The angle, position, and number of surfaces of a prism help define the type and function. To understand how the most popular prisms work and how each can best be used in light reflection and refraction applications, consider right angle prisms, roof prisms, and combination prisms. For the theory of how prisms work and a selection guide with over ten unique geometries, view Introduction to Prisms.
Right Angle Prism
Figure 1: 45° - 90° - 45° as a Right Angle Prism Showing Inversion (Left) and Reversion (Right) [View Larger Image]
By far the most commonly used prism is the 45° - 90° - 45° prism, known popularly as the right angle prism. It can be used in many ways to achieve different results pertaining to image parity or deviation and is named so for the angles on its triangular faces. The most common application of the 45° - 90° - 45° prism is to treat it as a right angle prism, which has only a single reflection that deviates the incident ray by 90°. The produced image will then become left-handed, but depending upon the position of the prism, can be inverted or reverted (Figure 1).
Using the hypotenuse face of the prism rather than the leg faces allows for another configuration known as the porro prism. This produces a right-handed image since two reflections occur. The ray's direction is reversed when using a porro prism since the object enters and the image exits the same face. The position of the prism determines whether a rotation or just a deviation occurs (Figure 2).
Lastly, a 45° - 90° - 45° prism can also be used as a dove prism. A dove prism rotates the image 180°, but since only one reflection occurs, it will become either reverted or inverted depending on the position of the prism (Figure 3).
Roof Prism
A prism roof consists of two reflecting surfaces located 90° from each other. It is equivalent in function when compared to any other reflecting surface, except handedness does not change. A good example of this is the amici (roof) prism, which is basically a right angle prism with a roof. Under this configuration, a deviation of 90° still occurs, but without changing parity. A roof prism is often used in conjunction with other prisms in order to achieve the desired parity.
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