Infrared (IR) imaging technology

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Thermography: Temperature measurement of an object by observation of its emitted radiation, primarily
in the infrared waveband.
How to Measure the Temperature
Of a Speeding Bullet
Infrared (IR) imaging technology
about 930 m/sec. We wanted to freeze the apparent
offers a way to measure temperatures of objects motion of the bullet, so we chose the shortest integration
that do not easily lend themselves to physical con- time (shutter speed) for this camera, which is 10 µsec.
tact. All objects emit electromagnetic radiation,
The bullet travels 9.3 mm in this time, about half of its
primarily in the IR waveband for objects at terres- length. One can see a blurred “tail” to the bullet caused
trial temperatures. One of the most useful wavebands by its motion. The camera has an indium antimonide
for thermography is the midwave IR, or MWIR band. detector with a cold filter that results in a 3 to 5 ␮m waveSmall changes in temperature produce large relative length sensitivity, and a 50 mm lens that was placed about
changes in flux, leading to increased thermal contrast 1 m from the gun’s muzzle.
over other IR wavebands, that in turns leads to more accuWe set up a digital data acquisition system to record
rate temperature measurement.
600 frames of 12-bit digital image data at 60 frames/sec.
The technique is as follows: A thermographically cal- We would then empty a 30-round magazine through the
ibrated IR camera is pointed at an
gun over the 10 sec recording
object of interest and an image is
time. We would go back through
taken. Software takes the digital
the sequence frame by frame to
data from the detectors in the
find images of the bullet emergcamera’s sensor and converts
ing from the muzzle gas cloud.
these data into temperature valThe bullets cooled as they travues. In a typical calibration proeled away from the gun. Any bulcedure, the camera is pointed at a
let within the gas cloud is effecblackbody source at known temtively invisible to this camera.
perature. The response of the
Once we had located the best
camera is repeatedly recorded as
image in our data set, we measthe source temperature is varied
ured the signal level average on
over a range of interest for a spethe pixels right in the center of
cific application, and a polynomithe bullet in the image. This sigal is fit to the data. This polynonal level was found to be 3,800
mial function can be used to
digital counts out of a possible
calculate the temperature of an
4,096. We then pointed the camobject for any temperature within This MWIR image shows a bullet that has
era at a high-temperature cavity
the range of the calibration.
blackbody source that had a
just emerged from the cloud, where the temThe camera lens itself can perature would be at its highest. A close-up
copper penny placed inside of it.
contribute to the flux of IR radia- of bullet shown in inset.
The penny has a surface of oxition reaching the detector array.
dized copper, and emits MWIR
The lens contribution can be calibrated out at a particular light in a similar manner to a copper-jacketed bullet in
temperature, but if the camera environment changes, then flight. When the penny temperature reached 267°C, the
the apparent temperature of an object as viewed by the signal level averaged over a cluster of pixels within the
camera can change. Therefore, a thermographic camera penny’s boundaries was the same as the signal level
should have temperature sensors in the lens housing or from the bullet.
lens interface itself, so that the software that calculates the
This type of calibration is the most accurate way to
temperature of a source compensates for the IR contribu- measure the temperature of an object that is not a pertion from the lens itself.
fect blackbody emitter, as is the case with a bullet. We
Recently, we measured the temperature of a rifle bul- concluded that the bullet’s temperature was around
let in flight. This is an excellent
267°C. The bullet gets very hot as it
example of an object whose temperis forced down the rifle barrel at
ature is impossible to measure with
supersonic speeds.
—Austin Richards, PhD
standard thermometers. The bullet
Richards is the senior applications engineer at
was a 5.56 mm NATO round (fired
Indigo Systems Corp.
from an AR-15 carbine) traveling at
R&D Magazine, June 2002