Temperature Measurement; General - Jenoptik VARIOSCAN high resolution Series Manual

Temperature measurement

9 Temperature measurement
9.1

General

VARIOSCAN high resolution is a measuring slow-scan thermographic
system scanning the object field in a raster using a single element
detector.
The detector signal is amplified, digitised with 16 bits, and visualised
with an 8 bit resolution (256 colours). Every colour of the displayed
thermogram represents a defined temperature. Between two image
scans, a reference radiator (chopper) is moved into the optical path for
a short time, and the detector signal clamped to a defined voltage
value. In this way, the detector signal is referenced to an absolute
temperature. The temperature of the chopper and of the detector chip
are measured by sensors. To expand the temperature range of the
camera, different apertures can be swung into the optical path in front
of the detector.
The relationship between detector signal and object temperature is
non-linear. It is described by a family of characteristics based on
chopper and detector temperature. This family of characteristics is
saved in a data record of camera-specific SPC files in the form of
backing points at different object, chopper and detector temperatures.
These files are the result of a calibration. During calibration, every
camera head is measured point-by-point over the full temperature
measuring range at different ambient temperatures and using the
individual apertures. From the calibration points, a special interpolation
method generates a solid characteristic. All temperature-relevant
calculations are based then on this characteristic.
Temperature calculation on VARIOSCAN high resolution is performed
in three steps:
1. The current characteristic of the camera head is calculated based
on calibration data (SPC file) and depending on current ambient
conditions (chopper and detector temperature). As instrument
temperature changes relatively slowly only, the characteristic is
recalculated only on system start and at defined time intervals
(refresh).
2. Next, based on this characteristic the relationship is set up between
pixel signal (digitised voltage value of the amplified detector signal)
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