Abstract
The relationship between the visual threshold and the size of the surrounding field is investigated. Using different surround luminances as parameter, the visual threshold of a test field has been determined as a function of the size of the surrounding field. Curves obtained by plotting the luminance difference threshold against the angle subtended by the surrounding field are shown to have a flat minimum in the size range 3 to 10°. The increase of the threshold for larger surrounding fields is caused by light scattered in the media of the eye while the increase in smaller fields remains unexplained as far as pupil effects are concerned and will be the subject of further studies.

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