A right-angle prism is one of the most common and versatile optical prisms, defined by its two 45° acute angles and one 90° right angle. Its simple, robust geometry allows it to perform several critical functions in optical systems, primarily based on the principle of Total Internal Reflection (TIR).

A right-angle prism can be used in three primary configurations:

1. 90° Beam Deviation (Most Common): A light beam entering one of the rectangular faces perpendicular to it will strike the hypotenuse face at a 45° angle. If the prism is in air and the incidence angle is greater than the critical angle (which it is for most optical glasses), the beam undergoes Total Internal Reflection (TIR) at the hypotenuse. This reflects the beam and causes it to exit through the second rectangular face, deviating its path by exactly 90 degrees. This is highly efficient as it requires no metallic coatings.

2. 180° Beam Retroreflection (Dove Prism Orientation): If a beam enters through the hypotenuse face, it can undergo two TIR reflections—first from one rectangular leg, then from the other. This flips the image and sends it back out along the hypotenuse, parallel to its original path but displaced. This acts as a retroreflector.

3. Image / Erecting Prism: When used in the 180° configuration, the prism also rotates and inverts the image. This property is used to erect an inverted image in optical systems like telescopes or binoculars.

• Materials: Typically made from high-quality optical glass like N-BK7 or Fused Silica, chosen for their transmission properties across UV, Visible, or IR wavelengths.

• Precision: Critical manufacturing parameters include:

  • Accuracy of the 90° angle: Essential for precise beam steering.

  • Surface Flatness: Often specified as λ/4 or λ/10 at 632.8nm to prevent wavefront distortion.

  • Surface Quality: Measured by scratch-dig specifications (e.g., 20-10) to minimize light scatter.

• Coatings: While TIR is efficient, sometimes an aluminum or protected silver coating is applied to the hypotenuse face for applications where TIR conditions cannot be met (e.g., the prism is immersed in liquid or for use with light rays at acute angles).

• Beam Steering: The most frequent use, for folding an optical path by 90° to save space or align components in laser systems, lab setups, and imaging systems.

• Optical Imaging: Found in telescopes, microscopes, and periscopes for both bending light and erecting images.

• Retroflection: Used in interferometry, metrology, and alignment tools where a beam must be returned to its source.

• Reference Mirror: Serves as a highly stable and accurate 90° reference mirror in measurement instruments.