Glossary
Absolute Zero
Theoretical temperature at which all molecular activity stops. Deep
space is about 3º C above Absolute Zero.
Absorption
The ratio of how much infrared radiation is absorbed by a surface,
and later emitted, as a percentage of the total amount of energy
exposed to the surface. The percentage of absorption is basically
equal to its emissivity.
Accuracy
A measure of the similarity of an instrument reading to the actual
value for that reading. The accuracy of temperature measurement indicators
on thermal imagers is affected by emissivity, the distance from the
object, the angle of the object and a number of other factors.
Ambient Temperature
The temperature of the surrounding air and environment, which can
impact the heat transfers around an object. Most temperature indicators
on thermal imagers do not measure ambient air temperatures.
Amorphous Silicon
A material used to create infrared detectors. These types of detectors
are used in a number of fire service thermal imagers.
Angstrom
A unit of measure equal to one-thousandth of a micron.
Aperture
A hole or opening that limits the amount of infrared radiation that
reaches a detector. Bullard’s Thermal Throttle on the TIx and TI
Commander is also called an aperture control.
Aspect Ratio
The ratio of the horizontal width to the vertical length of a display.
Many video displays are 4:3, as are the infrared detectors. This
is the ratio of the common television.
Atmospheric Attenuation
The amount of signal reduction that occurs when infrared radiation
travels through the atmosphere between the target and a thermal imager.
Dust, humidity and precipitation can all reduce the effectiveness
of the thermal imager, and the accuracy of any surface temperature
measurement device.
Background Noise
The noise naturally present in an infrared detector, independent
of the signal strength or ambient temperature. This is usually masked
by software, but it may occasionally appear as image graininess in
very bland scenes.
Binocular
A display device providing a separate display for each eye.
Blackbody
An ideal thermal radiator, normally used as a testing standard. The
most common can be set to a specific temperature as the device emits
almost 100% of the infrared radiation expected at a given temperature.
Emissivity ratings are essentially a percentage value, compared to
the blackbody. (Emissivity = 1.0, Reflectance = 0.0, Transmittance
= 0.0)
Bland Scene
A very stable scene viewed by a thermal imager. The scene is characterized
by little temperature variation and a lack of strong emitters. Bland
scenes can be difficult for thermal imagers to generate quality images.
Bolometer
An temperature measuring instrument using a strip thermistor to achieve
higher sensitivity than a simple thermistor. Unlike thermistors which
are used for contact temperature measurements, bolometers have been
used to measure radiation levels.
BST (Barium Strontium Titanate)
A material used to create infrared detectors. These types of detectors
are used in a number of fire service thermal imagers.
Calibration
The process of adjusting an instrument to read accurately under specific
conditions.
Celsius (Centigrade)
A scale for measuring temperature, where Absolute Zero is -273.2º
C, water freezes at 0º C, and water boils at 100º C.
Chopper Wheel
A small wheel that rotates rapidly in front of a BST-based infrared
detector. The wheel has a spiral cut out that partially blocks different
portions of the detector momentarily. This process generates fluctuation
in the amount of infrared energy reaching the detector, allowing
it to create an accurate thermal image.
Conduction
The transfer of heat energy through a solid.
Convection
The transfer of heat energy through a liquid or gas due to the motion
of that liquid or gas.
Crosshair
The intersecting vertical and horizontal line superimposed on the
thermal imager display. It is commonly used to indicate the approximate
area from which a thermal imager is taking a surface temperature
reading.
Degree
An increment of temperature measurement.
Detector (Infrared)
The individual chip or wafer that senses infrared energy.
Distance to Spot Ratio
A measurement of the area a pyrometer or radiometric thermal imager
views from a specific distance. A 10:1 ratio indicates the surface
temperature measurement taken at 10 feet is averaging an area of
1 square foot.
Electromagnetic Radiation
The field effects given off by accelerating a charged particle in
a magnetic field. Depending on field strength and speed of acceleration,
many types of electromagnetic radiation are created.
Electromagnetic Spectrum
A plot of the range of wavelengths and types of electromagnetic radiation
found to exist from subsonic waves to cosmic rays. Radio waves, infrared
energy and visible light are all portions of the spectrum.
Emissivity, Emission
The ability of an object to absorb and then radiate infrared energy.
High emissivity ratings indicate good absorbers of infrared energy.
Good reflectors of infrared are poor absorbers.
