imagopus@xxxxxxxxxxxxxxxx writes:
<< But back to IR film: do we agree that it is temperature difference,
manifested as a difference in radiation wavelengths, that gives IR
film (or the near-IR consumer films) their distinctive sensitivity?
..... or am I still way off the mark?
>>
Chris,
My understanding is the IR film is only sensitive to very near infra red
wavelengths just outside the visible range in the region of 700nM (0.7uM) or
so. A black body radiator radiates almost no energy at these wavelengths
until the temperature gets pretty high so the film can only detect
temperature of pretty hot things like a cigarette. Otherwise what it detects
is reflected energy from other hot sources like the sun. You can think of it
this way : to detect low temperatures just above room temperature you need to
detect wavelengths in the region of 8uM, that is more than ten times longer
wavelengths than where the film is sensitive. At high temperatures (1000's
C) the peak emission wavelength moves down to the 1uM - 2uM region and then
there is enough emission at visible and near IR for the film to work. (The
Physicists will describe this in terms of Planck's law, Stefan-Boltzman law
etc which describes radiation energy distribution with wavelength) . The
hotter the radiator the higher the "tail" radiation in the visible spectrum
and near IR. (compare a 3500 deg Kelvin projector lamp to a 2800K normal room
lamp). Adding a red filter then emphasises the near IR over the visible.
Usually the IR detectors used for high temperatures are chosen to
be sensitive to shorter wavelengths than the 8uM used for low temperatures
because both the detector sensitivity and the energy emitted can be many
orders of magnitude better at those wavelengths. There are a variety of
semiconductor detectors that are used, some using exotic materials and these
are what are probably used in the weapon systems you are familiar with.
Silicon photocells (without the filters used for camera cells) will work
quite well at near IR wavelengths up to 950nM or so. Glass lenses of course
filter out long IR wavelengths so that even if the film worked for much
longer wavelengths and the other problems of storage etc were overcome the
lenses would be opaque and not work.
Regards,
Tim Hughes
>>hi100@xxxxxxx<<
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