Radiometric Calibration - Teledyne FLIR A68 Series User Manual

If a window is added in front of the lens as part of the design (window enclosure for
example), both slope/gain and offset need to be readjusted to compensate for
transmission loss. If the window material does not have sufficiently high transmission,
window temperature and transmittance need to be considered in the correction values
applied. The lower the window transmission the higher the impact on temperature
accuracy, also factoring in the window temperature.

Radiometric Calibration

The A68/A38 camera allows the user to compensate for mounting and enclosure
elements by updating both gain and offset coefficients.
Before performing radiometric calibration, the camera should be at
normal operating temperature; acquire images from the camera for a
minimum of 30 minutes to allow the internal temperature to stabilize. A
longer delay may be required when using the camera in an enclosure.
To compute the necessary gain and offset to apply, a high-quality heat source that
emits radiation at a specific temperature, known as a blackbody, is necessary.
For gain calculation, at least two different temperature values between 35 and 45 °C
are required for gain calibration. To simplify the procedure, two or more blackbodies
are preferred; if only one blackbody is available, at least two different temperature
settings must be used. The blackbodies should be placed at about 1.5m or more from
the camera, centered in the frame as much as possible and correctly focused. For
proper measurements, a blackbody should occupy a region of interest (ROI) of at least
20x20 pixels in the image.
For radiometric calibration, the procedure depends on whether a blackbody reference is
available in the final deployment environment.
• Blackbody not available: a one-time calibration of the gain and offset values
is performed using blackbody reference(s) in conditions that are as close to the
deployment environment as possible.
• Blackbody available: automatic offset correction can be performed
dynamically, providing more accurate results (gain calibration can be performed
manually as required, after the initial calibration).
Note: An external reference temperature source in the camera Field Of
view (FOV) is required by IEC 80601-2-59 (or FDA guidelines). This
allows stable temperature information to compensate for the temperature
reading if airflow is coming on the camera.
In addition to this external reference, IEC 80601-2-59/ISO-TR 13154 also
requires installation of the camera far away from any external high
temperature source or movement of air.
50 • Features and Configuration Options FLIR A68/A38 Series IR Camera User's Manual