Delta T - The temperature difference between two targets usually of comparable targets under comparable conditions.

Detector, Infrared - A transducer element that converts incoming infrared radiant energy impinging on its sensitive surface into a useful electrical signal.

Diffuse Reflector - A surface that reflects a portion of the incident radiation in such a manner that the reflected radiation is equal in all directions. A mirror is not a diffuse reflector.

Diffusivity, Thermal, [a] - (Note: same symbol as absorptive, may be confusing.) The ratio of conductivity (k) to the product of density (r) and specific heat (Cp) [a = k/rCp cm² sec­‑1]. The ability of a material to distribute thermal energy after a change in heat input. A body with a high diffusivity will reach a uniform temperature distribution faster than a body with lower diffusivity.

D* [Detectivity Star] - Sensitivity figure of merit of an infrared detector--detectivity expressed inversely so that higher D*s indicate better performance; taken at specific test conditions of chopping frequency and information bandwidth and displayed as a function of spectral wavelength.

Direct Thermography - Thermal imaging and measurement of a surface whose thermal signature is, or is directly affected by the target of concern. That is, the target of concern has little or no thermal insulation between it and the surface measured.

Display Resolution, Thermal - The precision with which an instrument displays its assigned measurement parameter (temperature), usually expressed in degrees, tenths of degrees, hundredths of degrees, etc.