A
Aberration
Any undesired effect that distorts an infrared image. Can result from noise (e.g. signal crosstalk), optical defects (e.g. chromatic aberration), or video issues like barrel distortion and pixel sampling errors.
Absolute Temperature Scale
A temperature scale starting from absolute zero. Includes the Kelvin and Rankine scales.
Absorptance (Absorptivity)
The ratio of energy absorbed by a surface compared to the total incident energy. Absorptance equals emissivity.
Accuracy
How closely an instrument reading matches the true value. Affected by drift, environment, temperature, and operator.
Active Element
The responsive part of a detector that converts IR radiation into signal. Not all parts of a detector are active.
Active IR System
An infrared system using an external IR source to illuminate a scene, similar to a floodlight. The scene is then imaged like a passive system.
Ambient Temperature
The surrounding environmental temperature. Used as a baseline for heat transfer measurements.
Anti-Reflective Coating
A surface coating that reduces backscattering and improves light transmission through optical elements.
Angstrom
A unit of measurement equal to one ten-billionth of a metre, often used for electromagnetic wavelengths.
Apparent Temperature
The temperature calculated by a remote IR sensor, assuming blackbody emission.
Aspect Ratio
The height-to-width ratio of an image or display. For example, 3:4 in standard television.
Atmospheric Windows
Wavelength regions where atmospheric gases don’t absorb IR radiation, allowing clearer transmission.
Atmospheric Attenuation
The loss of IR signal strength as it travels through the atmosphere. Heavier on foggy or dusty days.
Attenuation
Reduction in radiation intensity when passing through filters or media. Measured in decibels (dB).
Azimuth Angle
The horizontal angle from the centre of a scene. Positive and negative angles extend left and right.
B
Backscattering
IR energy reflected from clouds or the underside of layers. Can also describe unwanted reflections off lenses.
Background Noise
Signal interference present in the detector, independent of ambient temperature or signal strength.
Background Temperature
Average surrounding temperature, which contributes to reflected IR radiation.
Blackbody
An ideal object that perfectly emits and absorbs thermal radiation. Emissivity is 1.0.
Bolometer
A high-sensitivity thermal detector using thermistors. Detects IR radiation without contact.
British Thermal Unit (BTU)
The heat needed to raise one pound of water by one degree Fahrenheit at near-freezing conditions.
C
Calibration
The adjustment of an instrument to ensure accurate readings, often using reference standards.
Celsius (Centigrade)
Temperature scale where water freezes at 0°C and boils at 100°C. Absolute zero is -273.2°C.
Certified
As in "certified infrared thermographer": someone who has completed at least 40 hours of IR training and passed an exam.
Clear Aperture
The usable area of a lens, excluding edges affected by mounting or optical distortion.
Colour Scale
A visual scale showing how colours are assigned to temperature levels on a display.
Colour Thermogram
An IR image where temperature differences are displayed using assigned colour values.
Conduction
Heat transfer through solids without movement of the material itself.
Conduction Band
The energy level in a semiconductor where electrons can move freely and conduct electricity.
Convection
Heat transfer through a fluid (gas or liquid) by movement of the medium.
Coolant
A substance (gas or liquid) used to lower temperatures through expansion or circulation.
Critical Angle
The minimum angle of incidence at which total internal reflection occurs.
D
Degree
A unit of temperature measurement.
Delta-T
The difference between two temperature readings.
Depth of Field
The range over which objects appear in focus around a set focal distance.
Depth of Focus
The amount a lens can move without defocusing an object already in focus.
Detectivity
A measure of a detector's sensitivity. Higher detectivity means better signal-to-noise performance.
Detector (IR)
A device that converts infrared radiation into electrical signals.
Detector Element
The specific area on a detector that senses IR photons. Multiple elements form an array.
Diffuse Reflections
Scattered, non-shiny reflections that remain uniform regardless of viewing angle.
E
Edge Effects (Optical)
Image distortions near the edges of a lens, often from vignetting or lens curvature.
Electromagnetic Radiation
Energy released by accelerated charged particles. Includes IR, visible light, radio, and X-rays.
Electromagnetic Spectrum
The full range of electromagnetic wavelengths, from subsonic to gamma rays.
Emissivity
The ratio of radiation emitted by an object to that of a blackbody at the same temperature.
Endothermic
Describes a process or reaction that absorbs heat.
Equilibrium
A thermal state where temperature variations have stabilised.
Exothermic
Describes a process or reaction that releases heat.
F
Fahrenheit
Temperature scale where water freezes at 32°F and boils at 212°F. Absolute zero is -459.7°F.
Far Infrared (LWIR)
Long-wavelength infrared radiation: 8–100 microns.
Field of View (FOV)
The angular area an IR system can image or measure.
Filter (Optical)
A lens element that alters or limits the wavelengths passing through.
Focal Length
The distance between a lens and its focal point.
Focal Plane
The surface where focused rays converge. Detector elements must align with this for clarity.
Front Surface Mirror
A mirror with the reflective coating on the incident side, maximising reflection in IR systems.
G
Geometric Concentration
The ratio of lens aperture size to detector size. Indicates how effectively a lens focuses IR radiation.
Greybody
A thermal emitter that radiates less than a blackbody, but consistently across frequencies.
Grey Scale
A visual scale showing image intensity levels, often used for calibration.
H
Heat Capacity
An object’s ability to store thermal energy. Based on mass, density, and specific heat.
Heat Transfer
Movement of heat via conduction, convection, or radiation.
I
Ice Point Reference
A precise 0°C standard used to calibrate temperature measurement instruments.
Image Enhancement
Techniques used to improve visibility or readability of thermal images.
Imager
An IR device combining optics, detector, and scanner to capture and display thermal scenes.
