# 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.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
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." This is where the Eye Check comes into its own.
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 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. Br J Ophthalmol 1994;78(11):842-4.
* 9 (25%) patients had deviation within 10 prism dioptres on prism bar cover test.
Relation between Duration of Presentation and Effect of Botulinum Toxin: Of 21 patients presenting within 1 month of duration, mean deviation before treatment was 30.10 prism dioptres. After treatment at 1 week, mean deviation was 2.58 prism Diopters i.e., 27.52 prism diopters reduction was observed at 1 week after injection.
At 1 week after treatment, mean deviation was 5.47 prism diopters i.e., 29.39 prism dioptres reduction was observed at 1 week after injection.
* Idopathic cases were 11, in them mean deviation before treatment was 31.43 prism dioptres. At 1 week after treatment, mean deviation was 6.
In them mean deviation before treatment was 41.25 prism dioptres. At l week after treatment, mean deviation was 13.45 pirsmdioptres i.e., 27.80 prism dioptres reduction was observed at 1 week after injection.
In them mean deviation before treatment was 30.25 prism dioptres. At 1 week after treatment, mean deviation was 13.25 prism diopters i.e .17 prism diopters reduction was observed at 1 week after injection observed at I week after injection.
In them mean deviation before treatment was 31.50 prism dioptres. At 1 week after treatment, mean deviation was 3 prism dioptres i.e., 28.50 prism dioptres reduction was observed at 1 week after injection.
Prism ballast usually employs 1 or 1.5 prism dioptres base down.

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