Incident electromagnetic radiation impinging on a body may be transmitted through the body. The transmissivity describes the proportion of electromagnetic radiation which is transmitted through the body, and has values between 0 and 1. The remaining portion of the radiation is either reflected or absorbed by the body. This gives the relationship:
α = absorptivity, ρ = reflectivity and τ = transmissivity
Most solid objects do not display transmission in the infrared range. The exception is some plastics as well as transparent ionic crystals and semi-conductor materials such as silicon, which is transparent in the infrared wavelength range (1.2 – 15 µm) and therefore allows transmission.