Apparatus and a corresponding method for the diagnosis and alleviation of symptoms of visually induced physiological defects and/or pathological conditions is provided. A plurality of narrow-band light sources are combined to constitute a colour controllable lamp. A method for adjusting the settings of this lamp permits the optimum illumination for a particular subject to be found, whilst the latter carries out a task such as reading or writing. By use of the lamp to simulate the expected visual stimulus, to which the subject would be exposed if provided with viewing aids such as tinted spectacles and the like, an optimal selection from a database of such aids may be made or a new formulation defined. Inter alia, the symptoms of visual dyslexia, macular generation and visually induced migraine may be alleviated.