Maintenance, Test & Measurement, Calibration

How are infrared cameras calibrated?

October 2021 Maintenance, Test & Measurement, Calibration

Knowing the exact value of the radiation temperature is essential for the calibration process. It can be measured either by using a contact thermometer (in combination with the determination of the emissivity) or a transfer standard infrared thermometer. This value can then be used to determine the device constant for the initial calibration of the infrared sensors. The calibration temperature should be near the temperatures which occur at the respective applications.

Instrotech, the local supplier of Optris products, makes use of a transfer standard radiation thermometer CTlaser-PTB to measure the radiation temperature of a reference source. This instrument needs to be traceable to the international temperature scale from 1990 (ITS-90), so it is calibrated regularly by the PTB (German National Metrology Institute).

ITS-90 is a good approximation of thermodynamic temperature. It is based on 17 well-reproducible fixed values such as melting points of highly pure metals. Within the framework of ITS-90 the CTlaser-PTB is compared to national temperature standards from the PTB. This comparison within a closed chain of comparative measurements with a known uncertainty in measurement takes place regularly.

Based on the CTlaser-PTB, Optris produces the CTlaser- DCI as a high-precision reference IR thermometer for its customers. The DCI units are produced with pre-selected components supporting high stability of measurement. In combination with a dedicated calibration at several calibration points, the CTlaser-DCI achieves higher accuracy than units from series production.

Optically, an IR thermometer is described by the distance-to-spot ratio (D:S). Depending on the quality of the optics, a certain amount of radiation is also received from sources outside the specified measurement spot. The maximum value here equals the radiation emitted by a hemispheric radiant source. The respective signal change in correlation with a resize of the radiation source is described by the Size-of-source effect (SSE). As a result of this correlation, manufacturers of IR thermometers use accurately defined geometries for the calibration of their units, meaning that depending on the aperture of the radiation source (A) a distance (a) between the IR thermometer and the reference source is defined. Thus, the value specified in datasheets and technical documentation as measurement field is in general a certain defined percentage of this radiation maximum – values of 90% or 95% are common.

Optris has up-to-date in-house laboratories which fulfil the mandatory requirements for calibration stations. When issuing calibration certificates, it is not only the laboratory temperature and humidity that is documented but also the measurement distance and source diameter (calibration geometry).

Ambient temperature referencing source BR20AR

To improve the specified camera accuracy of the PI 450i T010 camera, a reference source with a high emissivity and a stable and known temperature must be positioned close to the subject to be scanned. The BR 20AR Ambient referencing source is equipped with a temperature probe with 0,1°C accuracy.

By integrating this highly accurate reference signal with the PIX Connect software, camera uncertainties resulting from device adjustment, ambient temperature drift and short-term stability, can be reduced to a system accuracy of 0,5°C. Other BR20AR features include:

• Reference radiator with high emissivity ideal for IR camera-based fever screening applications.

• Integrated 16-bit digital temperature sensor with 0,1 °C accuracy.

• Mounting bracket – adjustable for either ceiling or wall mounting.

• Plug-and-play installation with 20 m cable and suitable PIF connector.


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