Background Temperature, Instrument - The apparent temperature of the radiant energy impinging on an object that is reflected off the object and enters the instrument. Originates from the scene behind and surrounding the instrument, as viewed from the target. The reflection of this background appears in the image and affects temperature measurements. Good quality quantitative thermal sensing and imaging instruments provide a means for correcting measurements for this reflection.

Background Temperature, Target - Apparent ambient temperature of the scene behind and surrounding the target, as viewed from the instrument. When the FOV of a point sensing instrument is larger than the target, the target background temperature will affect the instrument reading. Also called surroundings temperature or foreground temperature.

Blackbody, Blackbody Radiator - A perfect radiator, one that radiates the maximum number of photons in a unit time from a unit area in a specified spectral interval into a hemisphere that any body in thermodynamic equilibrium at the same temperature can radiate. It follows that a blackbody absorbs all radiant energy impinging on it and reflects and transmits none; thus a surface with emissivity of unity (1.0).

More information on blackbody radiation.

More information on blackbody types and emissivity.

Blackbody Curves - Plots of radiant power spectral exitance (W/m²/mm) vs. wavelength for various temperatures according to the Planck equation. These curves show the maximum amount of energy at any given wavelength that can be radiated by an object due solely to its temperature. Also called Planck curves.

Bolometer, Infrared [Micro-Bolometer] - A type of thermal detector commonly used in uncooled radiometers.

Calibration - Checking and/or adjusting an instrument such that its readings agree with a standard. Calibration removes instrument systematic error and quantifies the instrument random error.

Calibration Check - A routine check of an instrument against a reference to ensure that the instrument has not deviated from calibration since its last use.

Calibration Accuracy - The accuracy, to which a calibration is performed, usually based on the accuracy and sensitivity of the instruments and references used in the calibration.

Calibration Source, Infrared - A blackbody or other target of known temperature and effective emissivity used as a calibration reference.

Capacitance, Thermal - This term is used to describe heat capacity in terms of an electrical analog, where loss of heat in analogous to loss of charge on a capacitor. Structures with high thermal capacitance lose heat more slowly than those structures with low thermal capacitance.

Capacity, Heat - The heat capacity of a material or structure describes its ability to store heat. It is the product of the specific heat (c­­­p) and the density (r) of the material. This means that denser materials generally will have higher heat capacities than porous materials.

Celsius [Centigrade] - A temperature scale based on 0°C as the freezing point of water and 100°C as the boiling point of water at standard atmospheric pressure; a relative scale related to the Kelvin scale [0°C = 273.12 K. 1 C° (DT) = 1 K. (DT)]

Color - A term sometimes used to define wavelength or spectral interval, as in two-color radiometry (meaning a method that measures in two spectral intervals); also used conventionally (visual color) as a means of displaying a thermal image, as in color thermogram.

Colored Body - See non-graybody.

Conductance, Thermal - A measure of the ability of a material of defined thickness and cross-sectional area to conduct heat. Related to the material property, thermal conductivity. The inversed of thermal resistance (C = 1/R).

Conduction, Thermal - The only mode of heat flow in solids, but can also take place in liquids and gases. It occurs as the result of atomic vibrations (in solids) and molecular collisions (in liquids and gases) whereby energy is transferred from locations of higher temperature to locations of lower temperature.

Conductivity, Thermal,  [K] - A material property defining the relative capability to carry heat by conduction in a static temperature gradient. Conductivity varies slightly with temperature in solids and liquids and with temperature and pressure in gases. It is high for metals (copper has a K of 380 W/m-°C) and low for porous materials (concrete has a K of 1.0) and gases.

Convection - The form of heat transfer that takes place in a moving medium and is almost always associated with transfer between a solid (surface) and a moving fluid (such as air), whereby energy in transferred from higher temperature sites to lower temperature sites.

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.

Effective Emissivity [e] (also called emittance, but emittance is a less preferable term because it was formerly used to describe radiant exitance). - The measured emissive value of a particular surface under existing measurement conditions (rather than the generic tabulated value for the surface material) that can be used to correct a specific measuring instrument to provide a correct temperature measurement.

Effusivity, Thermal [e] - A measure of the resistance of a material to temperature change
E = ÖkrCp cal Cm2 °C-1 sec½
where:
K = thermal conductivity
r = bulk density
C­­­­­­­­p = specific heat

Emissivity [e] - The ratio of a target surface’s radiance to that of a blackbody at the same temperature, viewed from the same angle and over the same spectral interval; a generic look-up value for a material. Values range from 0 to 1.0. Alternatively, the ratio of a flat, optically polished, opaque target surface radiance to that of a blackbody at the same temperature, viewed from the same angle and over the same spectral interval. The latter definition characterizes the property of the material. When used this way, emittance is used to characterize the material when it is other than flat, optically polished and opaque.

More Emissivity information.

Emittance [e] - The ratio of a target surface’s radiance to that of a blackbody at the same temperature, viewed from the same angle over the same spectral interval; a generic look-up value for a material. Values range from 0 to 1.0.

EMI/RFI Noise - Disturbances to electrical signals caused by electromagnetic interference (EMI) or radio frequency interference (RFI). In thermography, this may cause noise patterns to appear on the display.

Environmental Rating - A rating given an operating unit (typically an electrical or mechanical enclosure) to indicate the limits of the environmental conditions under which the unit will function reliably and within published performance specifications.

Exitance, Radiant [Also Called Radiosity] - Total infrared energy (radiant flux) leaving a target surface. This is composed of radiated, reflected and transmitted components. Only the radiated component is related to target surface temperature.