Infrared thermometer limitations:
As it is the surface of an object that emits infrared, an infrared thermometer will not measure its internal (core) temperature. You cannot accurately measure through any covering glass, polythene, clingfilm etc. Any surface you are measuring must be clean and dust free. Air temperature cannot be measured by an infrared thermometer.
Emissivity is a measure of efficiency in which a surface emits thermal energy. It is defined as the fraction of energy being emitted relative to that emitted by a thermal black surface (a black body). A black body is a material that is a perfect emitter of heat energy and has an emissivity value of 1. A material with an emissivity of 0 would be considered a perfect thermal mirror. For example, if an object had the potential to emit 100 units of energy but only emits 90 units in the real world, then that object would have an emissivity value of 0.90. in the real world there are no perfect “black bodies” and very few perfect infrared mirrors so most objects have an emissivity between 0 and 1.
The table below is just a small selection of different emissivity values.
- Aluminium (anodised) 77 Plastic (black) 0.95
- Brass (oxidised) 61 Porcelain (glazed) 0.92
- Brick (red) 92
- Rubber 95
- Cement 54 Skin (human) 0.98
- Copper (oxidised) 65 Soil (dry) 0.92
- Glass 92 Stainless steel 0.59
- Paper (white) 68 Water 0.95
- Perspex 86 Water (ice) 0.96
- Pipe (galvanized) 46 Water (frost) 0.98
- Plastic (white) 84 Wood (planed) 0.90
Care must be taken with the infrared thermometer’s lens. The infrared waves are focused and filtered by the lens, therefore if the lens gets dirty or damaged in any way (even light scratches) then accuracy can change at some temperatures.