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Introduction to Optical Prisms
19/05/24

Prisms are solid glass optics that are ground and polished into geometrical and optically significant shapes. The angle, position, and number of surfaces help define the type and function. One of the most recognizable uses of prisms, as demonstrated by Sir Isaac Newton, consists of dispersing a beam of white light into its component colors. This application is utilized by refractometer and spectrographic components. Since this initial discovery, prisms have been used in "bending" light within a system, "folding" the system into a smaller space, changing the orientation of an image, as well as combining or splitting optical beams with partial reflecting surfaces. These uses are common in applications with telescopes, binoculars, surveying equipment, and a host of others.

Optical-Illusions-2

PRISM MANUFACTURING

First, a block of glass of a specified grade and glass type is obtained. This block is then ground, or generated, by a metal diamond bonded wheel into a near-finished product. A majority of the glass is removed quickly in this stage resulting in flat, but still coarse surfaces. At this point, the dimensions of the prism-to-be are very close to the desired specifications. Next is a fine grinding process that removes sub-surface breaks from the surface; this stage is known as smoothening. Scratches left from the first stage are removed in the second stage. After smoothening, the glass surfaces should appear cloudy and opaque. In both the first two stages, the prism surface must be wet in order to expedite glass removal and prevent overheating of the glass itself.

The third stage involves polishing the prism to the correctly specified surface flatness. In this stage, the glass is rubbed against a polyurethane polisher wet with "slurry," an optical polishing compound typically comprised of water mixed with pumice or cerium oxide. The exact duration of the polishing stage is highly dependent on the surface specifications required. Once polishing is completed, chamfering can begin. In this fourth stage, the edges of the prism are subjected to a spinning diamond plate in order to slightly dull the sharp edges it obtains throughout the aforementioned steps. After chamfering, the finished prism is cleaned, inspected, and coated with anti-reflection (AR) and/or metallic mirror coatings, if necessary, to further aid in overall transmission and/or reflection. Though the process is much more involved and may require more iterations or operations due to the number of surfaces on a prism, the Generating, Smoothening, Polishing and Chamfering Stages are roughly outlined as below.

Prism Manufacturing Process: Generating Stage
Prism Generating Stage

Prism Manufacturing Process: Smoothening Stage
Prism Smoothening Stage

Prism Manufacturing Process: Polishing Stage
Prism Polishing Stage
Prism Manufacturing Process: Chamfering Stage

Prism Chamfering Stage

 

THEORY: LIGHT AND REFRACTION
Understanding how a prism works is key to deciding which type of prism fits best for a specific application. In order to do so, it is important to first understand how light interacts with an optical surface. This interaction is described by Snell's Law of Refraction: n1sin(θ1)=n2sin(θ2)n1sin(θ1)=n2sin(θ2)
Where n1 is the index of the incident medium, θ1 is the angle of the incident ray, n2 is the index of the refracted/reflected medium, and θ2 is the angle of the refracted/reflected ray. Snell's Law describes the relationship between the angles of incidence and transmission when a ray travels between multiple media.

Snell's Law and Total Internal Reflection
A prism is notable for its ability to reflect the ray path without the need for a special coating, such as that required when using a mirror. This is achieved through a phenomenon known as total internal reflection (TIR). TIR occurs when the incident angle is higher than the critical angle θc: sin(θc)=n1n2sin(θc)=n1n2
Where n1 is the index of refraction for the medium where the ray originates, and n2 is the index of refraction for the medium where the ray exits. It is important to note that TIR only occurs when light travels from a high index medium to a low index medium.

At the critical angle, the angle of refraction is equal to 90°. Referencing above figure, notice that TIR occurs only if θ exceeds the critical angle. If the angle is below the critical angle, then transmission will occur along with reflection as given by Snell's Law. If a prism face does not meet TIR specifications for the desired angle(s), then a reflective coating must be used. This is why some applications require coated versions of a prism that would otherwise work well uncoated in another application.

THEORY: IMAGE HANDEDNESS
A significant aspect of imaging through a prism is image handedness, otherwise referred to as the orientation of the image. This is introduced every time the ray path hits a plane mirror, any flat reflective surface, or a prism surface at an angle that produces TIR. There are two types of handedness: right and left. Right handedness describes the case where an image undergoes an even number of reflections, resulting in the ability to read it clearly in at least one position. Left handedness describes the case where the image undergoes an odd number of reflections, leading to an irregularity in the position of the image that is comparable to what one sees in a mirror.

