Originally posted by MJNewton
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The only time you might need to do something like this would be an unpainted antique cast iron radiator - but even then an oxidised surface still has a very high emissivity compared to polished metal.
Also make sure you hold the sensor right up to the surface - they essentially average across an imaginary 'cone' of IR input and hence the further away you are the more background radiation you'll pick up (lowering the apparent temperature).
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