See correction induced convergence Prentice's law. The amount of prism power is given by Prentice's law. Induced prism Prismatic effect created when the patient's visual axis does not pass through the optical centre of an ophthalmic lens. These Press-On Fresnel prisms can be cut to any desired shape and are used commonly in orthoptics treatment. Large optical effects can thus be provided in a much thinner and lighter form.
See erector.įresnel Press-On prism A trade name for a thin disc of transparent plastic consisting of one flat surface which can adhere to a clean lens surface when pressed in place, and another surface on which are incorporated small prismatic elements laid parallel to one another. Light enters one side, is then refracted onto the base surface where it is internally reflected and refracted again through the other side.Įrecting prism A prism designed to invert an image in an optical system with no change of size or shape. It can be used with the cover test or even to measure fusional responses when determining the zone of clear, single, binocular vision if rotary prisms are not available.īase-in prism base-out prism See base setting.Ĭompensating prism See associated heterophoria relieving prism.ĭissociating prism A prism which, when placed in front of an eye, produces dissociation.ĭove prism An isosceles reflecting prism used to invert the image in an optical system.
Prism definition series#
Prism bar Clinical device consisting of a series of prisms of increasing strengths arranged in a convenient mount for rapid positioning in front of an eye. Prism adaptation See vergence adaptation prism adaptation test.Īligning prism See associated heterophoria.
It consists of two prisms, usually one of crown glass and the other of flint, of equal angular dispersions and mounted so that the apex of one is against the base of the other. See vergence adaptation base setting prism dioptre prism power minimum deviation of a prism ophthalmic prism recumbent spectacles spectroscope.Īchromatic prism A prism that deviates light without dispersion. D = i + i′ − awhere i is the angle of incidence, i′ the angle of emergence and a the prism angle (Fig.
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