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An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides.
The prism cover test (PCT) is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. [1]
Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base". The base is the thickest part of the lens and is opposite from the apex. Light will be bent towards the base and the image will be shifted towards the apex.
Prism and Base Prism refers to a displacement of the image through the lens, often used to treat strabismus and other binocular vision disorders. The prism value is measured in prism diopters, and Base refers to the direction of displacement. PL is an abbreviation for plano or plain, meaning no prescription
In the fields of optometry and ophthalmology, the Hirschberg test, also Hirschberg corneal reflex test, is a screening test that can be used to assess whether a person has strabismus (ocular misalignment). A photographic version of the Hirschberg is used to quantify strabismus.
The Four Prism Dioptre Reflex Test (also known as the 4 PRT, or 4 Prism Dioptre Base-out Test) is an objective, non-dissociative test used to prove the alignment of both eyes (i.e. the presence of binocular single vision) by assessing motor fusion.
The parameters appraised by a lensmeter are the values specified by an ophthalmologist or optometrist on the patient's prescription: sphere, cylinder, axis, add, and in some cases, prism.
Dissociation of the deviation is brought about by presenting a red line image to one eye and a white light to the other, while prisms are used to superimpose these and effectively measure the angle of deviation (horizontal and vertical).
The separation of colours by a prism is an example of normal dispersion. At the surfaces of the prism, Snell's law predicts that light incident at an angle θ to the normal will be refracted at an angle arcsin(sin (θ) / n).
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted, when entering a material.