Calibration is also a prerequisite for
adjustment
, which is the set of operations carried out
on a measuring system such that the system provides prescribed indications corre-
sponding to given values of quantities to be measured, typically obtained from measure-
ment standards. Simplified, adjustment is a manipulation that results in instruments that
measure correctly within their specifications. In everyday language, the term “calibration”
is widely used instead of “adjustment” for measuring devices.
27.6
Non-uniformity correction
When the thermal camera displays ”Calibrating…” it is adjusting for the deviation in re-
sponse of each individual detector element (pixel). In thermography, this is called a ”non-
uniformity correction” (NUC). It is an offset update, and the gain remains unchanged.
The European standard EN 16714-3, Non-destructive Testing—Thermographic Testing
—Part 3: Terms and Definitions, defines an NUC as “Image correction carried out by the
camera software to compensate for different sensitivities of detector elements and other
optical and geometrical disturbances.”
During the NUC (the offset update), a shutter (internal flag) is placed in the optical path,
and all the detector elements are exposed to the same amount of radiation originating
from the shutter. Therefore, in an ideal situation, they should all give the same output sig-
nal. However, each individual element has its own response, so the output is not uniform.
This deviation from the ideal result is calculated and used to mathematically perform an
image correction, which is essentially a correction of the displayed radiation signal.
Some cameras do not have an internal flag. In this case, the offset update must be per-
formed manually using special software and an external uniform source of radiation.
An NUC is performed, for example, at start-up, when changing a measurement range, or
when the environment temperature changes. Some cameras also allow the user to trig-
ger it manually. This is useful when you have to perform a critical measurement with as
little image disturbance as possible.
27.7
Thermal image adjustment (thermal
tuning)
Some people use the term “image calibration” when adjusting the thermal contrast and
brightness in the image to enhance specific details. During this operation, the tempera-
ture interval is set in such a way that all available colors are used to show only (or mainly)
the temperatures in the region of interest. The correct term for this manipulation is “ther-
mal image adjustment” or “thermal tuning”, or, in some languages, “thermal image optimi-
zation.” You must be in manual mode to undertake this, otherwise the camera will set the
lower and upper limits of the displayed temperature interval automatically to the coldest
and hottest temperatures in the scene.
#T810253; r. AL/84068/84441; en-US
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