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.