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Glossary of Infrared non-contact temperature measurement terms
Posted: 6 Nov 02
Glossary of Terms used in Infrared Non-contact Temperature Measurement
This glossary of terms provided by www.Raytek.com defines vocabulary and nomenclature commonly used within the infrared temperature measurement industry.
Absolute Zero The temperature of 273.16° C, 459.69° F, or 0° K; thought to be the temperature at which molecular motion vanishes and a body would have no heat energy. [Ref.1]
Accuracy The maximum deviation in a set of measurements between the temperature indicated by a radiation thermometer and the known temperature of a reference source, including the uncertainty of the reference temperature source. [Ref. 3] The accuracy can be expressed in a variety of ways including temperature, percentage of temperature reading, or percentage of full scale temperature of an instrument.
Ambient Derating Derating or decrease in accuracy of an instrument due to changes in its ambient temperature from that at which it was calibrated. See also Temperature Coefficient.
Ambient Operating Range Range in the ambient temperature over which the instrument is designed to operate.
Ambient Temperature The temperature of the instrument. Can also refer to the temperature that gives rise to the background. See Background Radiation.
Ambient Temperature Compensation (TAMB) See Reflected Energy Compensation.
ASTM American Society for Testing and Materials.
ASTM E 1256 ASTM E1256 - 88, Standard Test Methods for Radiation Thermometers (Single Waveband Type). A standard by which Raytek products are tested and calibrated for accuracy, repeatability, resolution, target size, response time, warm-up time, and long-term drift.
Atmospheric Windows The spectral bands in which the atmosphere least affects the transmission of radiant energy. The spectral bands are 0.4 to 1.8, 2 to 2.5, 3 to 5, and 8 to 14 micrometers.
Background Radiation Radiation that enters an instrument from sources other than the intended target. Background radiation can enter due to reflections from the target or scattering within the instrument.
Blackbody An ideal thermal radiator that absorbs all of the radiation incident thereon, and the radiant emission from which is quantified by Planckís Radiation Law. [Refs. 2,3]
Calibration Procedure A procedure that is performed to determine and set the parameters affecting an instrument's performance in order to ensure its designed function within prescribed limits.
Calibration Source A source for which the radiance temperature can be calibrated to within a known level of uncertainty in relation to some other parameter, and in which this relationship is sufficiently constant to enable it to be used for a reasonable period without calibration. [Ref. 4]
Carnot Cycle An ideal heat engine that converts thermal energy to mechanical work with the greatest efficiency that can be achieved.
Celsius or C The temperature scale in which the temperature in Celsius (TC) is related to the temperature in Kelvin (TK) by the formula; TC = TK 273.15. The freezing point of water at standard atmospheric pressure is very nearly 0°C, and the corresponding boiling point is very nearly 100°C. Formerly known as centigrade temperature scale. [Ref. 1]
Color Temperature The temperature of a black body from which the radiant energy has the same spectral distribution as that from a surface.
Colored Body or Non Gray Body A source of thermal emission for which the emissivity depends on wavelength and is not constant.
Comparison Pyrometry Method of radiation thermometry wherein the temperature of a calibrated source is changed until the radiation received from the source is the same as that from the target to determine the temperature of the target.
Current-Loop A form of communications wherein a pair of wires is used to transmit the signal as a current. Levels of 4 to 20 mA are often used to indicate the minimum and maximum signal level, respectively. Sometimes, for digital applications, various magnitudes of mA current are used to indicate a logical 1 and 0 mA. The current loop is often characterized by a maximum impedance of the device that is connected to the loop.
D:S Optical resolution expressed as a ratio of the distance to the resolution spot divided by the diameter of the spot.
Deadband Temperature band (±) about the set point, wherein an alarm output or relay cannot change state, thus providing hysteresis.
Detector Transducer which produces a voltage or current proportional to the electromagnetic energy incident upon it. See also Thermopile, MCT, Thermoelectric Cooled, Pyroelectric, and Lead Selenide and Si detectors.
Dielectric Withstand Voltage (Breakdown Voltage) The maximum voltage an insulator of electricity can endure without electrical conduction through the material.
Digital Data Bus A pair of electrical conductors connecting several transmitters and receivers of digital data.
Digital Image Processing Converting an image to digital form and changing the image to enhance it or prepare it for analysis by computer or human vision. In the case of an infrared image or thermogram, this could include temperature scaling, spot temperature measurements, thermal profiles, image addition, subtraction, averaging, filtering, and storage.
