Dispersive Prisms, Reflective Prisms and Distortion Prisms

 One of the most famous images in the history of optics is Sir Isaac Newton's image of a beam of white light passing through a glass prism with a rainbow coming out the other side. In this article, we will try to describe four types of prisms: dispersive prisms (like the one used by Newton), reflective prisms, rotating prisms, and displacement prisms.

Reflection Prisms

Reflection prisms can be used in imaging systems. Due to total internal reflection, light entering the prism undergoes multiple reflections until they reach the output surface. Reflective surfaces can be added so that the prism acts as a beam splitter. Reflective prisms are used to reduce the physical size of an optical system, to change the direction of light, and to change the orientation of an image.

Reflective prisms have lower optical power losses than equivalent systems made of mirrors and are often easier to align because they use a single element rather than multiple elements.

Right-angle prisms are typically used to deflect the direction of light by 90 degrees. A right-angle prism can be used in a Porro configuration when light is incident through the beveled side of the prism. The light will be deflected 180 degrees and flipped.

Dowell prisms are right-angle prisms with the top removed. They can be used to invert the image. For optical sensing applications, a coating can be applied to the side where the light is reflected.

Right-angle roof prisms are typically used in binoculars or when a right-angle deflection of the image is required. The image is deflected from left to right rather than from top to bottom.

Pentagonal roof prisms deflect the beam by 90 degrees without deflecting it from left to right or top to bottom.
Rhombic prism, a prism for reflection with a diamond-shaped cross section. The resulting output beam is displaced from the input beam, but it does not change the direction of the beam and does not invert the image. The rhombus has a pair of angles of 45°, and can be used as a combination of two rotating prisms and a rectangular transparent glass.







A quartz right-angle prism is often used as a reflector, which bends the light path by 90° for total internal reflection, or as an image retroreflector, which bends the light by 180°. Right-angle prisms are also used in applications such as image combining and beam deflection.


Dispersive prisms

A dispersive prism, usually in the shape of a triangular prism, is used to split the different wavelengths of an incident beam into different light paths. Each wavelength will be deflected by the prism shape by a different angle, thus changing the refractive index of each wavelength. Flint is a very common glass used to make prisms because it is highly dependent on the refractive index relative to the wavelength of light. Typical applications of dispersive prisms can be found in spectroscopy, but they can also be used for laser tuning and beam combining.


Transformation Prisms

An interesting application of prisms is to change the size of the incident beam. This is caused entirely by the geometry of the prism (e.g., the angle of incidence to the refractive plane), rather than by the focusing element or collimation effect as in a lens. Morphing lenses are usually configured in pairs to keep the beam propagating along the optical axis.
Cylindrical lenses are mainly used to change the size required by the imaging design. For example, converting a light spot to a line spot or changing the height of an image without changing the width of the image. Cylindrical lenses are used for linear detector illumination, bar sweeps, holographic illumination, light information processing, computers, and laser emission. Optical cylindrical lenses are widely used in intense laser systems and synchrotron beamlines.






Process

The fabrication of a prism usually involves several steps. Starting with the selected glass, a series of cuts are made to form the basic prism shape. This stage usually ends with a rough draft of the final product. The prism will have the required shape, but with poor optical properties.
After that, a series of polishing and smoothing steps need to be performed on the optical surface. This may require several iterations depending on the customer and the optical tolerances required by their application. At this stage, anti-reflective coatings, filters and metal layers may be added to achieve the required performance.
A technician oversees and evaluates each stage. Some prism geometries can be purchased off-the-shelf, but for specific applications or custom optics, significant time is usually required for testing and fabrication.


If you need to use a custom prism and what your application is, please contact us and BOFA will provide you with a custom solution.