Contrast sensitivity is the visual system’s ability to detect faint targets, or to detect an object against its background. Visual acuity tests the system’s ability to discriminate small detail of high contrast (e.g. black on white) i.e. it is one extreme of our visual capability. Very little in the real world is of high contrast. Most of the tasks we do in every day life involve seeing relatively large objects against low contrast background (seeing white bread on a light coloured plate, avoiding a dark chair seen against a carpet). So contrast sensitivity is a much better indicator of our capabilities in the real world.

Conditions exist which reduce visual acuity while leaving contrast sensitivity intact and yet other conditions reduce both acuity and contrast sensitivity. It is even possible for visual acuity to be normal while contrast sensitivity is reduced. So measuring contrast sensitivity is essential in defining visual problems, for both diagnosis and for management.

Look at the following three pictures of Maggie Woodhouse, who developed the Cardiff range of tests. The first is a typical photo, as seen by a normally-sighted person. The second has fine detail removed, such as might happen when visual acuity is reduced. In the third picture, contrast is reduced, so this is how the photo would appear to someone with a loss in both acuity and contrast sensitivity.

A child with visual impairment who retains good contrast sensitivity (second photo) will benefit from enlargement of tasks – he or she sees large faint objects well. A child with an equivalent acuity but with reduced contrast sensitivity (third photo) has a much more severe impairment, and will not get the same benefit from enlargement. Large objects as well as small objects are difficult to see. So a measurement of contrast sensitivity can help determine appropriate management of a child’s impairment.

Treatment for visual deficits (including amblyopia therapies) can result in improved contrast sensitivity sometimes in the absence of increased visual acuity. Contrast sensitivity can therefore be an essential tool in monitoring therapies.

Some progressive eye conditions will result in changes in contrast sensitivity, so that monitoring contrast sensitivity alongside visual acuity (and other visual functions) may be essential in determining progress of the condition.

The Cardiff Contrast Test uses the same pictures as the Cardiff Acuity Test, but the pictures all have the same level of detail throughout the test. The outlines that make up the pictures are light and dark grey and these grey levels vary to give lower and lower contrast as the test proceeds.

The pictures are placed either at the top of bottom of the each card, so that the observer simply notes the child’s eye movements to determine whether the picture is visible. Thus the Cardiff Contrast Test can be used as a Preferential Looking Test. Alternatively, older children can point to or name the pictures.

How it works

The Cardiff Contrast 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 contrast sensitivity in people who cannot communicate well enough to name a letter 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, like that of the Cardiff Acuity Test, is that of the vanishing optotype. The targets are pictures drawn with a light grey band bordered by two darker grey 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 contrast sensitivity is good enough to allow discrimination between the light and dark bands, the picture will be visible and the child can look towards it. If the contrasts within the picture are too faint, 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 Contrast 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 differ in the contras difference between the light and dark bands. The contrast sensitivity is given by the lowest contrast for which the picture is visible (contrast sensitivity is the reciprocal of contrast threshold).

An important feature of the preferential looking technique is that the examiner should not know in advance the position of the target. The Cardiff Contrast Test includes three cards at each contrast 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 contrast level) has been presented, the examiner and child cannot predict the position of the next card.