Digital Output Interval (DOI) The time interval between transmission of packets of digital data containing temperature and system status information.
DIN (Deutsches Institut für Normung) The German standard for many instrumentation products.
Drift The change in instrument indication over a period of time not caused by external influences on the device. [Ref. 3]
EMI/RFI Electro-Magnetic Interference/Radio Frequency Interference, which affects the performance of electronic equipment.
Emissivity At a given wavelength the ratio of infrared energy radiated by an object at a given temperature to that emitted by a blackbody at the same temperature The emissivity of a blackbody is unity at all wavelengths.
Environmental Rating A rating given (usually by agencies and regulatory bodies) to indicate the severity of the environment in which the unit will function reliably.
External Reset (Trigger) Initialization of an instrument to its state at power up including signal conditioning features (Peak Hold, Valley Hold, Sample Hold, Average, 1-way RS232, etc.) via the external reset input.
Fahrenheit (F) Temperature measurement scale where, at standard atmospheric pressure, the freezing point of water is 32°F and the vaporization point of water is 212°F. To convert from Celsius, use F = (C x 1.8) + 32.
Fail-Safe Operation A feature designed to alert the operator via display, and to bring a process to a safe shutdown via output, in the event of a particular control system or process failure.
Far Field A measurement distance sufficiently large (typically greater than 10 times the focal distance) whereby the spot size of an instrument is growing in direct proportion to the distance from the instrument, and the field of view is constant.
Field of View (FOV) The area or solid angle viewed through an optical or infrared instrument . Typically expressed by giving the spot diameter of an instrument and the distance to that spot. Also expressed as the angular size of the spot at the focal point. See Optical or Infrared Resolution.
Focal Point or Distance The point or distance from the instrument at which the object is focused onto the detector within the instrument. The focal point is the place or distance at which the optical or infrared resolution is greatest.
Full Scale Accuracy The temperature measurement accuracy expressed as a percentage of the maximum possible reading of an instrument.
Gray Body A source of radiant emissions for which the emissivity is less than 1 but constant and, therefore, independent of wavelength.
IEC (International Electrotechnical Commission) A European organization that coordinates and sets related standards among the European Community.
IEEE-488 A standard developed by Hewlett-Packard Corporation and adopted by the IEEE for digital interface between programmable instrumentation. It uses a 16-bit bus to interconnect up to 15 instruments. The standard comprises hardware and protocol options. It is also called the Hewlett-Packard Interface Bus (HPIB) or General Purpose Interface Bus (HPIB) or General Purpose Interface Bus (GPIB). The present standard is ANSI/IEEE-4881-1987.
IFOV (Instantaneous Instantaneous Field of View is the angular resolution of an imaging Field of View) instrument that is determined by the size of the detector and the lens. For a point instrument the IFOV and FOV are the same.
Image Processing Converting an image to a digital form and further enhancing the image to prepare it for computer or visual analysis. In the case of an infrared image or thermogram, this could include temperature scaling, spot temperature measurements, and thermal profiles, as well as image addition, subtraction, averaging, filtering, and storage.
Indium Antimonide (InSb) A material used to construct photon detectors that are sensitive in the spectral region from 2.0 to 5.5 µm and used in infrared scanners and imagers. These detectors require cryogenic cooling.
Infrared Radiation (IR) Radiation within the portion of the electromagnetic spectrum which extends from 0.75 to 1000 µm.
Infrared or Optical Filter See Spectral Filter or Neutral Density Filter.
Infrared Thermometer An instrument that determines the temperature of an object by means of detecting and quantifying the infrared radiation emitted therefrom. Types include total power, wide band, narrow band, and multiple wavelengths.
Insulation Resistance The property of a material to resist the flow of electrical current and expressed in Megohms (M) as the ratio of an applied electrical potential divided by the flow of electrical current resulting therefrom.
Interchangeability (of heads) The ability for a head sensor to be interchanged with another of the same type without the need to recalibrate the system (also referred to as Universal Electronics). Some monitors support the interchangeability of different types of heads.
Intrinsically Safe A standard for preventing explosions in hazardous areas by limiting the electrical energy available to levels that are insufficient to cause ignition of explosive atmospheres during normal operation of an instrument.
