# prism dioptre

## dioptre, prism

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 Χ
The exact formula for any angle α less than 90º is
α in Χ = 100 tan α
Note: 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.
Fig. D3 Effect of an 8 Χ prism
References in periodicals archive ?
The smallest that we can perceive is a difference of about one millimetre in asymmetry," explained Dr Barnard, "but one millimetre is a 20 prism dioptre squint.
It calculates how many prism dioptres the eyes converge or diverge for each dioptre increase or decrease change in accommodation; this can be measured by adding lenses in front of the patient's corrected prescription using a Thorington near card or Maddox wing and calculating the change in phoria per dioptre increase or decrease in what is known as the gradient method.
The angles are characteristically large, ranging from 30 to 100 prism dioptres (PD) and increases gradually over time as long as the cause of visual deficit remains active.
The quantitative data including age at surgery and pre-operative amount of deviations in prism dioptres at distance and near and amount of surgery in millimetre of lateral rectus recession and medial rectus resection were represented in Mean [+ or -] SD.
8 mm bimedial rectus recession in infantile esotropia of 8090 prism dioptres.
9 (25%) patients had deviation within 10 prism dioptres on prism bar cover test.
39 prism dioptres reduction was observed at 1 week after injection.
At 1 week after treatment, mean deviation was 3 prism dioptres i.
The preinjection deviations ranged from 18 to 60 prism dioptres.
9(25%) patients has deviation within 10 prism dioptres on prism bar cover test.

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