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Glaucoma can be diagnosed by sensing excess pressure in the eyeball, but by then it’s often too late to save someone’s sight. Now Vision RX of Elmsford, New York, has developed an early-warning test based on the discovery that eyes at risk find it more difficult to recognise rapidly changing colour patterns (WO 02/09578). The test display switches 40 times a second between blue and yellow patterns, while gradually reducing the brightness. Healthy eyes continue to see colour long after diseased eyes see only grey, says Vision RX. By focusing the test image on different parts of the retina, the damaged parts can be mapped.