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How are digital cameras able to render violet?

 
 
Paul Ciszek
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      02-13-2013

In article <(E-Mail Removed)>,
RichA <(E-Mail Removed)> wrote:
>
>Violet isn't the major problem colour with sensors, green is.


I don't seem to have a problem with it:
http://www.flickr.com/photos/35853148@N05/5929247242/


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Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

 
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Paul Ciszek
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      02-13-2013

In article <kfe8b3$sh6$(E-Mail Removed)>,
Me <(E-Mail Removed)> wrote:
>>

>The red filter in the Bayer array probably passes some violet light.
>Here are some transmission charts (for photographic filters).
>This red filters shows a (second) "transmission peak" at the violet end
>of the visible spectrum. Perhaps - more correctly - the chart should be
>inverted so you look at it as an absorption peak rather than
>transmission peak.
> >

>http://www.schneiderkreuznach.com/pd...ion_curves.pdf
>(see page 2)


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--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

 
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Paul Ciszek
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      02-13-2013

In article <(E-Mail Removed)>,
Alfred Molon <(E-Mail Removed)> wrote:
>In article <(E-Mail Removed)>, Chris Malcolm says...
>> In part it's an artefact of a three primary colour space,

>
>Would the situation improve with four primary colours? Or more?


I seem to remember reading that someon tried a red-yellow-green-blue
mosaic filter, and it didn't help any. I'm not sure what you would
actually do with the extra information. I suppose you could eventually
work out improved versions of the different white balance modes, or the
phony color filters that some cameras and post-processing programs are
able to apply to RAW data.

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

 
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David Hare-Scott
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      02-13-2013
Alfred Molon wrote:
> In article <(E-Mail Removed)>, Chris Malcolm says...
>> In part it's an artefact of a three primary colour space,

>
> Would the situation improve with four primary colours? Or more?


Some women are tetrachromats, they have a rare mutation which gives them two
copies of the 'red' receptor with different sensitivity peaks. IIRC it
gives them slightly better discrimination in that end of the spectrum. The
interesting thing is that the brain integrates this into a coherent picture
of the world with no apparent effort.

David

 
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James Silverton
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      02-13-2013
On 2/13/2013 1:27 PM, Alfred Molon wrote:
> In article <afde856a-b41a-4960-9ebb-
> http://www.velocityreviews.com/forums/(E-Mail Removed)>, RichA says...
>> Violet isn't the major problem colour with sensors, green is.

> Why?

I heard Edwin Land lecture on his two color theory and was totally
convinced by his demonstrations. About the most spectacular was when he
used the two sodium D-lines (both yellow) to reconstruct color. I don't
know how Land's method could be used for color printing but it's a pity
that no-one has tried recently. Land, of course, used two projectors,
one for low and one for high wavelengths.

--
Jim Silverton (Potomac, MD)

Extraneous "not" in Reply To.

 
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Robert Coe
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      02-14-2013
On Tue, 12 Feb 2013 11:12:48 +0000 (UTC), (E-Mail Removed) (Paul Ciszek)
wrote:
:
: In article <(E-Mail Removed)>,
: RichA <(E-Mail Removed)> wrote:
: >
: >Human vision extends into the violet (about 1% efficiency) and can see
: >it. CMOS cameras are about 50% efficient in the violet and probably
: >renders it by combining red and blue filter elements.
:
: Being sensetive to a wavelength of light, and being able to distinguish it
: as a color are two different things. B&W film responds to many colors of
: light, but doesn't differentiate them. Color film used to respond to UV,
: but it would just show up as blue. IR response was weird.
:
: In order for a digital camera to render violet light as purplish, as in
: the rainbow picture I linked to, both the red pixels and the blue pixels
: would have to be responding to violet light. I understand why this
: happens in the human eye--as I explained, it's because we don't have a
: separate "red" detector as such--but in order for the bayer filter of a
: color digital camera to render violet as purple rather than blue, the
: "red" filters would have to pass the long wavelengths, block the green
: and blue wavelengths, and pass again at the very shortest wavelengths.
: This is not easy to do, especially if your filter has to be deposited
: onto a semiconductor surface rather than mixed up in a vat by Schott
: Glass.
:
: >There is no such thing as a reddish-blue colour, at least not
: >practically, since you cannot resolve the photons of red and blue
: >reflections so violet is what you see.
:
: There most certainly is such a thing as "reddish blue". Any purple you
: see printed, rendered on a monitor, or projected on a movie screen
: contains both red and blue light. Violet light exists in nature and
: can be produced by some light sources, but any form of media fakes it
: using purple--aka "reddish blue"--instead. Ironically, so do violets--
: they are purple rather than truely violet.

