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Re: [OM] infrared metering

Subject: Re: [OM] infrared metering
From: HI100@xxxxxxx
Date: Tue, 2 Nov 1999 03:00:05 EST
wright@xxxxxxxxxxxx writes:

<< ok, I hope someone can help me. Does the olympus metering system respond to
 infrared light? Can I do auto exposure on my OM-2n with the deep red filter
 on there and get an acurate result?  >>

Dirk,
      I can't answer this question directly but ...   

The OM uses a so called "silcon blue" photo cell (diode). These are sometimes 
reffered to as "blue enhanced" etc. A normal silicon photodiode has a 
spectral response that starts at the UV end and peaks out in the infra-red 
wavelengths at around 900nM or so. The normal (human ) visible spectrum is 
very approximately in the 400-700nM range  with a peak in the middle in the 
green range. This approximates the spectral peak of the sun reaching the 
earth's surface. Normal film has a spectral response that is designed to 
approximate the visible spectrum also although there is sensitivity further 
out. So to measure the light correctly the photo-diode is "enhanced" to 
reduce the infra-red sensitivity substantially so that it matches the film. 
They fundamentally add a filter to cut the infra-red end and they can also 
increase the sensitivity a bit at the blue end by not reverse biasing the 
diode in the circuit and by making the optical conductive layer something 
like a quarter wavelength in the middle of the range or at the blue end (this 
is like lens coating). Sometimes "blue enhanced" when used in 
non-photographic context refers to devices without the filter to cut the 
infra-red but in which the blue sensitivity is just a little better because 
of  the later features.
    So I would expect the sensitivity of the photocell to be low in the 
infrared region. If the infra-red film is mainly sensitive just outside the 
visible spectrum you may well be able to use the camera on auto.  (This is 
because the infra-red cell filter may not drop off very rapidly. )  Obviously 
the ASA would need to be determined by trial and error the first time and the 
accuracy would be lower as the filter drops off. (Also light from the 
eyepiece may cause errors, since it leaks past the mirror, so you might need 
to tape the eyepiece)   

Interestingly the Cds photocells of the manual meter circuit inherently have 
the right spectral response so do not need to be "blue enhanced" for normal 
photography use. 

Tim Hughes
Hi100@xxxxxxx

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