The visual stimulation responsible for seizures in patients with photosensitive epilepsy gives rise, in others, to aversion, distortions of visual perception, and to eye-strain and headaches, (click here for an example). Text can have aversive spatial properties and many individuals find reading uncomfortable, particularly those with a family history of migraine. The perceptual distortions they experience are often reduced when the text is illuminated by light of a particular optimal colour, different for each individual. When spectacles having this colour are worn, eye-strain and headaches are reduced. The reduction is greater than for spectacles with a very similar but sub-optimal colour, even under conditions in which the nature of the comparison is concealed. These findings have supported the development of the MRC System for Precision Ophthalmic Tinting.
Some people experience perceptual distortion and discomfort when they read. The purpose of the system is to establish whether coloured light will reduce these complaints, and, if so, to provide an appropriately coloured light source by means of tinted lenses.
Text is illuminated by coloured light in a device called an Intuitive Colorimeter. The device uses a patented system of colour mixture which allows the colour (hue) and depth of colour (saturation) to be varied independently and continuously, without any associated change in brightness (luminance). Effects of small changes in colour on perceptual distortion and visual discomfort can be assessed rapidly whilst the eyes are colour adapted and the luminance remains constant and appropriate. The colorimeter has no coloured surfaces in order to eliminate the potentially confusing effects of colour constancy and the spectral power distribution. Adaptation to the illuminating light allows an evaluation of the extent to which expectation and memory contribute to the subjective judgement.
Any colour setting (chromaticity) can be precisely matched by a combination of standardised trial lenses. The lenses have smooth spectral transmission (to reduce the influence of the illuminating source, metamerism). Any chromaticity can therefore be obtained by a lens combination having a smooth spectral transmission. The combination that matches the colorimeter setting is tried by the patient and the colour refined, if necessary. The chosen combination constitutes a "prescription". The prescription is sent to a dyeing company and used to guide the dyeing of matching spectacle lenses. Only two dyes are used and the dyeing can be guided by eye. The final spectral transmission is measured with a spectro-radiometer, and the colour checked using purpose-built software. The software calculates whether the lens is likely to interfere with the perception of coloured signal lights, and whether it would be appropriate to use the glasses as sunglasses. This information is provided in a digestible form for the optometrist and patient. The patient is invited to send comments to the Visual Perception Unit at the University of Essex. The clinical evidence for the efficacy of the system has been reviewed here.
The Intuitive Overlays are sheets of coloured plastic film suitable for placing over a page of text. They can be used for assessing the possible benefits of coloured text in the classroom.
The overlays are designed so that, when used singly or in pairs superimposed, a large range of colours can be systematically and efficiently obtained.
The overlays increase reading rate in many children.
A parsimonious explanation for the efficacy of tinted lenses and coloured overlays is offered elsewhere.