Abstract: A method of testing applicable to a vision aid, such as a tinted transparent overlay for placement over textual matter to be read, or selection of a tint for a vdu screen, to assist in amelioration of symptoms in dyslexia and other optical and related disorders, uses a quantitative approach to the identification of the colorimetric parameter within the three dimensions of color space applicable to the vision aid for optimal patient visual performance, and provides a test procedure in which a numerical value (the benefit value) is associated with a given value of each of the three calorimetric parameters, and a short series of similar tests on differing colorimetric parameters enables the optimum value to be interpreted whereby the optimal value may not be a value actually tested, with tasks strictly separated into array tasks and non-array tasks.