I really hate to write this post, but I’ve seen too much erroneous information presented both on the web and in print to let it go any longer. There seems to be a serious misunderstanding regarding the actual method of operation of IR thermometers, so let’s clear the air!

Sometimes referred to as non-contact thermometers or pyrometers, these devices measure the surface temperature of an object based on the emitted far-IR as guided by the principle of black-body radiation. All objects with a temperature above absolute zero emit some amount of energy in the far-IR spectrum. This lies beyond the infrared visible with night-vision goggles and below low frequency radio waves. We are all familiar with this sort of IR radiation, although we don’t think of it as such. What is called far-IR raditation, we know as heat. This IR radiation can be of three types: reflected, transmitted, and emitted.

The emitted IR energy of an object will determine its surface temperature and is affected by the emissivity of the object being measured. Truly accurate measurements require the device to be calibrated for the emissivity of the material being measured, but most units have a fixed emissivity value of 0.95 that corresponds to most organic matter and painted surfaces. Reflected energy can potentially come from an exterior light source, and is generally only a problem with shiny metals. In this case, the measurement area can be covered with masking tape to eliminate the reflected energy. Transmitted energy is related to the object’s internal temperature, and any object to which the heat is transferred to.

Now that be have a basic understanding of the device’s operation, let us now dispell a couple of myths.

 #1:IR thermometers DO NOT EMIT ANY IR!!!

Read that again…

You may be saying ‘but what about the laser?’. The laser on a non-contact thermometer is for AIMING PURPOSES ONLY. It provides the same function as a laser on a gun, but without the abrupt finale. The IR thermometer might just as well have an optical scope attached. Readings are made by the use of a simple optics system much like that of a camera that focuses IR heat onto a pyroelectric detector element. The meter takes a ‘picture’ of the IR being radiated from an object, but does not contribute any of its own.

 Because of this, the measurement area grows larger in proportion to the measurement distance much like a spotlight beam. Therefore, the field of view (FOV) must be considered when making measurements. If a measurement is being made from a considerable distance, the reading will actually be something of an average of temperatures within the optical field of view of the device.

 #2: IR themometers CANNOT measure ‘cold spots’!!

Non-contact thermometers rely upon heat emitted from an OBJECT (or matter of some sort) to make temperature readings. They are no more capable of measureing cold spots in a room than a thermal imager. There must be some sort of material providing radiated IR, and unless the local atmosphere contains a dense concentration of particulate matter (which would not be accurately measured, but only affect a measurement), IR thermometers cannot be used to ‘scan’ an area for ‘cold spots’. Remember, even if it appears that a reading is above or below the ambient temperature, if an IR thermometer is being pointed into a large area the reading can be affected by the three types of IR radiation: reflected, transmitted, and emitted. These devices simply are not desinged to measure the tempature of empty space. That’s what probe thermometers, thermistors, and thermocouples are for.

These devices were designed to read the surface temperature of objects in hazardous locations, objects that are dangerously hot or cold, and for other applications in which a contact temperature reading is simply not feasible. They perform well and are convenient, but not suited to every application.

Thanks for taking the time.