The Cardiff Acuity Test is designed for measuring visual acuity in young children aged 1 to 3 years, and in older children and adults who have intellectual impairment. It therefore allows us to measure acuity in people who cannot communicate well enough to name a letter or a picture or to describe what they see.

The Test works by ‘Preferential Looking’ – that is, the child simply looks towards the target, and the examiner watches this eye movement response to determine whether the child can see the target. If the child reliably looks towards the target, we assume that he/she can see it. If the child does not look at the target, we assume that he/she cannot see it.

The principle of the target design is that of the vanishing optotype. The targets are pictures drawn with a white band bordered by two black bands, all on a neutral grey background. The average brightness of the picture is equal to that of the grey background. If the child’s detail vision is good enough to resolve the white and black bands, the picture will be visible and the child can look towards it. If the target lies beyond the acuity limit, i.e. if the bands are too narrow for the child to resolve them, the picture merges with the grey background, and simply becomes invisible. Now the child cannot look at the picture, because the grey card appears completely blank.

In the Cardiff Test, each picture is located either in the top half or in the bottom half of the card. The examiner, watching the child’s eye movements, can judge the position of the target from those eye movements. The pictures are all of the same overall size, but decrease in width of white and black bands. The visual acuity is scored as the narrowest white band for which the picture is visible.

An important feature of the preferential looking technique is that the examiner should not know in advance the position of the target. The Cardiff Test includes three cards at each acuity level, although only two are usually presented. The three cards have the same picture, but two are at the top of the card and one at the bottom, or two are at the bottom and one at the top. This means that once one card (at a particular acuity level) has been presented, the examiner and child cannot predict the position of the next card.

How to carry out the test
(fuller instructions are in the manual supplied with the test)

Seat the child comfortably on a chair or on a parent’s lap. Sit in front of the child preferably at their eye level. A working distance of 1 metre is recommended. A closer working distance of 50cms may be used, if necessary to obtain a lower acuity, or to allow closer interaction with the child.

Beginning with the widest target (lowest acuity), shuffle the three cards, and present the first card at the child’s eye level, with the centre of the card at your own eye level. Keep the child’s attention by talking about the picture – turn the procedure into a game. From the child’s eye movement when he/she looks at the picture, estimate the position (top/bottom) of the picture. Once you have made your decision present the second card; you may then check both to corroborate your decisions. If you have made two correct estimates, proceed to the next level and repeat along the sequence.

Remember that a child’s response may be very brief – he/she may glance at the picture and then look away – usually towards you. Watch the child’s eyes carefully as you present the card. Judging eye position takes a little practice, but you can become very skilled very quickly.

Once you have made an incorrect estimate (or you cannot estimate the picture position) it is likely that the target is not visible to the child. Return to the next LARGER target, and repeat the tests at this and again at the ‘failed’ level. At this stage in order to avoid any expectations on the part of the examiner or the child, shuffle the cards between each presentation. The end-point should be taken as the highest acuity level at which two out of two presentations are scored correctly.

If you are unable to estimate the picture position because the child is no longer looking at the card, it may be that he/she has lost interest. Some children are not motivated to search for the picture when it becomes hard to see. In this case, return to a wide target (easy picture) to restore the child’s confidence. You may want to intersperse ‘difficult’ pictures with ‘easy’ ones. As with any clinical skill, each practitioner soon develops his/her own techniques for a successful result.

Although the test is designed for young children and those with intellectual disabilities, it can also be used, in different ways, for children of all ages. Older children may want to interact more by pointing at the picture, by naming it or by using sign language. (Note however, that the test does not incorporate ‘crowding’, and standard optotype tests may be more suitable for detecting mild amblyopia in older children.)