Empfaenger : /QWK/rec.photo.misc (Robert Long)
Absender   : W.J.Markerink@FOTOBANK
Betreff    : Re: Infrared redux
Datum      : Do 16.05.96, 18:48
Groesse    : 6106 Bytes
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On 14.05.96, Robert Long wrote about "Infrared redux":

RL>Yesterday I stumbled across a Kodak book called
RL>_Applied_Infrared_Photography_ that I'd forgotten I had.  My copy

If you forget again, please send it to me!....;-))

RL>dates from 1972, and formulations may have changed in the meantime
RL>(though the effective speed and apparently the HIE designation of the
RL>b&w material have not).  One illustration show some books between
RL>bookends that were photographed on HIE in total darkness using only
RL>the radiation from a pair of 750-watt flatirons.  The irons were 18
RL>inches from the books and exposure time at f/5.6 was 10 minutes.
RL>
RL>Presumably the irons put out not only heat, but some invisible IR
RL>radiation at shorter wavelengths as well; in addition, the film may
RL>be--as I've always assumed it was--somewhat sensitive to the longer
RL>heat wavelengths.  If the latter is true, however, it raises a
RL>question implied by one comment of the previous poster: If heat alone
RL>can expose the HIE emulsion, wouldn't you have to keep the camera
RL>refrigerated to avoid fogging the film?  Perhaps the answer is that
RL>the exposure threshold with the longer (heat) wavelengths is high
RL>enough that it takes a couple of 750-watt irons nearby to do any
RL>appreciable fogging.

You are confusing two things here:

1) *recording* heat on film is not the same as *spoiling* it with heat!  
You can do the latter with *any* film, it's just that HIE is more  
sensitive to it (as seems Ektachrome IR even more, considering its  
recommended storage of -18 to -23 degree Celsius; note that both HIE and  
EI share exactly the same spectral sensitivity!) [some of this might be 
related to maintaining the exact color balance of IE, a non-existing 
problem for HIE]

2) not the entire spectrum beyond the visible is heat, nor is everything  
below the heat spectrum visible! In spectral descriptions, one  
distinguishes *near* IR and *far* IR, only the latter being referred to as  
heat.

As for explaining the flatirons experiment: keep in mind that those things  
can come very close to glowing red.
Beyond that, if we consider such a flatiron as a perfect 'black body' with  
a given color temperature, you can calculate the energy it radiates in  
each specific wavelenght. The 'color temperature' only tells you at what  
wavelenght it radiates *most*. If you plot this energy/wavelenght  
correlation, you see a curve that travels long distances below and beyond  
this maxium energy wavelenght.
If you take a spectral energy chart of an ordinary light bulb, you might  
even find that its (relative) long wave spectrum overlaps with the  
(relative) short wave spectrum of your flatiron!

BTW light bulbs: since their color temperature is lower than daylight,  
they emit relativily more energy in the near IR!
(50% is visible, 50% is IR!)
This also reflects in the exposure recommendations Kodak gives for use  
with candescent light, you have to underexpose compared to daylight  
readings of you meter.




RL>
RL>But I also feel sure I've seen heat-loss studies made by
RL>photoghraphing the outside of a house on a winter night through a
RL>Wratten 87 (IR-only) filter to exclude any visible light.  Again, the
RL>exposure times may have been extremely long.

That is simply impossible. The spectral emission of a house can never come  
close to the spectral sensitivity of IR film.
You need specially cooled cameras to distinguish heat.



RL>Incidentally, an even older Kodak book on
RL>_Infrared_and_Ultraviolet_Photography_ gives no sensiometric data for
RL>HIE, but it does show a spectrum for the older (and slower) IR.  Its
RL>short-wavelength (blue) band begins around 38 Angstroms, reaches a
RL>maximum near 46, and effectively ends at 51.  The long-wavelength
RL>sensitivity band begins at about 67 in the red, reaches a broad
RL>maximum around 80-84, and falls off sharply by about 87.  (Add a zero
RL>to each of the above figures to convert to nanometers.)  If I remember
RL>the previous post, heat is at yet longer wavelengths (around 100 A?),
RL>so one could not expect significant exposture from heat--at least not
RL>short-term.

Near IR is not heat, far IR is.
Can't find a distinction between those in my books, but IR-A ranges from  
780-1400nm, IR-B from 1400-3000nm, and IR-C from 3000-10000nm.
Your skin (at 37C) has its maxium energy output at 9500nm....
A spectral emission chart of a 525C object shows a maxium output at  
4000nm, a long wave limit of 1mm (radar spectrum!), and a 'short' wave  
limit of 750nm. This is already visible. Going up in temperature makes it  
even more visible, going down will make it ultimately (at 250C) dissapear  
from the film spectrum (the 'short' wave limit goes up!)

FWIW, a grill has its maxium output at 2500-3000nm, light bulbs at 1000- 
1200nm.

Hope this helps more than it confuses.

Note that my best literature is of German origin, technically much better  
than the Kodak publications (for those interested, the author is Guenter  
Spitzing, one of his older editons has been translated to Dutch).
You can also verify the flatiron experiment with a real time trick:
night vision scopes are also IR sensitive, if you add a dark IR filter to  
block the visible spectrum, you can get a 1:1 simulation of the result as  
it would appear on film. I take a cheap and light weight scope with me all  
the time, to find the better compositions, and avoid the dull ones.

See my homepage below for a lot of IR tips and tricks, and a pointer to  
the IR FAQ.


bye,

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