Infrared thermometers (Aka I.R. Thermometers) measure temperature using blackbody radiation (generally infrared) emitted from objects. They are sometimes called laser thermometers if a laser is used to help aim the thermometer, or non-contact thermometers to describe the device’s ability to measure temperature from a distance. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can be determined.
The most basic design consists of a lens to focus the infrared energy on to a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons.
Some typical circumstances are where the object to be measured is moving; where the object is surrounded by an electromagnetic field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required. Infrared thermometers can be used to serve a wide variety of temperature monitoring functions. A few examples provided to this article include:
- Detecting clouds for remote telescope operation
- Checking mechanical equipment or electrical circuit breaker boxes or outlets for hot spots
- Checking heater or oven temperature, for calibration and control purposes
- Detecting hot spots / performing diagnostics in electrical circuit board manufacturing
- Checking for hot spots in fire fighting situations
- Monitoring materials in process of heating and cooling, for research and development or manufacturing quality control situations
What Factors Are Important To Consider When Selecting an Infrared Thermometer?
Distance from the object you're testing (D:S Ratio)
Typically the lower cost models have lower D:S ratios such as 4:1 or 6:1. As you get into higher quality models you see higher ratings such as 12:1 or even as high as 60:1. So what is the D:S ratio anyway? The sensor of an infrared meter picks up its radiation readings in a defined circular spot. The target must fill this spot or you will be measuring emitted infrared radiation from the area outside of the target which will cause an inaccurate reading. The D:S ratio is defined as the relationship between the distance of the measuring device from the target and the diameter of the testing spot. The higher the rating, then the smaller the spot you can measure from a set distance. For example, you have a I.R. thermometer with a 4:1 D:S ratio, then you can measure a target with a diameter no smaller than 40mm from a distance no farther than 160mm. If your I.R. thermometer had a D:S ratio of 12:1, then you could measure a spot with a diameter as small as 13mm from a distance of 160mm. Think of the D:S like the magnification power of the scope on a gun. (i.e. you can accurately hit a smaller bulls-eye with a more powerful scope). If you are testing close to the object or in a small space (such as a testing circuit board for hot spots), we recommend a small maneuverable model like the ThermoHAWK™. If you are testing objects with extreme temperatures or items that are hard to reach, then a model with a high D:S ratio is a good choice.
Reflectivity of the object you are testing (emmisivity)
Emmisivity is an important principle to understand. Some forms of matter emit infrared radiation differently at the same temperature compared to a different form of matter. This property is influenced by the reflectance of the substance. Metal, glass, and wood all have different emissivity ratings between 0-1. This is one of the key technology differences you will see in different models of infrared thermometers. Most commons substances such as wood, rock, rubber and organic substances have an emissivity rating around 0.8-0.95. Therefore, many lower grade infrared models are designed fixed for 0.95 emissivity. This is because 0.95 covers the vast majority of items you will be testing. However, it is important to understand that objects with higher reflectance like (shiny metal or tin foil) are going to have an emissivity ratting closer to 0.1. Therefore, a lower model with a fixed emissivity at 0.95 will not be as accurate when measuring a shiny metal substance. Therefore, if you feel you may use your device for shiny metal testing, we strongly recommend buying a unit with an adjustable emissivity between 0.1-0.95
Infrared Thermometer, http://en.wikipedia.org/wiki/Infrared_thermometer (last visited November 4, 2009).