Penta Prism: A Detailed Overview


A penta prism is a five-sided reflecting prism utilized to deviate a beam of light by a constant 90°, regardless of the entry beam’s angle relative to the prism. This unique characteristic is achieved through two internal reflections, allowing the transmission of an image through a right angle without inverting it. Unlike ordinary right-angle prisms or mirrors, penta prisms maintain the image’s handedness.

Working Principle

The penta prism‘s internal reflections are not due to total internal reflection, as the beams encounter the surfaces at angles less than the critical angle. Instead, the reflecting surfaces are coated to create mirror-like reflections. The two opposite transmitting faces often have an antireflection coating to minimize spurious reflections. The fifth face of the prism, which does not play an optical role, truncates the prism, simplifying its geometry and making it more practical to use.

Variants and Applications

  • Roof Pentaprism: Commonly used in single-lens reflex (SLR) cameras, a roof pentaprism corrects the lateral inversion of the image caused by the camera lens and reflex mirror. This variant includes a “roof” section with two surfaces meeting at a 90° angle, which reverts the image to its normal orientation.
  • Standard Penta Prisms: Used in image erecting assemblies, laser insertion prisms for range-finding, and advanced surveillance. Their consistent deviation angle, regardless of slight rotations, makes them valuable in applications requiring precise deviation without precise alignment.

Material Options

  • Corning Fused Silica 7980: This high-purity synthetic amorphous silicon dioxide is manufactured via flame hydrolysis. It has a very low thermal expansion coefficient, is colorless and non-crystalline, making it ideal for ultraviolet transmission.
  • JGS1: A high-quality ultraviolet grade fused silica, transparent in UV and visible ranges, with an intensive OH absorption band in the 2600-2800 nm interval. JGS1 penta prisms are suitable for deep UV and visible applications, offering minimal bubbles or inclusions.
  • JGS2: Similar to JGS1, this ultraviolet grade fused silica is typically bubble-free in micro prisms but may contain bubbles or inclusions in larger pieces. It is best used in applications less sensitive to these imperfections.

Customization and Coatings

At MOK Optics Co., Ltd., we provide penta prisms with aluminum or enhanced aluminum coatings, with optional black paint surfaces to increase reflection. Variations include:

  • Half Penta Prisms: Deviate a beam by 45°.
  • Roof Penta Prisms: Reverse the image’s parity.

Additional Information on Prisms

What is a Prism? A prism is a three-dimensional solid with two identical ends, flat faces, identical bases, and equal cross-sections. The faces are parallelograms or rectangles, excluding the bases. For instance, a pentagonal prism has two pentagonal bases and five rectangular faces.

Applications of Prisms Prisms are used in telescopes, periscopes, microscopes, and various scientific experiments to study light reactions. They are crucial in optical systems requiring precise light manipulation.

Rectangular Prism A rectangular prism, or cuboid, is a solid with six rectangular faces. It has a consistent cross-section along its length, classifying it as a prism.

Angle of Prism The angle of a prism is the angle between its two refracting surfaces through which light enters and exits after refraction.

How Does a Prism Work? A prism refracts light, separating it into its constituent colors due to different speeds of light within the material, resulting in a spread-out spectrum.

Contact Us

Our design team at MOK Optics Co., Ltd. is ready to collaborate on custom penta prisms and other optical systems tailored to your specific application needs. For more information, please contact us for a detailed consultation and quote.