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Posted by Toby on 07/04/06 04:35
"PTravel" <ptravel@travelersvideo.com> wrote in message
news:h5dqg.115236$H71.67801@newssvr13.news.prodigy.com...
>
> "Toby" <zdftokyo@gool.com> wrote in message
> news:44a8ddcd$0$4873$bb4e3ad8@newscene.com...
>>
>> "PTRAVEL" <ptravel88-usenet@yahoo.com> wrote in message
>> news:4grm5rF1o20foU1@individual.net...
>>>
>>> "Bill Farnsworth" <bill.farnsworth@verizon.net> wrote in message
>>> news:7u0qg.168$0G2.59@trnddc07...
>>>>
>>>>> PTravel wrote:
>>>>>> Sodium vapor lights are the common night time illumination in most
>>>>>> cities in the world, and put out a distinctive yellow light. Because
>>>>>> they work by exciting sodium atoms, the light output is a very narrow
>>>>>> spectrum, i.e. it's not just a question of being warmer or cooler --
>>>>>> there simply are no other bandwidths present. Naturally this results
>>>>>> in anything being videotaped as appear, essentially, monochromatic
>>>>>> (for an example, see here:
>>>>>> http://www.travelersvideo.com/amsterdam%20at%20night.wmv)
>>>>>>
>>>>>> Is there any way around this to get, if not a more accurate color
>>>>>> rendition, at least a more interesting one? Short of bringing my own
>>>>>> lights, which is not feasible at all, I can't think of anything that
>>>>>> will add spectra that simply aren't there.
>>>>
>>>> Nice stuff Paul.
>>>> There really isn't much you can do with limited resources other than
>>>> some correction in post. (bringing a chip chart and shoot that under
>>>> the light for reference can help later in post)
>>>> However............
>>>> Get yourself a rosco cinegel color correction sample set. (1 1/4" x 3")
>>>> Then you can experiment with white balance through different correction
>>>> filters.
>>>> Or add it as correction filtration
>>>> I'm thinking that something in the Tough Plusgreen family and Tough WF
>>>> green either added at the lens or subtracted through white balance
>>>> might bring you a wee bit closer to what you are looking for.
>>>>
>>>>
>>>> Bill F.
>>>
>>> Thanks, Bill!
>>
>> I've looked at sodium discharge lights through a viewing spectroscope.
>> The two dominant D-lines around 589 nm constitute well over 95% of the
>> light emission, and that's being generous. They So you have a two-fold
>> problem. The first is notch-filtering at 589 nm to get rid of the source
>> that is going to blow everything else out of the water. Then you have the
>> residual couple of percent to boost up to some level where it does you
>> some good. Kind of impossible....
>
> I'm not sure it's impossible. Notching 589 nm is certainly possible --
> it's the remaining 5% that I'm interested in playing with.
Here are a couple of interesting references. First is your way to notch
589 --with Didynium glass. A mild version of this stuff is what is used to
make red enhancing filters.
http://www.nezumi.demon.co.uk/nonad/nonad.htm
Next have a look here at the actual spectrum that you are dealing with:
http://ioannis.virtualcomposer2000.com/spectroscope/amici.html#colorphotos
Your comparison with an audio compander is disingenuous, because you start
with full spectrum sound, which is compressed and then reexpanded. At best
you get about 200% out of a good compander before the artifacts get too
objectionable.
If you were talking about reconstituing something approaching the full
spectrum from a Wratten OC filter, which peaks at about the same wavelength
as the sodium lights, I could buy your analogy, because you have a full
spectrum to begin with, just very attenuated, but the sodium spectum is a
whole different kettle of fish. They may look more or less the same to the
eye, but they are not.
Consider this: the visible spectrum extends from about 400-700 nm. The
D-line doublet falls at 588.9950 and 589.5924. The next strongest line, at
568.8205 is only 0.7% as strong as the stronger D-line, and all other peaks
are a factor of 2 or weaker. So all told, without the D-lines, you cut the
visible emission of sodium down by about 98.5 percent. To make matters even
more fun, most of that is in very narrow peaks and doublets.
Not only do you have an extremely--and I mean extremely--limited continuous
spectrum to deal with, you have to deal with peaks and doublets all over the
place. It is not just a matter of expanding what's there; it's a matter of
smoothing it out as well. Not only that, the artifacts that you would get
from the extreme amplification or expansion would be monumentous. We are not
talking about 100% here. Even if you were only going to try to expand the
stronger line, assuming a bandwidth of .05 nm (and it is not continuous over
that range), you are talking about something in the range of 1,000,000%. You
don't think you're going to get some pretty nasty artifacts at those levels?
Just wanted to point out what you are dealing with here.
In audio that would be equivalent, more or less, to taking one notch from
7000-7100 Hz, another, half as strong, at 7200-7300, another from 6000-6100
or so, at less than one percent of the strongest, and a few others at half
that strength, and trying to reconstitute something from that.
Do you think it will be intelligible?
Toby
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