Re: IR Exposure meter

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 21 Jul 96 at 19:10, kenneth.r.rockwell@xxxxxxxxxx wrote:

> -------------
> Original Text
> >From tom@xxxxxxxxxxxxxxxxxxxxxx (Tom Benedict), on 7/19/96 3:25 PM:
> To: <kenneth.r.rockwell@xxxxxxxxxxxxxxxx>
> 
> So has anyone gone out and made an IR light meter?  Most silicon photo
> diodes that have been optimized for IR performance usually cut off
> just past 9500A anyway, so it seems like it would be pretty well suited
> for most of the IR films.  So long as you metered through the same filter
> that's on the camera, and calibrated the meter through careful bracketing,
> it seems like it would save on film in the long-run.
> 
> Tom
> Tom and all - 
>                  I was just thinking about an IR light meter just last 
> night.  Designing and making one would be as simple as designing any other 
> light meter.  Since IR is such a teeny market no has bothered to introduce 
> one comercially.
> 
> Even better, some conventional light meters based on silicon photo diodes 
> (which are more sensitive to IR than visible light) actually have IR CUT 
> filters in them to optimize them to conventional film.  A real great thing 
> is for someone to find a meter like that and pull out that filter!
> 
> Almost all CCD imagers for video have IR cut filters for exactly that 
> reason; the colors would really look bad if that filter wasn't there.  That 
> is the first part to crack if a video camera is left pointed at the sun to 
> long, obviously.

Yep, and you can even find this kind of filter in the regular B+W 
program; it has a slight green tint.
 
> Has anybody tried the obvious of taping an 87 filter over a conventional 
> meter and changing the ASA accordingly?

Well, I have a Gossen Sixtomat Digital, one of the smallest digitals 
around, and surprise surprise: it shows exactly the right corrections 
when metered through any kind of IR filter. Red shows neatly +3 (unlike 
many camera meters), darker stuff is also right on spec. 
For those knowing this Gossen meter: it has a sliding dome, to change 
from incident to reflective metering. The dome can slide off the cell 
towards both sides....so adding a square piece of filter on the other 
side is possible. The only problem is that I can't find any screws on 
the housing to open it and slide the square piece inbetween. But that 
has never stopped me before with other projects....;-))
I have also been pondering about adding an IR filter under the dome, 
but I doubt whether this incident metering results in a reliable reading, 
since IR reflection is most often the big variable in the calculation....a 
reflective reading seems most appropriate.
 
 
--
Bye,

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The difference 
between men and boys
is the price of their toys

<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 25