any other chart type. Indeed, the spectral distribution of the red and green backgrounds encountered in projection and backlit charts shows considerable variation leading to a lack of standardisation of the dioptric interval between the red and green foci despite the existence of a British Standard specifying the spectral distribution of the filters required for the test.

Although projection charts contain an astigmatic dial which can be used to determine the axes of regular astigmatism, the cylinder correction cannot be determined using the fan and block technique as rotational blocks cannot be incorporated into the design of a projection chart. The fan and block test chart is often found on a backlit chart and on computer driven charts where precise rotation of the arrow and blocks is possible to one degree intervals. Cylinder correction during subjective refraction is most commonly determined using the cross cylinder method. The choice of target used in the cross cylinder technique can influence the quality of response given by the patient. When the cross cylinder is presented with the circle of least confusion on the retina, the size of the blur circle is altered between the two positions while its shape remains unchanged. There are magnification differences between the two positions which can make judgements of clarity difficult for the patient. Circular letters and Verhoeff circles are often used as cross cylinder targets but can make the separation of clarity from cross cylinder distortion difficult to separate by the patient. These problems are exacerbated when the circle of least confusion is not precisely on the retina. Targets consisting of a collection of dots tend to minimise the distortion effect of the cross cylinder, making the blackest and sharpest decision required of the patient easier. Dot targets are available as an option on all test chart types. With the exception of projection charts they can also be used in the duochrome test. Dot targets found on a backlit chart contain polarised filters oriented vertically and horizontally which enable them to be used under binocular refraction and for binocular balancing. Horizontal black and white bars are seen by both eyes and act as a fusional binocular lock (Figure 1).

Tests of binocularity

Polarising targets are a useful facility which can be used for binocular refraction and tests of binocularity. Backlit charts most commonly use polarising axes set at 90 and 180 degrees while those targets found on projection charts are oriented at 45 and 135 degrees which corresponds to the polarising filters found in refractor heads. When using backlit charts, the most commonly used targets are the spot for Maddox rod evaluation and the Mallet fixation disparity chart. The latter is rarely encountered on projection charts and use of the Maddox rod is impaired by the low luminance of the spot target, especially when projected to 6m. There are, however, a number of polarised targets which are used to evaluate heterophoria (Figure 2). The luminance of a white spot on a computer monitor is often insufficient to gain an adequate streak when carrying out the Maddox rod test. In computer-driven displays, the spot can be substituted for a bar which is the same colour as the Maddox rod and oriented at 90 degrees to the axis of the rod. Prismatic power is applied until the bar seen with one eye passes through the bar seen on the screen.

The associated phoria tests differ from fixation disparity tests in that in the former, no part of the target is seen by both left and right eyes, ie there is no binocular lock. In an associated phoria test, the size of the disparity between the targets is measured rather than the prismatic power required to overcome the disparity which is assessed in a fixation disparity test. There is little correlation between measures of associated phoria and fixation disparity.

The use of polarised targets in computer-driven displays requires the addition of a polarising filter in front of the monitor. Polarising filters substantially reduce the luminance of the screen which is generally lower than encountered on backlit charts which can significantly impair visibility. Another possible approach is to use a red filter before the right eye and a green filter before the left.

In computer-driven charts two red bars found on a backlit chart fixation disparity test are replaced by red and blue-green bars respectively (red and green on a backlit chart). The colour of the bars is chosen to match the filters in front of the eye so that the red bar merges with the grey background when viewed through the red filter and the blue-green filter merges with the grey background when viewed through the green filter (Figure 3).

Testing distance

Projection charts suffer from the disadvantage that the luminance of the test type is reduced by multiple reflection through mirrors for a testing distance of 6m since it is normal for the projector to be mounted beside the consulting chair. Consequently, many practices have circumvented the mirror and employed direct projection to 3m. Testing at 3m has many negative implications for subjective testing and thus should be undertaken with caution. Testing at 6m is considered a good approximation to a viewing distance at infinity since the vergence of a target at 6m is -0.17D, which is substantially less than the 0.25D prescribing interval and symptoms of over plussing the distance correction are rarely reported by patients. When a test chart is viewed at 3m, the vergence at the eye is -0.33D. If the subjective refraction result was prescribed at this test distance, the resulting distance correction would be +0.25D over-corrected leading to refraction induced myopia.

Consider a -1.00D myope being tested at a distance of 6m. Unaided vision will be expected to be 6/18. The far point of the patient lies 1m in front of the eye. The further an object is away from this point, the more blurred it will become. Although the angular subtense of letter target calibrated for a test distance of 3m is the same as one positioned at 6m, the smaller blur circle of the letter at 3m is less degraded than the blur circle for the equivalent size letter at 6m. Figure 4 shows a camera image of charts at 3 and 6m. The exposure settings were constant for each image and the focus was set at 1m to simulate a -1.00D myope. The 6/18 line is shown and although the camera image is not a true representation of what a patient sees, it can be seen that the chart at 3m gives a higher acuity than the chart at 6m.

Hypermetropes may be able to employ accommodation to view a chart at 3m which may be beyond their amplitude of accommodation at 6m. It is common practice to employ 6m Snellen notation when testing at 3m (6/60, 6/6, etc) which leads to erroneous interpretation by a professional who does not work at the practice and assumes subjective refraction was carried out at 6m. This could have potential legal implications when recording vision and visual acuities on medical reports of vision for licensing and job entry requirements. Best practice would be to record the Snellen fraction, eg 6/60 = 0.1, 6/6 = 1.0, etc and the test distance at which subjective refraction was carried out as it is representative of the conditions under which subjective refraction took place. Table 1 shows the equivalent decimal for Snellen notation at 6m.

The biggest problem with testing at 3m is the greater degree of accommodation in non-presbyopic patients, which leads to the distance refraction being myopic by 0.33D. Subjective refraction at 6m does not require any modification of the final prescription, whereas testing at 3m requires final modification for the test distance. This can either take the form of a mathematical correction to the final prescription of -0.25D or using the duochrome test to judge the end point of the examination.

At 6m the subjective end point is taken where the duochrome targets are blackest and sharpest on the red background or equally against the red and green backgrounds. At 3m, the subjective end point is taken when the targets are just boldest against the green background, inferring a -0.25D addition to refraction at 3m. However, lack of standardisation between duochrome tests means that some caution should be taken with this method as the dioptric interval between red and green foci varies between charts, although familiarity with a particular chart gives the practitioner a good indication of the end point. Testing at 3m also confounds assessment of distance binocular status since heterophoria and fixation disparity are not being measured at infinity (defined as 6m testing) and thus distance normative values do not apply since these measurements are in fact being carried out at a far intermediate distance. Prescribing of prism for distance vision from a chart at 3m may not translate to the true prism required for a testing distance of 6m and mathematical corrections are not possible. Distance subjective refraction should therefore be carried out wherever possible at a standardised test distance of 6m. Space-saving charts are available from a number of manufacturers (see Figure 5), which negates the need for a mirror mounted 3m in front of the patient. Instead, the image is projected to 5 or 6 m by means of a system of mirrors contained within a box which gives versatility in positioning the chart as little as a metre in front of the patient, eg as part of a refraction unit.

Although computer monitors suffer from lower luminance levels than other test charts, their infinite customisability facilitates the design of new tests which can be used in subjective refraction. This feature combined with future developments in display technology suggests that they are likely to become the dominant test chart of the future.

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