Many manual lenses have an IR focus mark slightly offset from the main
focus mark. The intention is you focus visually as normal - but then
shift the focus from the normal position to the IR position marked on
the lense to allow for the slight difference in refractive power at that
Again, like the CCDs this won't give you long IR, so will not be
of any use for detecting IR emitted by objects at room temperature.
Think about it - if it did, how would you stop the film from
going off the minute it's produced?
This did bother me a lot but as the Kodak example is a picture of a tree
which shirley must be colder than a house I assumed that some sort of
black magic must be happening. Film you keep in the freezer? Cameras you
keep in the fridge? Err ...
Unrelated to IR specifically - but films do keep better colour accuracy
if you keep them cool. This is more true of the "pro" films rather than
the more consumer oriented versions.
In our local camera shop they have a display fridge for all of the
You gone and done it now - you made me think about it.
Gut feeling says you'll get next to nothing under a micron at 300K.
This is backed up by this graph:
I dusted off the old calculator, and calculated the emitted
power per square metre (black body) using the Planck energy
For 300k and 1um, and it came to ~6x10-7 W/m^2. I don't reckon
any film has the sensitivity to pick that up out of the overall
background. I may be wrong, but I remain to be convinced.
Well, I have seen a few IR photos, and they definitely seem to show
people and faces as rather bright.
Probably the only thing to do is to try it.
Film can pick up just one photon you know. It is sensitive...the point
being does the near infra red actually give any meaningful correlation
with heat? I also vaguely remember flash guns usd in dark places coverd
with IR filters.
Even if it was, it wouldn't be very useful in a camera, the glass in the
lens would block the IR .... I believe that's how windows work.
Special gallium something or other lenses are required which are mega
expensive - one of the main costs of a thermal imager.
IR film *mainly* detects the shorter (near visible) wavelength radiation
that is mainly reflected - not emitted - from objects (unless they are very
Dave Gibson wrote:
In the past I have done work on avionics spec thermal imaging kit (i.e.
the good stuff - not just the images they let you see on telly!)
Looking round the lab with a steerable thermal imager with built in
thermal telescope was "educational"!
Needless to say, you could get into serious trouble if any of the ladies
in the lab even thought you were pointing it in their direction!
(although this is where video autotrackers have uses - you can be well
away from the controls and leave it to follow the "hot body").
That's right, machined from germanium, totally opaque to normal light.
The telescope fitted to the setup I was using cost about 100K on its
own. Ordinary glass looks black, opaque and a little reflective on a
thermal image - no use at all for lenses.
As you say, IR film is a world away from real thermal imaging. The best
high resolution kit, is sensitive enough to see the thermal impression
of foot prints left after someone has just walked normally over a bit of
floor (in shoes)!
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