ABSTRACT
The full Infra-red (IR) region of the electromagnetic spectrum may be taken as the 700 nm to 20 µm band. The limits are set by spectral sensitivity limits of detectors such as the eye and pyroelectric vidicon detectors at the short- and long-wavelength ends respectively. In contrast to their ultra-violet (UV)-transmitting properties, most optical glasses and polymers transmit freely in the near IR. Catadioptric lenses may be achromatic into the near IR, depending on design, and visual focusing is correct for IR use. Catadioptric lenses using coated, diamond-turned metal mirrors with germanium refractive elements give both large apertures and long focal length for IR video use. General-purpose lenses with multi-layer coatings may be less suited to IR use due to reduced transmission in the 700–1000 nm region. Specialist IR-sensitive photographic materials record up to 900 nm, extended to about 1300nm for astronomical materials, with a theoretical limit of 1350 nm.
