Objective, Objective Lens - The primary lens of an optical system, on an infrared instrument, usually the interchangeable lens that defines the total field of view.

Opaque - In thermography, an opaque material is one that does not transmit thermal infrared energy, ( t = 0).

Optical Element, Infrared - Any element that collects, transmits, restricts or reflects infrared energy as part of an infrared sensing or imaging instrument.

Oversampling - Collecting samples at a rate higher than the Nyquist critical frequency, fc = 1/(2D), where D is the sampling interval. Applies to both time and spatial domains.