FLIR Photon User Manual

36
Chapter 
a camera’s overall range. Selecting a
narrower scale allows better resolution
of the images and higher accuracy in
the measured temperatures. Therefore,
the images will better illustrate smaller
temperature dierences. On the other
hand, a broader scale and/or higher
maximum temperature range may be
needed to prevent saturation of the
portion of the image at the highest
temperature.
It’s important to understand how the
camera’s calibration and temperature
measurement processes aect its
response time. IR cameras measure
irradiance, not temperature, but the
two are related. When an IR camera
is thermographically calibrated, it
can measure temperatures based on
standard blackbody radiances at specic
temperatures. As will be discussed later,
the emissivity of the target object being
measured is vital to achieving accurate
temperature readings. (Emissivity or
emittance is the radiative properties of an
object relative to a perfect blackbody.)
When an IR camera is calibrated at the
factory, calibration factors are stored
internally as a table of values based
on the camera’s A/D counts from the
temperature/radiance measurements of
a standard blackbody. When the system
makes a measurement in an application,
it takes the digital value of the signal at a
given moment, goes into the appropriate
calibration table, and calculates
temperature. Before the nal result is
presented, due consideration is given
to other factors, like emissivity of the
target objects, atmospheric attenuation,
reected ambient temperature, and the
camera’s ambient temperature drift.
Figure 5. Creating a temperature scale narrower
than the camera’s full range improves image
resolution and may improve defect detection.
As an adjunct to major temperature scale
selections, most IR cameras allow a user
to set up a color scale or gray scale for a
temperature range that’s even narrower
(Figure 5). This should be done where
practical, not only because of improved
image resolution, but also because of
response time considerations. A narrower
temperature range can reduce the A/D
converter’s processing load and overall
response time of the system.
Another complexity is the fact that
each individual pixel in the camera’s
focal plane array has a slightly dierent
gain and zero oset. To create a useful
thermographic image, the dierent
gains and osets must be corrected
to a normalized value. This multi-step
calibration process is performed by the
camera rmware (Figure 6). The non-
uniformity correction (NUC) factors are
also stored in a table.
IR cameras also have dierent
measurement modes: spotmeter and
area measurements in the case of the
FLIR A320 Series. The spotmeter nds the
temperature at a particular point whereas
the area function isolates a selected area
of an object or scene. In the latter case,

FLIR Photon Specifications

General

Brand	FLIR
Model	Photon
Category	IP Camera
Language	English