Optical prism : a complete guide

Most of us remember the famous rock band Pink Floyd&#;s &#;Dark side of the moon&#; album  cover showing a light beam been diffracted in a &#;rainbow&#; through a triangular prism. That is a very good summary of how light beam travel through a prism, below you&#;ll find a complete guide to look further in optical prisms.

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What is a optical prism ?

Definition

An optical prism is a bloc of optical material cut in geometrical shape for the specific purpose of modifying the incident light beam by affecting it&#;s direction, rotational angle or diffraction.

In optical systems they are used to turn a signal or separate the signal according to it&#;s wavelengths.

Specifications

Many different optical materials can be used to manufacture a prism, depending of the type of prisms and the wavelength range they will be used in. Optical prisms can be provided with below materials  :

Below table shows the typical specifications / tolerances for optical prism manufacturing :

Specification Detail Dimensional range 0.3mm to 300mm Dimensional tolerances +/- 0.1mm (normal) or +/-0.05mm Surface flatness up to Lambda /10 Surface quality S&D 40/20 Angle tolerance up to +/- 1 arcmin

Prisms can be coated, AR to reduce signal reflection on the surface or Reflective coatings to enhance the signal reflection at specific wavelengths.

Different types of optical prisms

Type of prism Description Usage Image Right angle prism Prism with a right angle triangular cut shape with both sides opposite to the hypotenuse of equal dimension. They are used to deviate a light path to 90° or 180° by TIR (Total Internal Reflection) Penta prism Prism with a pentagone cut shape, with specific angles. Penta prisms are used to redirect a signal by 90° without inverting nor reversing the image. Retroreflector Round base prism with three face pyramidal sphape. Return the light at 180° even without alignment of the incident light and the retroreflector. Used in topograply and other applications. Dove prism Parallelepiped which correspond at a truncated right angle prism. Rotate the image with no deviation of the beam. The rotation angle is equal to twice the prism optical axis angle compare to the incident beam. Roof prism Roof shaped prism. Used to deviate the signal at 90° while inverting it. Equilateral prism Prism made of high diffractive Abbe number glass in a shape of an equilateral triangle. Used to disperse the light with different angles for each wavelength. (think of the Pink Floyd album cover). Pellin Broca prism Quadrilateral prism with angles of 90°, 75°, 135° and 60°. Deviate the light by 90°, used in spectroscopy and for harmonic separations. Wedge prism Prism made of a plan surface and and other tilt surface. Used to deviate light signal (mostly for laser beams) Micro prism Prism with dimensions between 0,3 and 3mm of side. Used in optical fiber sysems and endoscopy. Glued prism Assembly of custom prism made by gluing / cementing, can include air space in between parts. Custom made to fit customer applications.

1. What are optical prisms?

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In optical devices, optical prisms can cut a piece of glass or other transparent materials at a precise angle and plane, which can be used to analyze and reflect light. Ordinary triangular prisms can separate white light into its constituent colors, called frequency spectrum. Each color or wavelength that makes up white light can be bent or refracted, but the amount is different. Shorter wavelengths (wavelengths toward the purple end of the spectrum) bend the most, while longer wavelengths (wavelengths toward the red end of the spectrum) bend the least. This type of prism is used in some spectroscopes, instruments for analyzing light and determining the identity and structure of materials that emit or absorb light.

2. Functions and applications of optical prisms 

Optical prisms refract light to reflect (reflection prism), disperse (dispersion prism) or split (beam splitter) light. The prism is usually made of glass, but any material can be used as long as the material is transparent and suitable for the design wavelength,with common materials including glass, plastic, and fluorite.

Optical prisms can reverse the direction of light through internal reflection, so those optical prisms for sale are useful in binoculars.

Optical prisms can be made in many different forms and shapes depending on the application. For example, the Porro prism consists of two prisms. The two prisms can invert the image and are used in many optical observation instruments, such as periscopes, binoculars and monoculars.

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