Infrared Thermometers for non-contact temperature measurement


Infrared thermometers work by measuring the amount of infrared light emitted at different wavelengths.

Objects glow with a characteristic spectral distribution, depending on their temperature. Objects with temperatures of interest around the house will have a maximum in spectral emission in the infrared.  The human eye cannot see wavelengths greater than about 0.8 micro meters.

If an object is hot enough, it glows red-hot and is visible to the eye.  This red glow starts to become visible to the naked eye at about 900 degrees F.  Things that are at a temperature below 900 degrees F will also glow, but this glow is imperceptible to the human eye.  Instruments, however, can record it.  That is the basis of an infrared thermometer.

Infrared thermometers work by focusing light from an invisibly glowing object onto a sensor and measuring the intensity of that glowing light.

Two factors determine how bright an object will glow: temperature and emissivity.  The thermometer has no way of measuring the emissivity of a surface.  Most use a default emissivity value of 0.95, which is very close to the emissivity value of  many common surfaces including: asphalt, asbestos, paint, plastic, rubber, wood, and water.

Don’t use an infrared thermometer to measure the temperature of bare metals like copper, lead, aluminum, or iron unless you read the manual and understand how to change the emissivity on the meter.  On cheaper meters, you cannot change the emissivity.

You can use an infrared thermometer as normal for painted metals, but bare metals will give a bad reading unless you change tell the meter what type of material you are pointing at. If you cannot change the emissivity levels, you can instead put some black electrical tape on the metal’s surface and measure the temperature of the black electrical tape to get an accurate reading.  The surface that you point the thermometer at is what matters, not what’s behind the surface.

Imaging Detector Technology

The technology behind today’s infrared imaging thermometers was classified military technology up until the early 1990’s.

The detector is based upon an array of microbolometers.  A microbolometer is a tiny piece of material  thermally isolated from its substrate.  A lens focuses infrared light onto the microbolometers and this light causes the tiny piece of material to increase its temperature.  This increase in temperature, although also small, leads to a change in its electrical properties.  Sensitive electronics can read this change in electrical properties and interpret it as an image.

Three classes

  • Low priced.  Typically fixed emissivity values. Etekcity lasergrip 1080
  • Mid range. Fluke 566
  • Imaging. FLIR TG165 (FLIR ONE phone adapter, 160×120 12 with micron pixels)

The FLIR TG165 is built upon the FLIR Lepton camera package.  The Lepton is an 80×60 array of microbolometers sensitive to longwave infrared (8 to 14 microns) with a 17 micron pixel size.  You can get the Lepton from Digikey.


You can buy the FLIR TG165 at amazon