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Infrared Thermometers Emissivity

An article on infrared (IR) thermometers accuracy, spot size, and emissivity.

Infra red thermometers receive IR radiation from the target and convert that to a small voltage which is displayed as a temperature reading. This method is very convenient and has numerous applications. However, it is important to be aware of some basics to obtain meaningful results.

Spot size:

Some IR meters have a laser beam to indicate the centre of the area measured, but the area measured is much larger. Meters come with a specification that gives a ratio of distance to spot size. The spot size may be 8:1 or just 1:1. The former means that a measurement is made 1.6m from the object, then the measured area is 20cm, but with only a 1:1 spot size the measured area would have a diameter of 1.6m and in most cases the operator would need to take the measurement closer in.

A measurement can only be accurate if the spot size is smaller than the measured object.

IR meters may be specified to be within 2ºC or within 2% (whichever is greater), accuracy is not great, but it can be improved - considering:

At the same temperature a black body will emit more IR than a white body; these values are referred to as 'emissivity' and they are defined as the ratio of the energy/heat given off through radiation by an object, compared to that radiated by a black body (perfect radiator) at the same temperature. I.e. dark objects have a high emissivity (~0.8), where as a white object will have a low emissivity (~0.1). The surface finish also plays a role - see emissivity table below.

Most IR thermometers have a set emissivity of 0.95. Adjustable emissivity allows an operator to calibrate the instrument for different surfaces and then setting the emissivity to the most appropriate for any particular surface. Alternatively, without an adjustment an operator may learn that depending on the surface, the IR gun will read a little high or a little low. If most of the time the temperature is read from the same type surface, i.e. milk, or a bearing housing, a reading with an accurate thermometer may be made and compared with that of the IR gun. Then simply apply an adjustment (for instance minus 2ºC) whenever that particular surface is measured.

Measuring the same type of surface and often at a similar temperature should, with a calibration adjustment, achieve more accurate readings.

A sample of emissivity coefficients

Aluminium foil 0.04
Aluminium heavily oxidised 0.2 - 0.31
Aluminium highly polished 0.039 - 0.057
Black body matt 1.00
Black enamel paint 0.80
Carbon filament 0.77
Concrete 0.85
Glass 0.92
Gold pure and highly polished 0.018 - 0.035
Ice 0.97
Iron polished 0.14 - 0.38
Iron plate rusted red 0.61
Mild steel 0.20 - 0.32
Plaster 0.98
Porcelain, glazed 0.92
Plastics 0.91
Stainless steel, weathered 0.85
Stainless steel, polished 0.075
Tungsten polished 0.04
Tungsten aged filament 0.032 - 0.35
Water 0.95 - 0.963
Wood oak 0.91
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