All about Prisms- Easy to Understand
This paper describes about introduction to prisms used in ophthalmology, its optics and its calculation.
Partha Haradhan Chowdhury1* and Brinda Haren Shah2
Prasikshan Sansthan, Pauri, Affiliated to Uttarakhand State Medical Faculty, India optometrypublish@gmail.com Keywords: Prism; Prentice Rule; Risley Prism
Introduction
Prism is defined as a portion of transparent, non parallel, isotropic material and two polished surface. Usually it is helpful for making all ophthalmic lenses and it is very helpful for vergence also.
Ophthalmic prisms have mild curved surface and the meaning of isotropic is “from the area of the prism, have equal amount of scattering effect” is present. The prism consists of Apex or Apical Angle and Base. Whenever the light is passing through the prism, then light is bend towards the Base and image will be shifted towards the Apex. Apex is denoted in degrees and whatever the light is passing through the prism that is denoted with cm [1, 2, 3].
So, the Prism Diopter can be defined as “A deviation of 1 cm at a distance of 1 m” 1 PD = 0.57 degrees All the ophthalmic lenses are made up of prisms, so if spherical lenses are decentered from its proper position then it can create prismatic effect. It is explained by “Prentice Rule.” P = D*F Where D is Decentration and F is Refractive Power of the lens.
Basic Instruction for Prentice Rule
In case of minus refractive error, base direction will be opposite compared to decentration of spherical lens.
In case of plus refractive error, base direction will be on the same side compared to decentration of spherical lens.
Always mm should be converted to cm. Eg : a) If , spherical lens is -8.00 Ds and decentration is 5 mm towards nose, then according to prentice rule, P = D*F 5mm = 0.5 cm (mm should be converted to cm) P = 0.50 * (-8.00) = 4.0 PD Base out. Here, base direction towards ear became decentration towards nose, opposite due to minus lens. b) If, spherical lens is +9.00 Ds and decentration is 4 mm towards nose, then as per prentice rule, P = D*F 0.4*(+9.00) = 3.6 PD Base In
Here, base direction is on the same side compare to decentration due to plus lens.
Risley Prisms
R = Risley prism R = Rotary prism This prism is used in Refractor/ Phoropter, to measure phoria and fusional vergence. Risley prism consists of two prisms, one behind the other. It consists of a knob, when the knob is oriented vertically, then Base in prism and Base out prism is imposed, when knob is oriented horizontally, then Base up and Base down prism is imposed.
References
-
Atchison DA, Smith G (2005) Optics of the Human Eye.
-
Fannin TE, Grosvenor TP (1996) Clinical Optics.
-
Benjamin WJ (2006) Borish’s Clincial Refraction.
- Screening of Hospital Staff During World Glaucoma Week in a Tertiary Eye Care Centre
- Angioid Streaks with Macular Neovascularization: Clinical Insights from Two Cases
- Giant Kissing Naevus: An Oculoplastic Challenge
- Why Freedom of Vision Should Not Cost the Freedom of Feeling - LASIK in the Climate of Change
- Asymmetric Optic Nerve with Small Disc and Large Cup: A Rare and Challenging Case of Unilateral Optic Nerve Hypoplasia
- Large Angle Exotropia in a Child: A Case Report