tangent screen

screen, tangent 

A large plane surface for detecting and plotting the central visual field (about 50º in diameter) by moving the position of a stimulus (e.g. a white 1 mm pinhead). It consists of dull black cloth or other material perpendicular to the line of sight and placed usually 1 m away from the subject (2 m gives more accuracy). In the centre of the screen is a white spot that provides a fixation point and a series of radial and circumferential lines are sewn or drawn to facilitate the localization of the stimulus. Syn. Bjerrum's screen. See campimeter; perimeter.