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
Types of thermal infrared cameras
In general, there are two types of thermal infrared cameras in 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 energy in a specific waveband, typically the
midwave IR band at around 3 to 5 μm. The photons strike the pixels and
are converted into electrons that are stored in an integration capacitor.
The pixel is electronically shuttered by opening or shorting 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 temperature 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.
Time constants and a thought experiment
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 accurately 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