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Assuring accurate temperature measurement

Infrared instruments capture the invisible infrared energy naturally emitted from all objects. There are several important factors that determine accurate measurement. The most important are emissivity, field of view and distance to spot size.


Emissivity – All objects reflect, transmit and emit energy. Only the emitted energy indicates the temperature of the object. When IR units measure the surface temperature they sense all three kinds of energy, therefore they have to be adjusted to read emitted energy only. Some units have adjustable emissivity, others are preset at 0.95. The value of emissivity for various materials can be looked up online in a published material emissivity table. If you are using a unit with a fixed emissivity to measure the surface temperature of a shiny object you can compensate by covering the surface to be measured with masking tape or flat black paint. Allow time for the tape or paint to reach the same temperature as the material underneath. For the true temperature, measure the temperature of the taped or painted surface.


Distance to spot ratio – The optical system of an infrared unit collects infrared energy from a circular measurement spot. Optical resolution is defined by the ratio of the distance from instrument to the object compared to the size of the spot being measured (D:S ratio). The larger the ratio number, the better the instrument's resolution and the smaller the spot size that can be measured. Laser sighting only helps to aim at the measured spot.


Field of view – Make sure the target is larger than the spot size the unit is measuring. The smaller the target, the closer you should be to the target. When accuracy is critical, make sure the target is at least twice as large as the spot size.

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