Thermal Imaging Cameras and Temperature

I’m currently working on a project for a company to look at the heating behavior of certain materials and I ran into some fundamental misunderstanding of how thermal imaging cameras work from the company. Sounds like a great topic for a blog posting, if you ask me.

First off, thermal imaging cameras do not measure temperature. Of course, most temperature measurement tools don’t measure temperature directly either. A bulb thermometer, like those your mom used to stick under your tongue, measures the volume of a fluid such a mercury or alcohol which changes due to thermal expansion.

Alcohol Thermometer – http://en.wikipedia.org/wiki/File:SpiritTherm02.jpg

If you have a digital thermometer, it probably has a thermistor in it, which is a type of resistor whose resistance varies with temperature.  The thermometer you stuck in your turkey for Thanksgiving most likely has a bimetallic strip inside it, which turns the needle on the temperature dial.

Bimetallic Thermometer – http://commons.wikimedia.org/wiki/File:Thermometers_in_pitcher.jpg

The bimetallic thermometer has two strips of different metals that are joined together.  The metals expand at different rates when heated, causing the strip to curl as it heats up, thus turning the temperature needle.

Infrared thermometers use the same technology as infrared cameras.  Both measure the infrared light emitted by an object.  (Pet Peeve Alert:  An infrared laser thermometer does not use a laser to measure temperature.  The laser is just a guide so you know where the thermometer sensor is pointing).  All objects emit some infrared light due to their temperature.  This is referred to as black-body radiation (it’s called “black-body” because the light coming from a perfectly black object that doesn’t reflect any light would only be the black-body radiation).

The filament in an incandescent bulb is an example of a black-body (actually this is only an approximation, but it works for our purposes).  The more you heat up the filament the brighter it gets.  If you have any incandescent bulbs on a dimmer switch you can see this effect.  The reason that these bulbs are so inefficient (and the reason they are so darn hot) is because they are dumping a lot of energy into the infrared part of the spectrum that our eyes can’t see.  If you continued to heat the bulb up even higher you would eventually start to get some ultraviolet light as well (which isn’t so great).  This is why halogen bulbs, which do get hotter than incandescent bulbs, have ultraviolet filters on them.  For objects at typical human temperatures, the light emitted is in the infrared part of the spectrum.

The way the infrared camera works is that it converts the intensity of infrared radiation emitted by objects into an electric signal, which is converted into a colored pixel on the display.  You should end up with something that looks like this:

Haunted Basement? Elmo and Joy hang out in IR

There is one slight problem with this image.  Everything except Elmo and Joy (the glowing evil spirits that haunt my basement) are all at the same temperature as the room, but we can clearly see the boxes (can you spot the litter box?) and chair in the background.  What gives?  The reason we can see these objects, even though they have the same temperature is because they have different emissivities.  This means that have different “brightnesses” in the infrared.  Ironically, something that we see as black usually has a fairly high emissivity while something that is white to our eyes probably has a much lower emissivity and will appear “cooler” in the infrared than it really is.  The type of material and the thickness of the material can also affect the infrared light emitted so a sample of wood, plastic, and metal, all at the same temperature would look different in the infrared part of the spectrum.

So the problem with using a thermal imaging camera (or any infrared thermometer) is that you need to know something about the surfaces you are looking at.  To understand this point a little more clearly, take a look at the black-body radiation curve below.  The camera detects the infrared light near one particular wavelength.  In this graph here I’ve chosen the 2.0 μm wavelength (vertical line).

Black-Body Radiation – Image Credit: http://physics.schooltool.nl/irspectroscopy/images/planck_black-body_radiation.png

When the camera detects a particular intensity of radiation (say one of the points circled in red) it associates that intensity with a particular temperature curve.  If the camera detected an intensity of 2.0 (in arbitrary units displayed on the graph) it would say “aha, that looks like it belongs to the 6000 K curve for a black-body” so it reports a temperature of 6000 K (roughly 5727° C).  If the object the camera is looking at has a lower emissivity than the camera thinks (i.e. the object isn’t as bright in the infrared as its temperature would indicate) the actual temperature of the object would be much higher.  To go back to our incandescent bulb, imagine we are trying to determine how hot the filament is by measuring the brightness of the filament.  If the bulb is heavily frosted, the filament looks dimmer than it really is so we would think the filament is cooler than it actually is.  To determine the real temperature of the filament we would need to know something about how much light is blocked by the frosted bulb.  In infrared thermometry, the emissivity of the object takes the place of the amount of frosting on the bulb.

There is one other complication with measuring temperatures using an infrared camera.  Many surfaces are highly reflective in the infrared, so the infrared radiation you detect may not  be emitted by the surface, but may be a reflection from something else.  Notice Elmo’s reflection in the floor under him in the picture below.

Elmo hiding from the dogs

The camera thinks the floor in front of Elmo is much hotter than it really is because of his infrared reflection.  When you start making measurements of shiny metals, which have much lower emissivities (they tend to look much darker than their temperature would indicate) and higher reflectivities, and it becomes very difficult to accurately measure temperatures.  The reflection of the room temperature camera appears much brighter than a very hot piece of metal.

The idea of using infrared thermometers brings up another pet peeve of mine.  Students tend to think that tools that are more high-tech tend to be better than their lower-tech cousins.  This isn’t true and frequently the lower-tech devices are better.  To measure a temperature I’ll take a thermistor or thermocouple any day of the week.  Now that isn’t to say that infrared thermometry isn’t a very cool and very powerful tool, but it has its place. A great use for infrared thermometry is situations where a thermistor wouldn’t withstand the heat, like in a kiln, or when measuring temperatures over larger areas, like trying to find heat leaks on the outside of a house.  I’m sure there are many other uses I can’t think of.  Here is my “take-away” message for you: choose the simplest tool possible that gets the job done right.  Bells and whistles are cool (and believe me, this infrared camera is really cool), but more complicated tools come with a lot of built-in assumptions you might not be aware of.

Ellie takes a break from playing with her toy