Engine
The core component of a thermal imager. This includes the FPA, the
circuit boards that run the FPA, and the software that controls the
FPA and interprets the signals to generate a thermal image.
F-number (f/#)
The ratio of focal length to aperture for a lens assembly. Smaller
numbers represent faster lenses, which means that scene changes are
conveyed to the detector more rapidly.
Fahrenheit
A temperature measurement scale, in which Absolute Zero is -459.7º
F, water freezes at 32º F and water boils at 212º F.
Far Infrared
The longest wavelength of infrared radiation, measured as roughly
8 to 14 microns. This is the range used by fire service thermal imagers.
Also referred to as Long Wave Infrared.
Ferroelectric
The physical property of a material that leads to thermal detection.
This refers to the material’s ability to polarize as a direct result
of temperature change. The stronger the relationship between temperature
change and polarization, the better the material’s ability to function
as a detector. The most common ferroelectric TI is BST-based.
Field of View (FOV)
The total field, measured as an angle, within which objects viewed
by a thermal imager. Narrower FOVs generate more life-size images
and distances, while wider FOVs place more image on the display.
The most common FOV for fire service TIs is about 50º.
FPA (Focal Plane Array)
The infrared detector itself, usually a thin wafer less than 1”
square. The most common fire service FPAs are 320x240 pixels or 160x120
pixels.
Frequency
The number of cycles an operation occupies per period of time. The
normal unit of measurement for frequency is Hertz (Hz), or cycles
per second.
Heat
The energy or sensation that humans associate with infrared radiation.
Heat Sink
A device for dissipating heat; it absorbs heat by conduction from
heat producing devices and dissipates heat by means of convection.
Heat sinks are common inside fire service thermal imagers to help
maintain proper operating temperatures.
Heat Transfer
The flow of thermal energy from one object to another, by means of
conduction, convection or radiation.
Hertz (Hz)
A unit for measuring frequency. One hertz is one cycle per second.
Imager
A fully incorporated infrared detecting system that contains the
detector, optics, processor, power source and display.
Infrared (IR)
Electromagnetic radiation which occupies the band from 0.7 microns
to 100 microns. Infrared radiation is between the visible spectrum
and microwave radiation.
Infrared-window
A protective cover, transparent to infrared radiation, placed on
the front of a thermal imager to help protect the primary lens from
damage. Many fire service thermal imagers use a germanium window.
Kelvin
The temperature scale used by scientist. The scale is based on the
Celsius scale increments, but 0º is Absolute Zero rather than the
freezing point of water.
LCD (Liquid Crystal Display)
A flat panel display system, common on many modern thermal imagers.
Lead Scandium Tantilate
A type of material used on focal plane arrays.
Lens
An optical component constructed of transparent substance with one
or two curved surfaces of different curvature which has the ability
to change the direction of beam travel. Infrared lenses are used
for focusing the detector at a distance of interest and for modifying
the size and distance of the focused field of interest.
Light
The region of the electromagnetic spectrum which is visible to the
human eye. This is usually considered the region from 0.39 (violet)
to 0.77 (red) microns.
Long Wave Infrared (LWIR)
The longest wavelength of infrared radiation, measured as roughly
8 to 14 microns. This is the range used by fire service thermal imagers.
Also referred to as Far Infrared.
Microbolometer
A type of infrared detector that is capable of measuring absolute
energy levels on each pixel of the FPA. Surface temperature measurement
can be done directly from a microbolometer’s FPA.
Micron (micrometer)
A metric measurement equal to one-millionth of a meter.
Mid Infrared (or Mid-wave Infrared, MWIR)
Infrared radiation with wavelengths of roughly 3 to 8 microns.
Minimum Resolvable Temperature Difference (MRTD)
This number expresses the sensitivity of an infrared detector. It
defines the smallest temperature difference the detector can differentiate.
The lower the number, the more sensitive the unit is. Units with
high levels of sensitivity (lower MRTD) tend to produce better, more
defined images, especially in bland scenes.
Monocular
A viewing device made to produce an image for one eye only.
Near Infrared
The shortest wavelength infrared radiation, measuring approximately
1 to 3 microns. This is also referred to as Short Wave Infrared.
NETD (Noise Equivalent Temperature Difference)
This is a technical measure of sensitivity for a thermal imager,
similar to the MRTD. Generally, a lower NETD will equate to more
usable images, especially in bland scenes.
