What is the Function of Prisms in Binoculars?

Ever wondered why prisms are used in binoculars?. Well, this is a common query that most people have in their minds. Today we will look at what is the function of prisms in binoculars.

Binoculars play the function of magnifying objects and making them appear larger to our eyes than our naked eyes would see. These devices come in two common types of binoculars namely; roof prism and Porro prism binoculars.

The binocular type is determined by the type of prism used inside it. Prisms are just piece of glasses cut with precise angles and plane faces, useful for analyzing and reflecting light.

There are two main functions of prisms in binoculars:

  • The primary reason why prisms are used in binoculars is for correcting the image orientation of the object. When the image is formed by the objective lenses, it appears upside down. People don’t enjoy viewing their image that way. Hence the prism is used inside to turn the image upright for the viewer’s eyes.
  • The other function of a prism is to reduce the light path and make the binoculars smaller. They bend the light path thereby reducing the size of the binoculars. This makes binoculars become smaller than if there were no prisms used inside them.

how does Porro Prism Work in Binoculars?

Porro prism is named after its inventor Ignazio Porro. It is often found in optical instruments to change the image orientation in that device. It comprises of a block of glass shaped as a right geometric prism with right-angled triangular end faces.

The way they work is as the light enters the large rectangular face of the prism, it undergoes a total internal reflection twice from the loped faces and exit again through the large rectangular face.

What is the Function of Prisms in Binoculars
Porro Prism Design

An image traveling through a Porro prism is rotated by 180° and exits in the opposite direction offset from its entry point. The image still remains the same as it exists.

Roof Prism

Roof prism is also known as Dach prism and is from German. This prism is a  reflective optical prism containing a section where two faces meet at a  90° angle. This type of prism is named so because the two 90° faces resemble the roof of a building.

Reflection from the two 90° faces returns an image that is flipped laterally across the axis where the faces meet.

What is the Function of Prisms in Binoculars
Foor Prism (image: Wikipedia)

The simplest roof prism is that Amci roof prism and the most common ones are the Schmidt-Pechan prisms and the Abbe-Koenig prism. Roof prisms split the beam entering them in half with one half of the beam hitting the first one face then the other face while the other half is inverted.

Therefore, a roof prism can be used only with some distance to focal planes, or the “edge” of the roof would introduce slight distortions. The angle between the two faces has to be very close to 90°, or image quality would be degraded.

Coatings Use in Prisms

Phase Correction Coating

The multiple internal reflections cause a polarization-dependent phase-lag of the transmitted light. This is corrected by applying multilayer phase-coating to one of the roof surfaces to avoid unwanted interference effects and lost in image quality.

When roof prism is not coated with phase correction coating. s-polarized and p-polarized light each acquire a different geometric phase as they pass through the upper prism. When the two polarized components are recombined, interference between the s-polarized and p-polarized light results in a different intensity distribution perpendicular to the roof edge as compared to that along the roof edge. This causes a reduction in image quality.

There are two common coatings that are used in prisms today and they are;

  • BaK-4 – Created from Barium Crown Glass. Considered as the superior prism because it has a better light transmission.
  • BK-7 – Created from Borosilicate Glass. Good light transmission and more affordable.


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