46 | R&D Industry Guide | October 2014 www.rdmag.com
The Need for Speed
High-speed cameras aid in temperature measurements.
Traditional forms of temperature measurement, such as thermocouples and spot pyrometers, often don’t offer the resolution or speed required to fully characterize high-speed
thermal applications. This article explores the advantages of
high-speed thermal measurement with infrared cameras.
Types of thermal infrared cameras
In general, there are two types of thermal infrared cameras in
Time constants and a thought experiment
use today. These are high-performance cooled photon-counting
cameras and low-cost uncooled microbolometer based cameras.
The majority of thermographic cooled cameras on the
market today use a detector made from indium antimonide
(InSb). Cooled cameras work by counting the photons of en-
ergy in a specific waveband, typically the midwave IR band at
around 3 to 5 μm. The photons strike the pixels and are con-
verted into electrons that are stored in an integration capaci-
tor. The pixel is electronically shuttered by opening or short-
ing the integration capacitor. Typical integration times for - 20
to 350 C objects with a FLIR InSb camera range from about 6
msec to 50 μsec depending on the camera model. These short
integration times make it possible to “stop motion” and make
it possible to accurately measure very fast transients.
Uncooled cameras are lower cost, smaller, lighter and have
lower power consumption than the aforementioned cooled
counterparts. The pixels of an uncooled camera are made
from a material whose resistance changes significantly with
temperature. The most common materials for this application
are vanadium oxide or amorphous silicon. Thermal energy is
focused on the pixel and the pixel physically heats up or cools
down. The resistance of the pixel varies with temperature and
its value can be measured and mapped back to the target tem-
perature via a calibration process. Since the pixels have a finite
mass, they have a thermal time constant. Time constants for
modern microbolometer-based cameras are generally between
8 to 12 msec. However, this doesn’t mean that the pixel can be
read out every 8 to 12 msec and provide an accurate answer.
The rule of thumb for a first-order system responding to step
input is that it takes five time constants to reach steady state.
A fun way to think about a microbolometer detector’s time
response is to pretend that you have two buckets of water. One
bucket is full of well-stirred ice water at 0 C and the other is at
a rapid boil at 100 C. Allow the microbolometer to stare at the
ice water and then instantaneously switch to the boiling water
(a 100 C step input) and plot the resulting temperature. If we
covert the thermal time constant of 10 msec into a half time
to make for easier math, we get something around 7 msec.
Here the microbolometer reports 50 C at 7 msec or one half
time, 75 C at two half times and 87.5 C at three half times. What
would happen if users tried to read out this microbolometer at
the equivalent of 100 fps or 10 ms? The camera would report
back 63 C and have an error of 37 C. The camera would accu-
rately report the temperature of the pixel, but the pixel wouldn’t
have reached the temperature of the scene that it was looking.
In general, it doesn’t make sense to run microbolometers faster
than about 30 fps.
Let’s take a look at a printing process that’s required to heat a
sheet of paper up to 60 C. The paper is coming out of rollers at
50 in/sec and must be uniform in temperature across both the
width and length (Figure 1).
Both a cooled photon counting camera and a microbolometer camera were used to capture side-by-side data. The result:
The data from the two types of cameras look dramatically
different (Figure 2). The data from the microbolometer shows
a big, relatively steady bump in temperature along the length.
The data from the photon counting camera shows significant
variations in temperature over time.
The InSb camera shows that the heated roller assembly
cooled down due to contact with the paper over the first revolution. The bang-bang controller sensed the temperature drop
and turned the heater controller fully on again in response. As
a result, the roller heated up until the set point was reached
and then shut off, and the process repeated. This study
convinces R&D engineers of two things: a photon counting
camera was required for testing the product and a PID control
system must be implemented on the heated roller instead of
the simple bang-bang controller if the desired design objectives were to be met.
—Chris Bainter, U.S. National Sales Director
Ross Overstreet, Sr. Science Segment Engineer
Figure 1: Thermal image of paper leaving heated rollers.
Figure 2: InSb versus microbolometer for thermal transients. Images: FLIR