Second Year 99/00
Clinical Optometry 2
Clinical Colour Vision Testing
1. Ishihara Pseudo-isochromatic colour vision test plates
This test consists of a series of plates designed to provide a quick and accurate assessment of colour vision deficiency of congenital origin. Most cases are characterised by a red-green deficiency which may be of two types; a protan type which may be absolute (protanopia) or partial (protanomalia) and secondly a deutan type which may be absolute (deuteranopia) or partial (deuteranomalia).
In protanopia the visible range of the spectrum is shorter at the red end compared with that of a normal person, and that part of the spectrum which appears to the normal blue-green, to those with protanopia appears as grey.
In deuteranopia the part of the spectrum which appears to the normal as green appears as grey. In protanomalia and deuteranomalia these parts of the spectrum appear as a greyish indistinct colour.
For those with red green deficiencies, blue and yellow colours appear to be very clear compared to red and green. This is made use of in the Ishihara test. An absolute failure to appreciate blue and yellow is termed tritanopia and if partial tritanomalia. The Ishihara plates are not designed to detect these deficiencies.
The plates are designed to be used in adequate daylight conditions and the introduction of direct sunlight or the use of electric light may produce some discrepancy in the results because of an alteration in the appearance of the colours.
The plates should be held 75cm from the subject and tilted so that the plane of the plate is at right angles to the line of vision. Each plate should be identified within three seconds. In practice for those patients unable to read numerals those plates near the end of the book with winding lines between two Xs should be used. The lines should be traced with a brush in less than ten seconds. For the purpose of this practical course the test should be conducted binocularly but in a real life clinical situation it may sometimes be appropriate to conduct the test monocularly and binocularly if an acquired colour vision defect is suspected.
The score sheet, which accompanies these instructions, can be used to determine the type of deficiency by noting which plates have been identified incorrectly.
Work in pairs and carry out a colour vision evaluation using the Ishihara colour vision test on each other under general room lighting conditions. Make a note of whether any plates were identified incorrectly. If any of the screening plates are incorrectly identified ask the observer to identify the classification plates.
2. The City University test (TCU)
The test comprises selected paper colour samples, which must not be touched. On each page, four colour samples surround a central spot and the subject must choose the one that most closely resembles the colour of the central spot. Each page provides the opportunity for a normal response-the patient identifies the normal spot as being the one that is identical in colour, or is most nearly similar in colour to the central spot. Each page provides possible protan, deutan or tritan confusions which may be mixed with a normal response since some observers may find more than one near match on a page. A consistent regime must be used, taking care to illuminate the test correctly.
Hold the booklet at 35 cm from the patient with the pages at right angles to the line of sight. Show the demonstration page 'A' and say 'here are four coloured spots surrounding one in the centre. Tell me which spot looks most near in colour to the one in the centre. Use the words top, bottom, right or left. Please do not touch the pages'.
Show the test plates 1 to 10 in turn. Allow about 3 seconds for each page. The plates are divided into six chroma 4 and four chroma 2 plates (smaller spots). The test can be scored by noting the response on the score sheet and then determining if the response is normal or abnormal. For the purpose of this course the test should be conducted binocularly but in a real life clinical situation it may sometimes be appropriate to conduct the test monocularly and binocularly.
Work in pairs and carry out a colour vision evaluation using the City University colour vision test on each other. Make a note of whether any targets are identified incorrectly and fill out the score sheet.
3. Farnsworth D-15 colour vision test
This test is a modification of the Farnsworth-Munsell 100 Hue test. It is intended for screening purposes, rather than for in-depth study of colour vision defect. Each set of discs contains a reference or 'pilot' cap and fifteen other discs, which make up an incomplete circle. A neutral is also included in the set but is not used.
Find the pilot cap and place it into the tray at one end. Arrange the other caps with the colour side up in a random sequence and keep the neutral on one side. For the purpose of this course the test should be conducted binocularly but in a real life clinical situation it may sometimes be appropriate to conduct the test monocularly and binocularly.
Ask the subject to select the cap that is the closest possible match to the pilot. This choice should be placed next to the pilot. The subject should continue to choose caps in this way until a sequence of colours has been built up and may change the sequence at any stage. A time of one to two minutes should be allowed to complete the task. Close the empty lid of the tray and tip it upside down. Open the tray again to expose the numbers on the undersides of the caps. Make a note of these numbers; if all the caps are in order 1 to 15 then no further action need be taken. If one or more inversions are made then the numbers should be recorded on the score sheets provided. Finally these numbers can then be transferred to the circle of dots at the bottom of the score sheet, joining the numbers which have been placed next to each other.
Conduct the test for each person in the pair with general room lighting conditions. Fill out the score sheet and comment on any anomalies.
Deutan subjects exhibit a confusion axis from green to purple; protans from red to blue-green while tritans show an axis from yellow to blue. Each of these axes tends to divide the circle in two, hence the test has been described as dichotomous. It also tends to show dichotomy between safe and potentially unsafe colour vision. The D-15 test does not however provide a formal identification of dichromats, anomalous trichromats or normals.