Re: Some points of ponder

Hi,

what about modifying a normal light meter for reading **UV** levels, does this sound possible. Or is there any other simple way to determine UV content of sunlight on a given day?

I ask because I print with the sun, sometimes shining from clear blue sky, sometimes through light cloud cover, at different times of day, etc ...

I've tried to use normal exposure meter, but of course it doesn't tell much about UV. Surely I use test strips, and with cyano and vandyke it's quite easy to judge the exposure from printing-out image, but it would be nice (and time saving) to measure the exposure times ;-)

regards
Jalo

Ryuji Suzuki kirjoitti 15.07.2004 kello 18:13:
> There are two issues here.
>
> Usual panchromatic b&w materials are typically sensitized to 650nm or
> so and doesn't even cover the longer end of red. By using IR filter of
> some sort, you are removing most of the visible spectrum and giving
> exposure in the wavelengths that the film is insensitive to. In order
> to get decent IR effects, you should use IR films that use sensitizing
> dyes that are suitable for 750-850nm range. You can buy 4x5 sheets
> made by Maco, or dye films yourself.
>
> Usual light meters are designed to approximate sensitivity of human
> eyes (by approximating CIE curve, etc.). Readings from such meters
> have nothing to do with the amount of infrared luminance/irradiation
> that you should be interested in. Relative amount of infrared to
> visible light varies a lot, but some photographers have empirical
> factors to estimate infrared exposure based on usual light meters.
> Such factors depend on time of day, time of year, weather, etc. and
> can't be generalized too much. Indeed, such factors depend on the
> particular light meter used. One way to overcome this problem is to
> build or modify a light meter to make it sensitive to IR but not to
> visible light and do some crude calibration. Such a meter will be
> valuable if you are a dedicated IR shooter, especially in LF.
>
> Alternatively, if you have some electrical engineering background, you
> can make your own meter. Bare silicon photodiodes have maximum
> sensitivity to infrared region (because of the property of silicon)
> and with a simple IR filter, such a sensor is ideal for IR exposure
> meter. One problem is very wide range of dynamic range is necessary to
> make a usable meter. When I checked a few years ago, TI, NS and a
> couple of other companies made IC's that incorporated a Si photodiode,
> a preamp of programmable gain, an I/V converter and a V/F converter,
> outputting the light intensity in terms of frequency.
>
> A simpler approach may be to buy a meter like Sekonic L-208 and remove
> the sensitivity compensating filter and glue a piece of IR filter cut
> to fit and recalibrate. I have dissected this meter somewhat, and took
> pictures inside. It's a very simple construction and I don't think
> much electrical engineering knowledge is necessary to modify this
> meter. (but needs some dexterity of course) Maybe same thing can be
> done with Digisix but I don't have that meter. (For this approach, one
> must begin with a straight silicon photodiode based meter, not CdS,
> selenium, GaAs, PIN, etc.)
>
> --
> Ryuji Suzuki
> "You have to realize that junk is not the problem in and of itself.
> Junk is the symptom, not the problem."
> (Bob Dylan 1971; source: No Direction Home by Robert Shelton)

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Received on Thu Jul 15 13:24:22 2004