IP Designation Grades of intrinsic safety protection pertaining to enclosures per the British Standard 4752. The type of protection is defined by two digits, the first relating to accessibility and the second to environmental protection. The two numbers are preceded by the letters IP. [Ref. 6]
Isolated Inputs, Outputs, or Power Supplies Inputs, outputs and power supply lines that are electrically insulated from each other, whereby arbitrary grounding of these lines cannot affect the performance of the instrument such as generate ground-loops or short out internal resistors.
Isotherm A continuous line (not necessarily straight or smooth) on a surface (or chart) comprising points of equal or constant temperature.
JIS (Japanese Industrial Standard) A technical governing body that sets standards for determining or establishing the accuracy of IR thermometers.
Kelvin (K) A temperature scale that is directly related to the heat energy within a body. Formally, a temperature scale in which the ratio of the temperatures of two reservoirs is equal to the ratio of the amount of heat absorbed from one of the them by a heat engine operating in a Carnot Cycle to the amount of heat rejected by this engine to the other reservoir. The temperature of the triple point of water (in this scale) is defined as 273.16° K. [Ref. 1] To convert from Celsius, K=C+273.16.
Lead Selenide (PbSe) A material used to make photon detectors that are sensitive in the 3 to 5 µm spectral band. These detectors require thermoelectric cooling and are used in IR thermometers, scanners, and imagers.
Maximum Current Loop Impedance Describes the size of a load that can be driven by an instrument with a mA output. For example a 500 ohm maximum loop impedance means that the instrument can supply 10 volts at 20 mA into this load.
MCT (Mercury Cadmium Telluride) or HgCdTe A ternary alloy material used to build photon detectors that are sensitive in the 3-5µm and 8-14µm regions of the spectrum and require TE cooling in the 3-5 µm region and cryogenic cooling in the 8-14µm region.
Minimum spot size The diameter of the smallest object for which an instrument can meet its performance specifications.
NEMA (National Electrical Manufacturer's Association) Among its activities, sets US standards for housing enclosures, similar to IEC IP.
NETD (or NET) Noise Equivalent Temperature Difference or the change in temperature of a blackbody target that fills the radiometer FOV which results in a change in the radiometer signal equal to the rms noise of the instrument.
Neutral Density Filter An optical or infrared filter for which the transmission is constant and not a function or wavelength.
NIST Traceability Calibration in accordance with and against standards traceable to NIST (National Institute of Standards and Technology, USA). Traceability to NIST is a means of ensuring that reference standards remain valid and their calibration remains current.
Optical or Infrared Resolution The ratio of the distance to the target divided by the diameter of the circular area (or spot) for which the energy received by the thermometer is a specified percentage of the total energy that would be collected by an instrument viewing a calibration source at the same temperature. The distance to the target is generally the focal distance of the instrument. The percentage energy is generally 90% to 95%.
Optical Pyrometer A system that, by comparing a source whose temperature is to be measured to a standardized source of illumination (usually compared to the human eye), determines the temperature of the former source.
Output Impedance Describes the impedance of the thermometer that is experienced by any device connected thereto. To achieve accurate readings, the input impedance of a device connected to the thermometer must be much greater than the output impedance of the thermometer.
Peak Hold Output of the maximum temperature measurement indicated by an instrument during the time duration for which this display mode has been active.
Photondetector or Quantum Detector A type of detector in which the photons or quanta of energy interact directly with the detector to generate a signal.
Pyroelectric Detector Thermal detector that has a signal generated by means of the pyroelectric effect wherein changes in temperature of the detector generates an electrical signal.
Pyrometer A broad class of temperature measuring devices. They were originally designed to measure high temperature, but some are now used in any temperature range. Includes radiation pyrometers, thermocouples, resistance pyrometers, and thermistors.
Radiance Temperature The temperature of a black body which has a radiance equal to the radiance of the object at a particular wavelength or wavelength band. [Ref. 5]
Radiant Energy The electromagnetic energy emitted by an object due to its temperature.
Radiation Thermometer A device used to measure the temperature of an object by quantification of the electromagnetic radiation emitted therefrom. Also, a radiometer calibrated to indicate a blackbody's temperature. [Ref. 3]
Rankine (R) The absolute temperature scale related to Fahrenheit in the equivalent manner Kelvin is to Celsius. R = 1.8 x K, or also R = F + 459.67.
Reference Junction or Cold Junction Refers to the thermocouple junction that must be known in order to infer the temperature of the other or thermocouple measurement junction.
Reflectance The ratio of the radiant energy reflected from a surface to that incident on the surface.
