OPHIR Pyrocam IIIHR, User Manual

Page: 22 / 42

Pyrocam III
HR
User Guide Document No. 50361-001 Rev E 7/18/2017 Page 22
Occasionally, the blade will jitter or make a little noise as the control loop makes slight
periodic adjustments to the chopper’s position.
The internal chopper is placed very close to the detector’s focal plane. Thus diffraction
effects are reduced and a sharper image is possible when an optional lens is fitted.
3.4.1.2 Beam Alignment
Chopped imaging is most accurate when the input laser beam is perpendicular to the plane
of the chopper/imager. Beams coming in at oblique angles will be chopped somewhat out of
phase. This can contribute to some loss in responsivity and/or response linearity.
3.4.1.3 Damage Concerns
In Chopped mode, with the gain set to minimum (or near the minimum setting), the output
signal from the detector will saturate at about 5 W/cm². This is very near the Pyrocam’s
damage threshold of 8 W/cm².
WARNING
With a low GAIN setting, it is easy to inadvertently exceed the Pyrocam
detector’s damage threshold. For example: A one (1) Watt input beam, with
a TEM
00
Gaussian beam width of 5mm will have a peak power density of 10
W/cm
2
. If left on the detector for a significant amount of time, the detector
will sustain damage in and around the region of peak power density.
Pyrocam detectors are very costly to replace, but if treated with care they
can last nearly as long as most other semiconductor devices.
NOTE: If the Pyrocam is placed into Pulsed operating mode, the internal
chopper will set itself to an open position. If you are illuminating the detector
with a CW laser and have placed the Pyrocam into Pulsed mode, the total
power falling on the detector will effectively double, thus increasing the
damage risk described above by a factor of 2.
3.5 Calibrate and Ultracal Cycles
Calibrate and Ultracal are two different operations but they are applied together when an
Ultracal operation is initiated in BeamGage. Both operations must be performed without a
laser on the imager and under subdued room lighting if fitted with a window that transmits
room light or if no window is installed. Here’s how they differ:
3.5.1 Calibrate Cycle
The Calibrate operation is performed in the Pyrocam when a Calibrate command (see
APPENDIX F) is sent to the Pyrocam firmware. This process establishes a zero set-point within
the Pyrocam that corrects for circuit and ADC offset errors. It also sign adjusts for the
pyroelectric crystal’s polarity and dynamic range.
A Calibrate cycle is initiated in the Pyrocam each time an
Ultracal cycle is initiated in
BeamGage.