Dichroic prism

A light beam enters the first prism (A), and the blue component of the beam is reflected from a low-pass filter coating (F1) that reflects blue light (high-frequency), but transmits longer wavelengths (lower frequencies).

The blue beam undergoes total internal reflection from the front of prism A and exits it through a side face.

The remainder of the beam enters the second prism (B) and is split by a second filter coating (F2) which reflects red light but transmits shorter wavelengths.

The red beam is also totally internally reflected due to a small air-gap between prisms A and B.

The remaining green component of the beam travels through prism C. The trichroic prism assembly can be used in reverse to combine red, green and blue beams into a coloured image, and is used in this way in some projector devices.