Photosensitivity is usually tested by intermittent photic stimulation. Photoparoxysmal responses in the EEG are enhanced when the eyes remain closed during stimulation. We tested the hypothesis that this is due to diffusion of light by the eyelids. In 25 photosensitive patients, conditions 'eye closure', 'eyes closed', 'eyes open' and 'eyes open with diffuser' were tested for frequencies of 2-60 Hz. Additional influences of a red filter and fixation were also examined. The photosensitivity range was maximal in the condition 'eyes open with diffuser', due to an increase of the upper limit to a median 60 Hz (range 25-60), from 35 (15-50) Hz with eyes open, 37.5 (25-60) Hz with eyes closed and 40 (23-60) Hz with eye closure (P = 0.0002). This effect was attenuated in patients on valproic acid and with the use of a red or white filter. Testing with a diffuser was more sensitive than without, except for one patient who was photosensitive only in the eye closure condition. The influence of the eyelids on photosensitivity can be explained by diffusion of light, attentuated by an intensity loss. Use of a diffuser may simplify testing for photosensitivity in the EEG laboratory. The diffusion effect may explain seizure susceptibility in front of 50 and 60 Hz television screens.