My wildass guess is that some radiation in the near-ultraviolet is perceived
in the human eye (and maybe even more so in digital sensors) as some higher
harmonic of red. This might not be inconsistent with the effect that others in
this thread have reported, viz. ultraviolet flower petals being captured as
pink.

Bob
 
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Me
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      02-14-2013
On 14/02/2013 10:39 a.m., Paul Ciszek wrote:
> In article <kfe8b3$sh6$(E-Mail Removed)>,
> Me <(E-Mail Removed)> wrote:
>>>

>> The red filter in the Bayer array probably passes some violet light.
>> Here are some transmission charts (for photographic filters).
>> This red filters shows a (second) "transmission peak" at the violet end
>> of the visible spectrum. Perhaps - more correctly - the chart should be
>> inverted so you look at it as an absorption peak rather than
>> transmission peak.
>>>

>> http://www.schneiderkreuznach.com/pd...ion_curves.pdf
>> (see page 2)

>
> "Die von Ihnen gesuchte Seite existiert leider nicht oder der Link ist
> veraltet. Bitte verwenden Sie die Suche oder kehren Sie zur Startseite
> zurück, um die gewünschten Informationen zu finden."
>

Hmmm - it was there yesterday...
Cached on my PC - here's a screen capture of the red filter chart:
> http://oi46.tinypic.com/23w4mll.jpg

 
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Paul Ciszek
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      02-14-2013

In article <kfhn67$irl$(E-Mail Removed)>,
Me <(E-Mail Removed)> wrote:
>>

>Hmmm - it was there yesterday...
>Cached on my PC - here's a screen capture of the red filter chart:
> > http://oi46.tinypic.com/23w4mll.jpg


The secondary peak at 500nm actually lies between green and blue. That
must screw things up a bit. The response does start ramping up again
just as the graph quits at 400nm, but that contribution is weaker than
the blue-green peak at 500nm. I would think that the false response from
blue-green would dominate the "purple" rendering of anything using this
filter--Anything blue is going to be considered kind of purple. And if
you try to correct for that, I think you will end up also eliminating the
weaker genuine response to violet.

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

 
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Me
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Posts: n/a
 
      02-14-2013
On 15/02/2013 8:54 a.m., Paul Ciszek wrote:
> In article <kfhn67$irl$(E-Mail Removed)>,
> Me <(E-Mail Removed)> wrote:
>>>

>> Hmmm - it was there yesterday...
>> Cached on my PC - here's a screen capture of the red filter chart:
>>> http://oi46.tinypic.com/23w4mll.jpg

>
> The secondary peak at 500nm actually lies between green and blue. That
> must screw things up a bit. The response does start ramping up again
> just as the graph quits at 400nm, but that contribution is weaker than
> the blue-green peak at 500nm. I would think that the false response from
> blue-green would dominate the "purple" rendering of anything using this
> filter--Anything blue is going to be considered kind of purple. And if
> you try to correct for that, I think you will end up also eliminating the
> weaker genuine response to violet.
>

We don't know what the absorption spectrum for the red dyestuff used in
Bayer RGB CFA is.(these are just red colour filters) If you can find
that out, then perhaps the answer will be obvious.
 
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Robert Coe
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      02-15-2013
On Wed, 13 Feb 2013 17:29:10 -0500, James Silverton
<(E-Mail Removed)> wrote:
: On 2/13/2013 1:27 PM, Alfred Molon wrote:
: > In article <afde856a-b41a-4960-9ebb-
: > (E-Mail Removed)>, RichA says...
: >> Violet isn't the major problem colour with sensors, green is.
: > Why?
: I heard Edwin Land lecture on his two color theory and was totally
: convinced by his demonstrations. About the most spectacular was when he
: used the two sodium D-lines (both yellow) to reconstruct color. I don't
: know how Land's method could be used for color printing but it's a pity
: that no-one has tried recently. Land, of course, used two projectors,
: one for low and one for high wavelengths.

My brother and I did a demo of the Land effect in my grandfather's dining room
50 years ago. It was pretty convincing, but it clearly (I think) depended on
the workings of the brain as much as on those of the eyes.

Bob
 
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