Archived posting to the Leica Users Group, 1999/09/23

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Subject: [Leica] Re: Low Pressure Sodium
From: "Michael D. Turner" <mike@lcl-imaging.com>
Date: Fri, 24 Sep 1999 02:00:17 +0000

Anthony Atkielski wrote:

> From: Mark Rabiner <mrabiner@concentric.net>
> Sent: Thursday, September 23, 1999 01:17
> Subject: Re: [Leica] Low Pressure Sodium
>
> > The most efficient form of lighting you can buy!
>
> The most efficient form readily available, perhaps.  I don't remember which type
> of lighting truly leads in the efficiency rate--I know that metal halide and
> ordinary fluorescent lights are right up there, but supposedly microwave sulfur
> lights are even more efficient (although they are really weird).
>
> The big problem with LPS is that it is truly monochromatic.  Tungsten is kind of
> orange, but still blackbody radiation, and with some tweaking you can adjust the
> color balance in a scan to make it look right, even with daylight film.
> However, in LPS illumination, there is literally nothing to adjust--you can't
> boost the blue channel because it's empty!  All you can do is convert to
> grayscale, or tint the entire photo a different color.  Really ugly if you
> originally wanted color.
>
>   -- Anthony

Yup. I once visited a semiconductor manufacturing facility which was using some
equipment I had designed in a cleanroom with sodium illumination. The color-coded
keys were useless--blue was black, yellow was white, white was white!

Similar situation with tungsten films shot in high K light or daylight films shot
in tungsten light sans filtration. You simply cannot correct what is not recorded
on the film.  Period. And if you scan and try to correct, you just get noise.
- -Mike