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- Newsgroups: sci.physics
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- From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
- Subject: Re: Simple Question on Color
- Message-ID: <1992Nov7.003729.24180@CSD-NewsHost.Stanford.EDU>
- Keywords: color, tv, violet
- Sender: news@CSD-NewsHost.Stanford.EDU
- Organization: Computer Science Department, Stanford University.
- References: <6078@tuegate.tue.nl> <31OCT199214124620@erich.triumf.ca> <1dcbcuINNj5s@smaug.West.Sun.COM>
- Date: Sat, 7 Nov 1992 00:37:29 GMT
- Lines: 62
-
- In article <1dcbcuINNj5s@smaug.West.Sun.COM> Richard.Mathews@West.Sun.COM (Richard M. Mathews) writes:
- >music@erich.triumf.ca (FRED W. BACH) writes:
- >> Yes, and speaking of color tv, since my favorite color family is
- >> the violets, it always bugs me when true violets (not blue+red phony
- >> attempts at it) are picked up by a color-tv camera. Now I don't
- >> know what *every* camera out there does, but some will simply *not*
- >> pick up this violet and display it. All you get is a pure bright
- >> blue. Is this because the true wavelength of the violet is outside
- >> (i.e., not between) the spectral responses of the phosphors used?
- >
- >I would think that you get a pure blue because the camera's red detector
- >does not have the same small peak in the violet that our eye's red cones
- >do. Only the camera's blue detector responds to the violet light, so the
- >camera produces a blue image.
-
- Is your reasoning here that the eye distinguishes violet from blue by
- the red-sensitive cones responding more strongly to violet than blue?
- I don't think this reasoning is correct. The eye's blue-sensitive
- cones have their main peak at 430 nm, well into the violet. The eye
- recognizes the transition from violet at 430 nm to blue at 480 nm by
- the red- and green-sensitive cones responding to blue *more* strongly
- than to violet, and by the blue-sensitive cones responding less
- strongly (it's downhill from their main peak at 430).
-
- The main peak of green is at 530 nm, and red at 560. At 430 nm the red
- output is a mere 10% of what it is at its 560 peak.
-
- There is indeed a violet peak, called the cis peak (I think Cis is
- German for C sharp, suggestive of a tweak), present in all three cone
- types. This gives all three a small violet boost, amounting to more of
- a tweak than a real peak that just attenuates the roll-off. I believe
- its role is not yet understood.
-
- The retina filters out UV. UV ordinarily looks like faint violet, but
- that's real violet you're seeing. People who have had their retina
- removed perceive UV not as violet but as blue. Not much is known yet
- about UV sensitivity of individual cones so we can only guess why, but
- plausibly it is due to the blue response decreasing more than the red
- and green responses for UV rather than some unexpected peak (F sharp?)
- in the latter responses.
-
- These numbers are based on measurements done at Stanford, in Denis
- Baylor's lab, of spectral sensitivity of single cones, which are
- micropipetted up and their electrical sensitivity measured. (No I
- didn't volunteer :-) The above numbers are the most recent from
- Baylor's lab. Wyszecki and Stiles "Color Science" is the "bible" in
- more than one sense: the 2nd edition (1982) reports (p.93) that visual
- pigments have only been extracted from rods, although of that date
- there was no shortage of indirect evidence for their presence in cones
- from work done in the 1970's. Extraction of cone pigments is a
- relatively new, tricky, and exciting business.
-
- >Without a TV which includes a violet phosphor, there is no way that any
- >camera can make your TV show violet.
-
- Yes, indeed. See my post of last week describing the gamut of a color
- CRT as a triangle in the interior of the CIE chromaticity diagram.
- Anyone care to hold forth on the main obstacles to a good violet
- phosphor for color CRT's? Seems like a great opportunity to make a
- buck or two!
- --
- Vaughan Pratt There's no truth in logic, son.
-