Re: IR light meter

[Stan McQueen <stan@xxxxxxxxxxx>]

At 12:31 AM 03/01/2000 +0100, you wrote:
>On 26 Feb 00 at 11:06, Foamb wrote:
>
> > Has anyone tried this? I'm thinking of using a Gossen Luna Pro digital
> > and trying it out unless someone has experience with the disassembly of
> > meters and can recommend a better choice. The pentax digital spot was
> > mentioned but required considerable effort to disassemble and modify,
> > not to mention that they aren't exactly cheap.
>
>I believe this one is named Sixtomat Digital (F) in Europe, and it is
>exactly what I did to mine....cut a square piece of #87C filter, popped it
>into the sliding rail that also holds the incident dome, and it
>now neatly reads the correct ASA values for HIE whenever I want it.
>
>The only boring thing is that on a bright sunny day, it *always*
>reads 1/60s @ f8....so I hardly use the meter anymore, while running
>brackets of 1/60s @ f5.6-f11....8-))
>--
>Bye,
>
>Willem-Jan Markerink

I tried this with my old, non-digital, Gossen Luna-Pro. On the low-range, 
you can indeed look into the meter and see the diode. On the high-range, 
there is a filter, presumably a neutral density filter, that slides over 
the opening. I slid the incident dome off to the side and covered the 
aperture with an 87C gel. On a bright sunny day I got nothing on the high 
range, but did get a meter reading (EV10) on the low range. I thought maybe 
I had something until I tried it again, replacing the filter with my thumb. 
I got the same reading! So either my thumb is equivalent to an 87C or what 
I am seeing is the dark current reading. I suspect the latter, especially 
since I get the same reading with the 87C pointed toward the sun as with 
the 87C pointed toward an incandescent lamp. Does anyone know what 
semiconductor is used in the photocells?

Here's some more info:
According to the Bogen site (www.bogenphoto.com/gossen_lunastar.htm), the 
Gossen Scout 3 and Pilot 2 use a Selenium photocell, the Luna-Pro S uses a 
Cadmium Sulfide cell, and the Luna-Pro Digital uses a Silicon photodiode.

The peak sensitivity for silicon is about 800 nm, whereas CdS is about 500 
nm. CdS also has a narrower response curve than Si, explaining why my an 
87C filter is about as good as my thumb when using this meter. I'm 
surprised that the Luna-Pro Digital doesn't have an IR-blocking filter on 
it; without one, it's readings would be heavily weighted toward the red end 
of the spectrum (relative response is above 60% between about 640 nm and 
960 nm). Anyway, it seems like this meter would be an ideal IR meter if 
suitably filtered. I haven't been able to find any spectral response data 
for Se. (Searching for selenium info is a pain, because it is also used as 
a dietary supplement, which leads to thousands of non-relevant matches. I 
think I would just as soon eat lead as selenium, but I guess there's no 
accounting for taste.) If it is sensitive to IR, that would be a better 
match for those of us who don't have the bucks for a Luna-Pro Digital. Does 
anybody have any spectral response info for selenium?

Stan

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