About calibration
36
36.1 Introduction
Calibration of a thermal camera is a prerequisite for temperature measurement. The cali-
bration provides the relationship between the input signal and the physical quantity that
the user wants to measure. However, despite its widespread and frequent use, the term
“calibration” is often misunderstood and misused. Local and national differences as well
as translation-related issues create additional confusion.
Unclear terminology can lead to difficulties in communication and erroneous translations,
and subsequently to incorrect measurements due to misunderstandings and, in the worst
case, even to lawsuits.
36.2 Definition—what is calibration?
The International Bureau of Weights and Measures
15
defines calibration
16
in the following
way:
an operation that, under specified conditions, in a first step, establishes a relation be-
tween the quantity values with measurement uncertainties provided by measurement
standards and corresponding indications with associated measurement uncertainties
and, in a second step, uses this information to establish a relation for obtaining a meas-
urement result from an indication.
The calibration itself may be expressed in different formats: this can be a statement, cali-
bration function, calibration diagram
17
, calibration curve
18
, or calibration table.
Often, the first step alone in the above definition is perceived and referred to as being
“calibration.” However, this is not (always) sufficient.
Considering the calibration procedure of a thermal camera, the first step establishes the
relation between emitted radiation (the quantity value) and the electrical output signal
(the indication). This first step of the calibration procedure consists of obtaining a homo-
geneous (or uniform) response when the camera is placed in front of an extended source
of radiation.
As we know the temperature of the reference source emitting the radiation, in the second
step the obtained output signal (the indication) can be related to the reference source’s
temperature (measurement result). The second step includes drift measurement and
compensation.
To be correct, calibration of a thermal camera is, strictly, not expressed through tempera-
ture. Thermal cameras are sensitive to infrared radiation: therefore, at first you obtain a
radiance correspondence, then a relationship between radiance and temperature. For
bolometer cameras used by non-R&D customers, radiance is not expressed: only the
temperature is provided.
36.3 Camera calibration at FLIR Systems
Without calibration, an infrared camera would not be able to measure either radiance or
temperature. At FLIR Systems, the calibration of uncooled microbolometer cameras with
a measurement capability is carried out during both production and service. Cooled cam-
eras with photon detectors are often calibrated by the user with special software. With
this type of software, in theory, common handheld uncooled thermal cameras could be
calibrated by the user too. However, as this software is not suitable for reporting
#T559954; r. AP/42311/42335; en-US
216
15.http://www.bipm.org/en/about-us/ [Retrieved 2017-01-31.]
16.http://jcgm.bipm.org/vim/en/2.39.html [Retrieved 2017-01-31.]
17.http://jcgm.bipm.org/vim/en/4.30.html [Retrieved 2017-01-31.]
18.http://jcgm.bipm.org/vim/en/4.31.html [Retrieved 2017-01-31.]
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