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Posted by PTravel on 07/03/06 01:25
"Ed Anson" <EdAnson@comcast.net> wrote in message
news:k9OdnUCbQaK70zXZnZ2dnUVZ_sudnZ2d@comcast.com...
> PTravel wrote:
>> I was thinking more in line with the video equivalent of an audio
>> compander -- something that can take the limited range that's available
>> and expand it. For example, if Na lights put out light limited to
>> 240-245 angstroms (I'm just making up numbers here), something that would
>> extrapolate on the video so that the lower range fell closer to red and
>> the upper range closer to blue would provide a more varied, albeit false,
>> coloration.
>
> You appear to be working against physics.
>
> According to Wikipedea (http://en.wikipedia.org/wiki/Sodium_vapor) "These
> lamps produce a virtually monochromatic light in the 589 nm wavelength. As
> a result, objects have no color rendition under a LPS light and are seen
> only by their reflection of the 589 nm light."
>
> Filters work by removing light at unwanted wavelengths. Given a
> monochromatic light source, no filter can do anything but reduce the
> intensity of what is there. Even if the light were simply a narrow
> spectrum, you would need special sensors to do what you suggest. You
> certainly couldn't do it with a normal camcorder.
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.
When I used the term "filter" I didn't mean an optical filter, but a
post-processing filter that would expand the bandwidth so as to create a
false-color effect.
>
> I'd be inclined to go with the monochromatic effect. You can replace the
> sodium color with any tone you want, and produce a pleasing image. But
> there's no way you can get a polychromatic image using monochromatic
> light.
Pure monochromatic light, no. Almost pure, yes.
>
>>
>> Hmmm. Have I just invented a new filter?
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