Operating Temperature Range
The acceptable temperature range for an infrared detector to remain
calibrated and function properly. This refers to the temperature
of the detector itself, not the ambient environment. The use of heat
sinks, insulation and construction will affect how well a detector
stays in its operating range, especially in a fire environment. Properly
designed fire service thermal imagers will have other components
that have a similar operating range as the detector.
PEV
See Pyroelectric Vidicon
Pixel (Picture element)
The smallest location size on a display or in memory. FPAs are measured
in pixels, with each pixel generating a small portion of the thermal
image.
PST (see Lead Scandium Tantilate)
A type of material used on Focal Plane Arrays.
Pyroelectric Vidicon
An outdated type of infrared detector. The sensor is based on hard
or soft vacuum tube technology. These sensors tend to "white
out" when exposed to strong infrared emitters and may become
permanently damaged if exposed for a long period of time. Fire service
thermal imagers are no longer manufactured with PEVs.
Pyrometer
An instrument used for non-contact measurement of surface temperatures.
BST-based thermal imagers with temperature measurement have a pyrometer
interlaced with the infrared detector.
Radiation
Heat transfer of energy in the form of electromagnetic waves. Forms
of radiation include cosmic rays, gamma rays, x-rays, ultraviolet
radiation, infrared, visible light, radio, audio, and subsonic.
Radiometric
The ability of an infrared detector to measure, pixel by pixel, the
actual scene temperature and display the results. Microbolometers
can be radiometric.
Reflectance (Reflectivity)
The amount of total infrared radiation reflected by a surface. This
value is frequently the inverse of the Absorption (Emissivity).
Saturation
The point at which an FPA or pixel cannot register any more infrared
radiation. A saturated pixel triggers will automatically be displayed
as the hottest indicator (white, red, etc.) the system has.
Sensitivity
A measure of the minimum amplitude of input signal change to which
an instrument will respond. On a thermal imager, this is a measurement
of the smallest temperature differences the unit will detect.
Sensor
The component which converts radiation into electrical signals. This
can refer to the detector itself, the engine, or the entire thermal
imager.
Short Wave Infrared (SWIR)
The shortest wavelength infrared radiation, measuring approximately
1 to 3 microns. This is also referred to as Near Infrared.
Shutter
A small device that repeatedly obstructs the FPA in a microbolometer
thermal imager. The shutter covers the FPA, stopping all infrared
energy from reaching the FPA. During the period the FPA is covered,
the thermal imager verifies its calibrations. When this occurs, the
image on the display will freeze momentarily. All microbolometers
have a shutter.
SWIR
See Short Wave Infrared
Temperature
An expression of thermal energy density, or how hot or cold an object
is.
Temperature Range
The maximum to minimum temperature display capability of a system.
This can be affected by the FPA, the engine’s software program or
manufacturer’s selection.
Thermal Radiation
Electromagnetic energy whose natural wavelength fall between .7 and
100 microns, also called infrared radiation.
Thermistor
A device which measures temperature. The sensor for the thermistor
is a semi-conducting resister whose resistance changes significantly
with temperature.
Thermocouple
A device which can measure temperatures by contacting the device
to be measured. The device is made from a junction of dissimilar
metals and as the junction changes temperature a voltage is created
which is read by a previously calibrated meter.
Thermoelectric Cooling
A solid-state device which converts current into a temperature difference
between two junctions. It is possible to put thermoelectric junctions
in series or parallel to increase either the overall temperature
drop or their power.
Thermography
The study of remote temperature measurement
TI (Thermal Imager)
A packaged, independently powered unit that detects infrared radiation
and portrays that information on a video display for the user to
interpret.
TIC (Thermal Imaging Camera)
See TI above.
Transmittance (transmissivity)
A measurement of the ability of a material to pass radiation from
one side to the other without absorbing or reflecting it. Infrared
transmittance for most materials is near zero. Therefore, absorption/emissivity
and reflectivity are usually inverse values.
Vanadium Oxide
A material used to create infrared detectors. These types of detectors
are used in a number of fire service thermal imagers.
Wavelength
The distance between the two peaks of an energy wave cycle. Very
long wavelengths may be measured in hertz, or how frequently the
peaks occur per second.
"White Out"
A condition that afflicted older infrared detectors. When the detector
was exposed to strong infrared sources, such as fires, the thermal
imager would generate completely white images on the display. Sometimes
this was intentional in an effort to protect the detector from damage;
sometimes it indicated damage to the detector. Modern thermal imagers
do not white out; they may experience saturation.