Top 3 Infrared Thermometers for non-contact temperature measurement

Three distinct classes of non contact infrared thermometers exist:

  • Low priced, budget models.  These typically have fixed emissivity values and measure temperature at one point. One of the better models in this class is the Etekcity Lasergrip 630
  • Mid range. These are professional tools. They have the ability to store measurements, chose emissivity values, and provide a USB connection to download stored data.  The Fluke 566 is representative of this class.
  • Imaging Thermometers. These provide an actual image, like a photograph, of the temperature. Two examples of instruments in this class are the FLIR TG165, a standalone infrared imager, and the FLIR ONE, which plugs into your Android or iPhone and provides you with an infrared image on your phone screen.

Theory of Operation

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

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 with a fixed emissivity value of 0.95 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. Before these MEMS based microbolometers, infrared imaging cameras required cryogenic cooling, usually using liquid nitrogen.

The infrared imaging camera’s detector is  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 the temperature of the object from which it originates.  A computer synthesizes the temperatures of the array of microbolometers into an image.

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.

Seek also makes thermal imaging sensors.  They have a 206×156 pixel sensor they pack into cameras that attach to your smartphone.

The FLIR TG165 is built upon the Lepton sensor