Reflected Energy Compensation Feature used to achieve greater accuracy by compensating for background IR energy that is reflected off the target into the instrument. If the temperature of the background is known, the instrument reading can be corrected by using this feature.
Relative Humidity The dimensionless ratio of the actual vapor pressure of the air to the saturation vapor pressure (abbreviated RH). Percent relative humidity is expressed as the product of RH and 100. For example an RH of 0.30 is a percent relative humidity of 30%. [Ref. 1]
Repeatability The degree to which a single instrument gives the same reading on the same object over successive measures under the same ambient and target conditions. The ASTM standard E 1256 defines it as the sample standard deviation of twelve measurements of temperature at the center of the span of the instrument. Generally expressed as a temperature difference or a percent of full scale value, or both. [Ref. 3]
Resolution See Temperature Resolution, Optical Resolution, or Spatial Resolution.
Response Time The time for an instrument's output to change to 95% of its final value when subjected to an instantaneous change in target temperature corresponding to the maximum temperature the instrument can measure (per ASTM E 1256). The average time required for software computation within the processor is also included in this specification for Raytek products.
RS-232 Recommended Standard (RS) 232 is a standard developed by the Electronic Industries Association (EIA) that governs the serial communications interface between data processing and data communications equipment and is widely used to connect microcomputers to peripheral devices. [Ref. 1] The present revision is EIA-RS-232-D, which defines the interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE) employing serial binary data interchange. The standard does not define the protocol or format of the binary stream. The standard comprises three parts: electrical characteristics, interface mechanical characteristics, and functional description of the interchange circuits. The equivalent international standard is Comite Consultatif International Telegraphique et Telephonique (CCITT) V.24.
RS-422 A recommended standard developed by EIA that defines a balanced interface and is an expansion of RS-423 that increases the data rate to 10 Mbps. see RS-423
RS-423 A recommended standard developed by EIA that defines an unbalanced interface and is an expansion of RS-232 and provides improvements included increased connecting cable lengths, increased data rates, and use of multiple receivers on line.
RS-485 A recommended standard developed by EIA that is an improvement over RS-422 in that it allows an increase in the number of receivers and transmitters permitted on the line.
RTD (Resistance Temperature Device) A contact measurement device whose resistance varies with temperature.
Sample Hold A temperature taken from a target and displayed or held for a set period of time or until the next external reset occurs.
Scatter Radiant energy reaching the detector of an instrument from the background other than that which is reflected from the target.
Set Point Process or measurement variable setting which when crossed by the measured value will trigger an event and/or cause a relay to change state.
Shock Test An impact test where an object or test unit is subjected to an impulsive force which is capable of exciting mechanical resonances of vibration.
Signal Processing Manipulation of temperature data for purposes of enhancing the data. Examples of signal processing functions include Peak Hold, Valley Hold, and Averaging.
Silicon (Si) Detector A photon detector used in measurement of high temperatures.
Size-of-Source Effect The effect by which the energy collected by, and temperature reading of, an instrument continues to increase as the size of a target increases beyond the field-of-view of the instrument. It is caused by two occurances: the remaining energy above the percentage used to define location and scattering of radiation as it enters the instrument such that energy from outside the FOV of the instrument enters it. The existence of this effect means that the accuracy of the instrument may be affected by targets that are too large as well as two small. This effect is also called Target Size Effect. [ASTM STP 895]
Slope The ratio of the emissivities for the two spectral bands of a 2-color radiometer. The emissivity of the shorter wavelength band is divided by the emissivity of the longer wavelength band. Slope can be greater than, equal to, or less than unity. Slope accounts for materials where emissivity varies with wavelength.
Spectral Filter An optical or infrared element used to spectrally limit the transmission of radiant energy reaching an instrument's detector.
Spectral Response The wavelength region in which the IR Thermometer is sensitive.
Spot The diameter of the area on the target where the temperature determination is made. The spot is defined by the circular aperture at the target which allows typically 90% of the IR energy from the target to be collected by the instrument. See also Size-of-Source Effect.
Stare or Lag A saturation effect whereby the signal from an instrument endures beyond the response time after the target has been removed from the field of view. Can be caused by exposing the sensor to a target of high temperature for an extended period. The effect is expressed as the increase in response time required for the sensor to return to within 5% of the correct reading.
Storage Temperature Range The ambient temperature range an instrument can survive in a non-operating mode and perform within specifications when operated.
Target The object upon which the temperature is determined.
