IR illumination night vision cameras allow you to see objects the
illumination is cast on at night without being spotted yourself
(UNLESS IR IS USED by the other side as well)
IR imagery cameras observe the natural emissions from any object and
do not need or gain anything from IR illumination.
One is reflected IR light from surfaces which allow you to delineate
them from the reflections. The other is a quantitative thermal image
composed from the levels of IR all the objects in view emit on their
own.
I just want to mention that these two different IR camera types work
from very different IR wavelength ranges and use different sensors.
Cameras that work from IR illumination, with at most very few exceptions
(I can't name any), work from infrared wavelengths generally in the IR-A
range, which is .7 to 1.5 micrometers.
For that matter, most of such work with illuminators is done with a
narrower range, since silicon IR photodetectors don't do well with
wavelengths longer than about 1.2 or so micrometers, and wavelengths a bit
longer than .7 micrometer (up to .82 easily, maybe .9 or so) are visible
enough for one to see the light source in the dark.
As for thermal IR - keep in mind that a 3000 K halogen lamp filament has
peak close to 1 micrometer and at .4 micrometer (borderline violet-UV)
output is down to about 7% of peak. At 300 nm output is close to or a
little over 1% of peak.
Keep in mind that this translates to 300 Kelvin radiation peaking close
to 10 micrometers, 7% of peak at 4 micrometers, and 1% of peak at or
hardly below 3 micrometers. I have a non-contact thermometer and IIRC
it works from the 7-14 micrometer range according to its documentation.
Please know that "room temperature thermal IR" is generally absorbed by
glass, water and most other transparent objects. A non-contact IR
thermometer will not see through these objects but read the temperature of
these objects.
Further digressing, many materials that absorb 10 micrometer ballpark
IR, even if transparent in the visible, are transparent at microwave and
radio frequencies. IIRC, Hertz managed some determination of an index of
refraction of some material for radio waves from some kind of spark
transmiter (presumably of frequency high enough to do this with) before
Tesla and Marconi, before there was electronics.
There are some materials with more limited rages of failing to be
transparent to medium/long IR wavelengths. What comes to my mind now are
some halides (such as potassium chloride - hygroscopic!) and (probably not
all) non-metallic elements and non-metallic isotropes of elements in the 4
and 5 column of the periodic table (where 3-8 are the last 6 and 1,2 are
the first 2), in periods/rows at least 4. Germanium and silicon come to
my mind as being mentioned as transparent to "mid-IR"/"far-IR", and
probably not transparent to all wavelengths in these ranges.
- Don Klipstein (
[email protected])