Incident Radiant Energy
The total energy striking a surface from all directions.
Infrared (IR)
Electromagnetic radiation between visible light and microwaves (0.7–100 microns).
Infrared Camera
A device that translates thermal radiation into a visible image. Used for analysis and diagnostics.
Infrared Thermography
The science of measuring and interpreting thermal energy using infrared imaging systems.
Infrared Thermographer
A trained professional certified in IR thermography practices and temperature analysis.
Infrared Window
A lens or shield that transmits IR radiation while protecting internal camera components.
Instantaneous Field of View (IFOV)
A system’s smallest resolvable angle, based on detector element size and lens length.
Irradiance
The rate of radiation impacting a surface area.
Isoradian
Lines or zones of constant IR radiation intensity.
Isotherm
Lines or zones of constant temperature within a thermal image.
Isothermal Windowing
Image processing that highlights a set temperature range for quick visual analysis.
J
Joule
A unit of thermal energy. One joule equals one newton-metre.
K
Kelvin
An absolute temperature scale. Zero Kelvin equals absolute zero; degree increments match Celsius.
Kinetic Energy
The energy of motion.
L
Linear Array
An evenly spaced row of detector elements used to capture thermal images.
Luminance
The brightness component of a displayed colour, as perceived by the human eye.
M
Micron (Micrometre)
A unit equal to one millionth of a metre. Commonly used to describe IR wavelengths.
Mid-Infrared (MWIR)
IR wavelengths between 2.4 and 7.0 microns.
Milliradians
A unit of angular measurement. One degree equals 17.4 milliradians.
N
Near Infrared (SWIR)
Short-wavelength IR radiation: 0.7–1.4 microns.
Neutral Density Filter
An optical filter that evenly reduces intensity across all wavelengths.
Noise
Unwanted signal interference in a detector or system.
O
Ohmmeter
An instrument used to measure electrical resistance.
Opaque
A material that does not transmit radiation. Transmissivity equals zero.
Operating Temperature Range
The ambient temperatures over which a system can accurately operate.
P
Passive IR System
A system that does not emit its own radiation source. It detects existing thermal radiation.
Photoconductor
A resistor that changes resistance when exposed to radiant energy.
Photon
A quantum of electromagnetic energy.
Photovoltaic Detector
A sensor that produces current in response to thermal radiation.
Pixel
The smallest display unit in a thermal image or digital memory.
Principal Plane
An optical reference used to simplify the geometry of lens systems.
Pyrometer
An instrument for remote spot-temperature measurement.
Q
Qualitative Analysis
Analysis based on relative structure or condition, without assigning temperatures.
Quantitative Analysis
Measurement-based analysis assigning numerical values to temperature or radiation levels.
R
Radian
A unit of angular measurement. One full circle = 2π radians.
Radiance
Total thermal intensity observable from a surface, factoring in angle and area.
Radiant Energy
See radiance.
Radiation
Heat transfer via electromagnetic waves.
Radiometer
An instrument that measures IR radiation levels, often in power or temperature.
Rankine
An absolute temperature scale using Fahrenheit increments. 0°R = absolute zero.
Reflectance
The proportion of IR energy reflected off a surface.
Repeatability
The ability of a system to produce the same result under consistent conditions.
Remote Sensing
Taking measurements from a distance without direct contact.
Resolution
The ability of an IR system to distinguish small objects or detail.
Responsivity
How much output a detector produces per unit of thermal energy input.
S
Scale Sensitivity
The system’s minimum detectable difference in thermal signal per step.
Selective Radiator
A source that emits in narrow spectral bands rather than across a wide range.
Sensitivity
The minimum temperature change a system can detect.
Sensor
The component (detector or system) that captures radiation and converts it into data.
Specific Heat
The amount of energy required to change the temperature of a given mass of material.
Spectral
Related to wavelength or frequency within the electromagnetic spectrum.
Spectral Absorption
The ability of a filter or gas to absorb specific wavelengths of radiation.
Spectral Emission
Thermal energy emitted in specific wavelength bands.
Specular
Highly reflective, mirror-like surfaces.
Spot Size
The smallest resolvable target area at a given distance.
Stability
How consistently a system maintains accuracy over time.
Standards
-
Primary: Maintained by NIST; used as a national reference.
-
Secondary: Calibrated from primary; used in high-end labs.
-
Reference/Working: Used in the field for everyday calibration.
T
Temperature
A measure of thermal energy or heat density.
Temperature Range
The minimum to maximum temperature a system can measure or display.
Thermal Conductivity
How well a material transfers heat. Metals conduct well; plastics do not.
Thermal Expansion
Size changes in a material due to temperature variation.
Thermal Gradient
A representation of how temperature changes over distance.
Thermal Radiation
Electromagnetic radiation in the 0.7 to 100 micron range.
Thermal Viewer
A remote IR sensing system that displays relative radiation and supports temperature analysis.
Thermogram
A 2D image of temperature distribution captured by an IR system.
Thermography
The field focused on measuring and interpreting infrared radiation.
Thermometer
A device for measuring temperature.
Thermopile
A series of thermocouples used to amplify voltage for sensitive temperature detection.
Transmittance
The proportion of radiation passing through a material without being absorbed or reflected.
V
Viewfinder
A small screen on an IR camera for aiming and adjusting.
Viewer (Infrared)
An IR device that displays radiation intensity only. Common in security and rescue, not for temperature measurement.
Vignetting
Reduced image intensity toward the edges due to optical limitations.
Void (Thermal)
An area with abnormal thermal characteristics compared to surroundings. May indicate structural issues.
W
Watt
A unit of power. One joule per second.
Wavelength
The distance between two peaks in a wave cycle, used to define radiation types.