In addition to parity, there are three types of image change. An inversion is an image-flip over a horizontal axis, whereas a reversion is an image-flip over a vertical axis. When both are done at the same time, an image rotation of 180° occurs and there is no change in parity. Another way to think of parity is defining it as being determined by looking back against the propagation direction towards either the object or image in its optical space.

When using a prism, consider the following four points:
1. Image Handedness Changes Every Time an Image is Reflected.
2. Any Point along the Plane of the Reflecting Surface is Equidistant from the Object and Its Image.
3. Snell's Law Can Be Applied to All Surfaces.
4. When Testing for Image Handedness/Parity, It is Best to Use a Non-Symmetrical Letter Such as R, F, or Q. Avoid Using Letters Like X, O, A, etc.

TYPES OF PRISMS
There are four main types of prisms: dispersion prisms, deviation, or reflection prisms, rotation prisms, and displacement prisms. Deviation, displacement, and rotation prisms are common in imaging applications; dispersion prisms are strictly made for dispersing light, therefore not suitable for any application requiring quality images.

Equilateral Prisms - Dispersion
Function
Disperse White Light into Its Component Colors
Application
Spectroscopy
Telecommunications
Wavelength Separation

Equilatera Prisms

 

Littrow Prisms - Dispersion, Deviation
Function
Uncoated: Disperse White Light into Its Component Colors
Coated: Deviate the Ray Path by 60°
Image is Right-Handed
Application
Spectroscopy (Uncoated)
Multi-Spectral Laser System Tuning (Coated)

Littrow Prisms

 

Right Angle Prisms - Deviation, Displacement
Function
Deviate the Ray Path by 90°
Image is Left-Handed
Used in Combination for Image/Beam Displacement
Application
Endoscopy
Microscopy
Laser Alignment
Medical Instrumentation

Right Angle Prisms

 

Penta Prisms - Deviation
Function
Deviate the Ray Path by 90°
Image is Right-Handed
Application
Visual Targeting
Projection
Measurement
Display Systems

Penta Prisms

Half-Penta Prisms - Deviation
Function
Deviate the Ray Path by 45°
Image is Right-Handed
Application
Pechan Erector Assemblies

Half Penta Prisms

Amici Roof Prism - Deviation
Function
Deviate the Ray Path by 90°
Image is Right-Handed
Application
Microscopes
Telescope Eyepieces

Amici Roof Prism

Schmidt Prisms - Deviation
Function
Deviate the Ray Path by 45°
Image is Right-Handed
Application
Stereo Microscopes
Pechan Erector Assemblies

Schmidt Prisms

Retroreflectors (Trihedral Prisms) - Deviation, Displacement
Function
Deviate the Ray Path by 180°
Image is Left-Handed
Reflects Any Beam Entering the Prism Face, Regardless of the Orientation of the Prism, Back onto Itself
Application
Interferometry
Boresighting
Rangefinding
Laser Tracking
Precision Alignment

Retroreflectors

Wedge Prisms - Deviation, Rotation
Function
Used Individually to Deviate a Laser Beam a Set Angle
Combine Two to Create An Anamorphic Pair for Beam Shaping
Application
Beam Steering
Tunable Lasers
Anamorphic Imaging
Forestry

Wedge Prisms

 

Rhomboid Prisms - Displacement
Function
Displace Optical Axis without Changing Handedness
Direction Remains the Same (No Ray Deviation Occurs)
Application
Binoculars
Rangefinders
Laser Instrumentation

Rhomboid Prisms

Dove Prisms - Rotation
Function
Uncoated: Rotate an Image by Twice the Prism Rotation Angle
Uncoated: Image is Left-Handed
Coated: Reflect Any Beam Entering the Prism Face Back onto Itself
Coated: Image is Right-Handed
Application
Interferometry
Astronomy
Pattern Recognition
Imaging Behind Detectors or Around Corners

Dove Prisms

Anamorphic Prism Pairs - Expansion
Function
Expand Incident Beam Diameter in One Dimension
Ideal for Making Elliptical Beams Circular
Application
Laser Diode Beam Expanders
HD Imaging Lenses

Anamorphic Prism Pairs

Light Pipe Homogenizing Rods - Homogenation
Function
Homogenize Non-Uniform Light Sources
Application
LED Illuminators
Micro-Projectors
Laser Speckle Reducers
OEM Illumination

Light Pipe Homogenizing Rods

 

Tapered Light Pipe Homogenizing Rods - Homogenation
Function
Homogenize Non-Uniform Light Sources While Reducing Output Numerical Aperture (NA)
Application
Projectors
Micro-Display Relay Systems

Tapered Light Pipe Homogenizing Rods
This introduction gave a look into the manufacturing process and the theory associated with prisms, as well as a selection to help you find the best prism for your application.