FLIR Photon Manual Book page 41

PC has the appropriate NIC driver and
application program. Network adapters
for fiberoptic and wireless connectivity
can extend Ethernet's scope (Figure 3).
Other hardware timing issues can be
minimized by using direct-wired digital
I/O and triggering between individual
cameras, PLCs, etc. Analog video
(NTSC and PAL) for conventional image
monitoring is probably most applicable
to qualitative applications where timing
is not critical.
IR Camera Hardware and
Firmware Issues
Thermal Time Constants for Cooled and
Uncooled IR Cameras. In general, time
constant refers to the time it takes for a
sensing element to respond to within
63.2% of a step change in the state of
a target that is being sensed (Figure
4). In IR sensing and thermography,
the thermal time constant of an IR
camera's detector is a limiting factor in
instrument performance as it relates to
response time.
100%
80%
63%
0
Thermal Time Constants
Figure 4. Thermal time constant concept
showing an integral number of time constants
on the X-axis.
1 2
Real-Time Control Issues
Older IR cameras have response times
similar to the human eye, so they are
unsuitable for capturing thermal images
of fast moving objects or those with
rapidly changing temperatures. Newer
IR cameras have detectors and digital
electronics with response times in the
sub-millisecond region. Cooled quantum
detectors are very sensitive and very
fast (sub-microsecond response times),
but their bulkiness and cost tends to
rule them out of many automation
applications. In addition, quantum
detectors have response curves
with detectivity that varies strongly
with IR wavelength. FLIR has made
recent improvements to its uncooled
broadband microbolometer detectors
and associated A/D converters so they
can continuously output images with
embedded temperature data at a
60Hz rate. This is satisfactory for most
temperature monitoring and IR machine
vision applications.
Temperature Measurement Range.
The overall temperature range of an
IR camera is primarily a function of
its detector and calibration. Camera
electronics, which include calibration
functions, can handle wide variations
in absolute detector sensitivities.
For example, the FLIR A325's overall
measurement range is divided into user-
selectable temperature scales that have a
measurement accuracy of ±2°C (±3.6°F) or
±2% of reading:
• –20°C to + 120°C (–4°F to +248°F)
• 0°C to +350°C (32°F to +662°F)
• Optionally, 250°C to +1200°C (482°F to
2192°F)
This is a valuable feature when a scene
has a temperature range narrower than
35