Posted by PTravel on 07/03/06 18:24

"David McCall" <david--------@techshop.net> wrote in message
news:2saqg.3042$b9.2152@trndny01...
>
> "PTravel" <ptravel@travelersvideo.com> wrote in message
> news:Pl_pg.114414$H71.48097@newssvr13.news.prodigy.com...
>>
>> Sodium lights contain impurities, so the output isn't a perfect 589 nm --
>> otherwise you could use it as a laser. If you put the output through a
>> spectrum analyzer, you'd see a sharp spike at 589nm, but there'd be
>> fall-off on either side of it, i.e. though most of the light output is at
>> 589nm, there may be light output as much as 10nm (or whatever) on either
>> side of it.
>>
> Have you seen a spectrum anaylisis that shows this "falloff"?

No, I haven't. I'd suspect the fall-off is fairly steep since the light is
produced by atomic excitation which, I'd think, would make it easier to
isolate spectra from the impurities.

>
> It's been a long time since I've seen one, but I seem to remember
> a chart with a a tiny bit of noise at the baseline, and the spikes
> were just thin vertical lines. They once used sodium vapor lights
> to light the background for process shots. It seems strange because
> the orange spike of the lights is so close to the colors in flesh tones,
> but they were still able to pull a matte. It is quite "pure".

Hmmm. The question, though, is how much "impure" light would be needed to
create false-color effect?

>
> Flourescent lights also have these extremely narrow spikes, but the
> phosphers added to the inside of the tube produce a glow with a
> wide enough spectrum to allow you to see colors. Depending on
> the phosphers used, the color rendering quality will be of better
> on some lamps than others (usually tracks with the price of the bulb :-).

My idea may not work at all. One thing I've noticed in my travels, though,
is that different cities that I've shot apparently use different "flavors"
of Na lamps, and some have somewhat broader spectra than others. For
example, the lamps in Copenhagen shoot white, as opposed to the distinctly
orange lights in the Amsterdam video. Though the Copenhagen light still
puts out limited spectra, and everything has a distinctive red/amber tint,
other colors, e.g. green, are discernible.


>
> David
>

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