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RE: IR talk, Now nano meters to color.

On Fri, 31 Mar 2000, ken wrote:

> IR talkFor those who care I thought I would post colors vs nano meters and
> silicon sensitivity info so that you may be able to use a light meter for IR
> metering, once any filtering is removed.
> 
> 632 nm    =Red
> 615 nm    =Amber
> 604 nm    =Orange
> 587 nm    =Yellow
> 525 nm    = Green
> 479 nm    = Blue
> 
> Silicon detector have a sensitivity of 350 nm to 1100 nm with the peak
> sensitivity at 500 nm. At 900nm, the emulsion sensitivity of Kodak IR film,
> the sensitivity is down by 50 % as compared to the peak.  So If you remove
> any filtering your light meter may have and place the same filter over the
> meter that you have over your lens, it might work out that you need to just
> open one stop to get correct exposure.

Well ... it usually isn't quite that simple.  Once you have modified the
meter, the calibration is no longer valid.  But you are thinking in the
right general direction.  

> More interestingly you can get IR LED's that emit at about 950 nm.  By
> clustering these together you can make a bright IR lamp. This would be
> similar to the LED traffic lights that are now being used except there would
> be zero visible light but the Kodak emulsion could see it.

Sure, but the current draw would be pretty high.  That translates into a
big battery supply.

For most IR photographers, simply taping a couple pieces of unexposed, but
developed pieces of E6 film over an existing flash would give them a
useable IR flash.

Some other corrections:

   - Silicon isn't a metal, but rather is a transition element.  In its
     pure crystaline state it is a shiny grey crystal.  It is opaque to
     visible light, but transparent in the IR region.  Silicon has been
     used to make lenses for imaging IR.

   - Silicon alone is not really light sensitive.  The light sensitivity
     comes from the doping of adjacent regions of the crystal to create
     lattice defects.  If you have a region doped with an atom with too
     many electrons to match the crystal lattice you get n-type material.
     Dopants with too few electrons give you p-type material.  The
     junction between a region of n-type and p-type material is called,
     not surprisingly, a pn-junction, and this is the structure of a
     diode. You create a transitor by adding another pn-junction.  Light
     incident on such a structure changes the generation and recombination
     rate of the electons and modifies the energy gap accross the
     pn-junction, essentially making it more or less conductive.

- Wayde
  (wallen@boulder.nist.gov)