4 Advice to Choose a optical prism

Optical Prisms Selection Guide

Optical prisms are solid glass optical components that can be used to manipulate light in many ways. From the minuscule dove prism in an endoscope to the porro prism in a space telescope, prisms can be found in many widely varying optical systems. Chosen wisely they can lead to more efficient, compact design and higher image quality.  But just how do prisms work, and what prism is best for your application? 

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This selection guide will provide an overview of the different geometries you may see in prisms, complete with details about prime applications and key functionalities. Knowing whether you need dispersion, deviation, displacement or rotation will help you decide what prism type is best for you. 

You should also be aware that prisms can be made of various substrates, and the substrate that is best for you will depend on the specific wavelengths of light you are targeting and the environmental conditions in which you work.

Right Angle Prism

A right angle prism is the basic geometry you probably think of when you think of a prism— a neat triangle wedge that deviates a ray path by 90°. These prisms are also called 45° – 90° – 45° prisms. 45° – 90° – 45° prisms are often used in combination to displace an image or beam. 

The standard way to use a right-angle prism is with light entering through one leg and exiting through another. The image you get is left-handed.  But if you change the orientation relative to the incident beam so that light enters through the hypotenuse, your prism is called a porro prism. The light will bounce against both legs and exit through the same side, the hypotenuse. This results in a right-handed image. 

Dove Prism

There are two kinds of dove prisms; one with a reflective coating and one without. The uncoated version rotates a prism by twice the prism rotation angle and produces a left handed image. The coated version reflects a beam back onto itself, and produces a right handed image. 

Both types of dove prism look the same; like a right angle prism with one corner truncated or cut off. 

Retroreflectors

Retroreflectors are also called trihedral prisms or corner cube reflectors; they reflect any beam that enters the prism face and sends it back at an 180 degree angle. The image resulting from a retroreflector is left handed.

Retroreflectors have many applications, and they aren’t all earth-bound.  When the Apollo 11, 14, and 15 landed on the moon, the astronauts left retroreflectors behind. Scientists on Earth aimed lasers at the prisms and, through time of flight calculations,  were able to calculate precisely the Moon’s orbit and shape.

Light Pipe Homogenizing Rods

Light pipe homogenizing rods are a special kind of prism that hardly looks like a prism at all. And rather than scatter light, these prisms make it tidier—- more homogenized. These square or hexagonal pipe-shaped prisms are also known as waveguides, light funnels, homogenizing rods or light guides. 

Light enters a waveguide through one end, and is kept within the prism with total internal reflection till it is emitted at the other end.  These light funnels are often used to transform the light from  non-uniform sources into clear, homogenized light. 

When checking a prescription, most opticians have an easy time finding and dotting a lens optical center. The center of the lensometer target is moved until it is in the center of the eyepiece reticle.

Often the prescription that includes prescribed prism ends up passed to someone else with the words "Here, you do this." It's really just as easy to center a lens on the point of prescribed prism as it is to center it at the optical center.

Prism is required when the line of sight must be changed to ensure binocular vision i.e., one fused image from both eyes. Prisms are used to move an image depending on whether the patient has a phoria (tendency of the eye to turn) or tropia (a turned eye). In the illustration on the right, the center of the lensmeter target (lines) is placed on the center of the reticle (concentric circles). This means that the lens is aligned with the on the lensmeter with the optical center along the optical axis of the lensmeter. Then the inking device can conveniently dot the optical center line, the place where the lens has no prism.

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When a prescription specifies prism, it specifies the amount of prism, in prism diopters, and the direction of the base (thickest part) of the prism. That means that the lens will be placed with that point of prism at the patient's PD, rather than the point of no prism, the optical center.

Remember, all lenses are prisms i.e., plus lenses are two prisms base to base, minus lenses are prism apex to apex.

The place where the prisms join is the point of no prism i.e., the optical center.Verifying prescribed prism is simple; locate the target center, the point where the mires cross at the point of prescribed prism. The target always moves in the direction of the base and position is dependent on whether its a right or left lens.

When patients look through plus lenses, the base is located at the optical center; in minus lenses, the base is located
at lens edge away from center. The place where prisms join is the point of no prism.

NO prism, lenses are centered at the lens' optical center.

The illustrations below show the location of the base in prescriptions with prescribed prism.

Verifying prescribed prism is simple; locate the target center, the point where the mires cross at the point of prescribed prism. The target always moves in the direction of the base and position is dependent on whether its a right or left lens.For example, in a right lens, 2 prism diopters, base out would look like this.

The illustrations below show the location of the base in a variety of prescriptions with prescribed prism. Remember the location of base in or base out is determined by right and left. In is always on the nasal side and out is always on
the temporal side of center.

PRISM AND CYLINDER Rx's

Locating and verifying prism, in a manual lensmeter, becomes more difficult when the target is formed by a cylinder prescription and even harder to visualize when there is an oblique cylinder axis. At axes near 90 and 180, the vertical and horizontal lines help to align the location of the prism. In cylinder lenses the sphere and cylinder lines are visible separately and the target center and point of prescribed prism must be found by rotating the two powers into focus. In the example below, the Rx is +1.00-2.00 x 90 and 1 prism diopter base down.

First, focus the sphere lines and move the center line to 1 base down, then focus the cylinder lines and align the center cylinder line at the center of the reticle. Rotate the power drum back to see if the sphere lines ahave moved from the original place and adjust them if necessary. Repeat the focusing back and forth so that the position where the two center lines cross are at the 1 prism diopter point on the reticle. Excellence comes from practice.

Again, in non-prism prescriptions the optical center or point of no prism is located at the patient's pupillary distance (PD); in prescriptions with prescribed prism, the point of prescribed prism is located at the PD.

FACTS ABOUT PRISM

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