A corner prism, more precisely known as a corner cube reflector (CCR) or retroreflector, is a specialized optical component designed to reflect any incident light ray directly back to its source, regardless of the angle of incidence. This unique property of automatic retroreflection makes it an indispensable tool in applications requiring precise alignment, distance measurement, and tracking.

The prism consists of three mutually perpendicular reflective surfaces meeting at a single corner (like the inner corner of a cube). This geometry is the source of its unique behavior:

• Retroreflection: When a light ray enters the prism, it undergoes three successive reflections—one from each of the three perpendicular surfaces. These three reflections reverse the direction of the light’s velocity vector, sending it back parallel to its original path. This happens even if the prism is slightly misaligned, making it highly tolerant to angular errors.

• Forms: Corner cubes exist in two primary forms:

  1. Hollow Retroreflector: Three flat mirrors are aligned precisely at 90° to each other.

  2. Solid Glass Prism: A single block of optical glass is precision-ground and polished to form three orthogonal quadrilateral faces. This is the more common and durable type for most applications.

• Materials: Typically fabricated from materials like N-BK7 glass for visible light or Fused Silica for UV applications. For high-power lasers, synthetic fused silica is preferred for its low thermal expansion.

• Extreme Precision: The critical 90° angles between the three faces must be held to exceptionally tight tolerances (often within arc-seconds). Any deviation from perpendicularity causes a wavefront error and spreads the return beam, reducing measurement accuracy.

• Coatings: The three reflective surfaces can be bare, relying on Total Internal Reflection (TIR), or they can be coated with a protected metal coating (e.g., aluminum or gold) for applications where TIR fails (e.g., non-normal incidence or immersion in liquid). The entrance face is always coated with an anti-reflection coating to minimize losses.

• High Precision Ranging: The core component in laser rangefinders, lidar (LiDAR) systems, and satellite laser ranging (e.g., tracking the distance to the moon using retroreflector arrays left by Apollo missions).

• Metrology and Alignment: Used in surveying instruments (e.g., total stations), industrial alignment tools, and vibration monitoring systems as a passive, reliable target.

• Optical Tracking: Serves as a robust target for tracking systems in virtual reality, motion capture, and robotics due to its unambiguous return signal.

• Safety and Transportation: The principle is used in mundane applications like bicycle reflectors and road signs, where plastic versions return light to car headlights.

• Wavefront Error: The three reflections can introduce wavefront distortion, which is problematic for interferometric applications. For this, more complex cat’s eye retroreflectors are sometimes used.

• Polarization Sensitivity: The series of reflections can alter the polarization state of the returning light.