On 18/04/2020 08:28, John wrote:
On Sat, 18 Apr 2020 07:37:34 +0100, alan_m wrote:

On 17/04/2020 21:50, Harry Bloomfield wrote:
Martin Brown explained :
For precise none contact work you put a black sticker on the target
that has the right emissivity for the measurement device you are
using. But for mass screening it is easier to fiddle the detector to
match "skin".

I knew about the black tape trick, but never saw them stick tape on the
people they were checking.


The black tape has to be the correct type. Some black tape may actually
act as a mirror to IR and so you are measuring the temperature of what
it is reflecting rather than what it is attached to.

What about people with varying degrees of black skin ?

Black in this sense means black in the 3-20um thermal IR band which is
quite unrelated to the apparent colour in the visible spectrum.

To a first approximation almost anything opaque and non-metallic is
almost black for thermal IR but when you are trying to measure a small
temperature difference precisely on a background of ~300K you have to
compensate for the fact that skin is not an ideal opaque emitter.

Notable examples where opaque things are transparent to thermal band IR
are black bin bags and thin pine cladding. Conversely window glass is
transparent in the visible (obviously) but opaque to far IR.

https://physics.stackexchange.com/qu...infrared-light

Note how his glasses are black in the IR photo.

--
Regards,
Martin Brown