1. A unit specifying the amount of light deviation by an ophthalmic prism. One prism dioptre (written 1 Χ) represents a deviation of 1 cm on a flat surface 1 m away from the prism. The surface is perpendicular to the direction of the original light ray (Fig. D3). Similarly, a 2 Χ prism deviates light 2 cm at a distance of 1 m, and so on. For small angles, conversion between prism dioptres and degrees is given by the approximate formula
7 Χ = 4º or 1 Χ = 0.57º or 1º = 1.75 ΧThe exact formula for any angle α less than 90º is
α in Χ = 100 tan αNote: the current British Standard regarding ophthalmic lenses specifies a deviation (in Χ) of a ray of light of wavelength 587.6 nm incident normally at one surface. 2. A unit of convergence of the eyes. See Prentice's law; prism power.