Monochromatic light (half-bandwidth (λ1/2) ≤ 13 nm) of different wavelengths (λmax 424, 456, 472, 496, 520 and 548 nm) and irradiances (0.70-65 μW cm−2) was administered for 30 min via a sphere (45 cm diameter, Apollo Lighting, Leeds, UK) coated with white reflectance paint (Kodak, Hemel Hempstead, UK). The sphere was illuminated via a fibre optic cable connected to a metal halide arc lamp light source (Enlightened Technology Associates Inc., Fairfax, VA, USA). The light source did not emit ultraviolet, infrared or electromagnetic radiation outside the visual range. The input port of the sphere housed the monochromatic (Coherent Ealing, Watford, UK) and neutral density (Kodak, Hemel Hempstead, UK) filters used to alter wavelength and irradiance, respectively. This light sphere provided constant uniform illumination of the entire retina in the pupil-dilated subjects. Light irradiance (μW cm−2) was measured at the subjects’ eye level (optical powermeter, Macam Photometrics, Livingstone, UK). Each wavelength was tested at five to eight irradiances (0.70-65.0 μW cm−2) and for each irradiance between three and seven subjects were studied. During light exposure subjects were instructed to keep their eyes open (verified by research observers) and to fix their gaze on a target dot in the centre of the sphere. A chin rest and head band limited movement of the subjects’ head. The design of the light sphere altered the spectral quality of the monochromatic filters slightly from the manufacturer's specifications. Measurements with a spectroradiometer (Spectrascan 650 portable, Photoresearch, Chatsworth, CA, USA) confirmed the actual λmax wavelengths at eye level to be 424, 456, 472, 496, 520 and 548 nm (λ1/2 5-13 nm) instead of 430, 460, 480, 500, 530 and 560 nm (λ1/2 10 nm), respectively. The measured wavelengths were used for all subsequent photon density calculations.