About calibration
17
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 cali-
brated by the user too. However, as this software is not suitable for reporting purposes,
most users do not have it. Non-measuring devices that are used for imaging only do not
need temperature calibration. Sometimes this is also reflected in camera terminology
when talking about infrared or thermal imaging cameras compared with thermography
cameras, where the latter are the measuring devices.
The calibration information, no matter if the calibration is done by FLIR Systems or the
user, is stored in calibration curves, which are expressed by mathematical functions. As
radiation intensity changes with both temperature and the distance between the object
and the camera, different curves are generated for different temperature ranges and ex-
changeable lenses.
17.4
The differences between a calibration
performed by a user and that performed directly
at FLIR Systems
First, the reference sources that FLIR Systems uses are themselves calibrated and trace-
able. This means, at each FLIR Systems site performing calibration, that the sources are
controlled by an independent national authority. The camera calibration certificate is con-
firmation of this. It is proof that not only has the calibration been performed by FLIR Sys-
tems but that it has also been carried out using calibrated references. Some users own or
have access to accredited reference sources, but they are very few in number.
Second, there is a technical difference. When performing a user calibration, the result is
often (but not always) not drift compensated. This means that the values do not take into
account a possible change in the camera’s output when the camera’s internal temperature
varies. This yields a larger uncertainty. Drift compensation uses data obtained in climate-
controlled chambers. All FLIR Systems cameras are drift compensated when they are first
delivered to the customer and when they are recalibrated by FLIR Systems service
departments.
17.5
Calibration, verification and adjustment
A common misconception is to confuse
calibration
with
verification
or
adjustment
. Indeed,
calibration is a prerequisite for
verification
, which provides confirmation that specified re-
quirements are met. Verification provides objective evidence that a given item fulfills speci-
fied requirements. To obtain the verification, defined temperatures (emitted radiation) of
calibrated and traceable reference sources are measured. The measurement results, in-
cluding the deviation, are noted in a table. The verification certificate states that these
measurement results meet specified requirements. Sometimes, companies or organiza-
tions offer and market this verification certificate as a “calibration certificate.”
Proper verification—and by extension calibration and/or recalibration—can only be
achieved when a validated protocol is respected. The process is more than placing the
camera in front of blackbodies and checking if the camera output (as temperature, for in-
stance) corresponds to the original calibration table. It is often forgotten that a camera is
not sensitive to temperature but to radiation. Furthermore, a camera is an
imaging
system,
not just a single sensor. Consequently, if the optical configuration allowing the camera to
“collect” radiance is poor or misaligned, then the “verification” (or calibration or recalibra-
tion) is worthless.
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