The colour required is determined using the Intuitive Colorimeter, designed by Prof. Arnold Wilkins of the University of Essex.
The Intuitive Colorimeter is essentially a black box that you put your head into. From here you view some text made up of random words, under a white light. The optometrist doing the test will change the colour and intensity of the light and ask you to tell them whether the first or second colour is better/clearer/more comfortable, in the same way an optician does with lenses in a standard eye test. From this, your optimum tint will be determined.
It is recommended that you be re-tested every few years to check you have the best tint for you. The colour I need changes every 6-18 months and I can tell when my lens colour needs changing, even just a 6 degree shift on the colour wheel, but I am extremely sensitive – not everyone will be able to tell that their optimum tint has changed.
While precision tinted lenses give a more accurate colour, some people may find the use of coloured overlays fit with their lifestyle better.
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Useful resources –
Precision Tinted Lenses, Cerium. http://ceriumvistechproducts.com/precision-tinted-lenses/. © Cerium Visual Technologies Limited 2015.