Target Size Effect See Size-of-Source Effect.
Temperature A property of an object which determines the direction of heat flow when the object is placed in thermal contact with another object (i.e., heat flows from a region of higher temperature to one of lower temperature). [Ref. 1]
Temperature Coefficient The change in accuracy of an instrument with changes in ambient temperature from that at which the instrument was calibrated. Usually expressed as the percent change in accuracy (or additional error in degrees) per change in ambient temperature. For a rapid change in ambient conditions, refer to Thermal Shock.
Temperature Resolution The minimum simulated or actual change in target temperature that gives a usable change in output and/or indication. [Ref. 3]
Temporal Drift The change in accuracy of an instrument over time. This effect may be due to aging of the instrument's components or calibration changes.
Thermal Detector Detector in which the photons of incident radiation are converted to heat and then into a signal from the detector. Thermal detectors include pyroelectric, bolometer, and thermopile types.
Thermal Drift See Temperature Coefficient.
Thermal Radiator An object that emits electromagnetic energy due to its temperature.
Thermal Shock An error due to a rapid change in the ambient temperature of an instrument. Expressed as a maximum error and the time required for performance to return to prescribed specifications.
Thermistor A semiconductor material whose resistivity changes with temperature.
Thermocouple A set of two junctions of two dissimilar metals used to measure temperature by means of the Peltier effect, whereby heat is liberated or absorbed by the flow of electrical current through a junction of two dissimilar metals such that an electrical potential develops between two such junctions in proportion to the difference in temperature of the junctions. A variety of types exist including: J (Fe / constantan)
K (chromel / alumel)
T (Cu / constantan)
E (chromel / constantan)
R (Pt / Pt - 30% Rh)
S (Pt / Pt -10% Rh)
B (Pt - 6% Rh / Pt - 30% Rh)
G (W / W - 26% Re)
C (W - 5% Re / W - 26% Re)
D (W - 3% Re / W - 25% Re)
Thermoelectric (TE) Cooling Cooling based on the Peltier effect. An electrical current is sent through two junctions of two dissimilar metals. One junction will grow hot while the other will grow cold. Heat from the hot junction is dissipated to the environment, and the cold from the other junction is used to cool. [Ref. 1]
Thermogram A thermal photograph generated by scanning an object or scene. [Ref. 1]
Thermopile A number of similar thermocouples connected in series, arranged so that alternate junctions are at the reference temperature and at the measured temperature, to increase the output for a given temperature difference between reference and measuring junctions. [Ref. 2]
Time Constant The time it takes for a sensing element to respond to 63.2% of a step change at the target.
Transfer Standard A precision radiometric measurement instrument with NIST traceable calibration in the USA (with other recognized standards available for international customers), used to calibrate radiation reference sources.
Transmittance The ratio of IR radiant energy incident on an object to that exiting the object.
Triple Point The condition of temperature and pressure under which the gaseous, liquid, and solid phases of a substance can exist in equilibrium. For water at atmospheric pressure, this is typically referred to as its freezing point.
Two-Color Thermometry A technique that measures the energy in two different wavelength bands (colors) in order to determine temperature. The 2 color technique has been shown to be effective for correcting errors due to partial blockage of the target caused by dust particles.
Valley Hold Output of the minimum temperature measurement indicated by an instrument during the time duration for which this display mode has been active.
Verification Confirmation of a design with regard to performance within all prescribed specifications.
Vibration Test A test where oscillatory or repetitive motion is induced in an object (as per MIL-STD-810 or IEC 68-2-6), which is specified as an acceleration in g's and power spectral density (PSD), after which the unit is tested for proper operation.
Warm-Up Time Time, after turn on, until the instrument will function within specified repeatability. [Ref. 3]
McGraw-Hill Dictionary of Scientific and Technical Terms, Fifth Edition, McGraw-Hill, Inc., 1994.
Compilation of ASTM Standard Definitions, Eighth Edition, ASTM, 1994.
ASTM E1256 - 88, Standard Test Methods for Radiation Thermometers (Single Waveband Type), ASTM, 1988.
ASTM STP 895, Applications of Radiation Thermometry,, Richmond and DeWitt, ASTM, 1984.
General Rules for Expression of the Performance of Radiation Thermometers, Japanese Industrial Standard, JIS C 1612, 1988.
Intrinsically Safe Instrumentation - A Guide, Robin Garside, Instrument Society of America, 1982.
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