From: Burt Kessler (bcomet@pacbell.net)
Date: Mon Jul 14 2003 - 14:59:36 PDT
Date: Mon, 14 Jul 2003 14:59:36 -0700 Subject: Re: pinhole light and pigments From: Burt Kessler <bcomet@pacbell.net> Message-Id: <758A25A9-B646-11D7-8336-000A27B74084@pacbell.net>
On Friday, July 11, 2003, at 03:55 PM, Mark Lawton wrote:
>
> 1) I assume that when two pigments mix they do not form a new
> compound. For example, blue paint and yellow paint will create green
> paint. The mixture (which is not a compound) still reflects blue
> light and yellow light and our eyes interpret these two frequencies as
> green. Is that right?
>
Color perception is more complicated than that. Our eyes have 3 types
of cones that respond to red, green, and blue light respectively.
These are the 3 additive primary colors of light and are used in
monitors. In this system, blue and yellow are complimentary colors.
Blue light and yellow light combined will create white light. This is
because yellow is perceived when both red and green receptors are
stimulated. Therefore combining blue and yellow will stimulate all 3
types of cones the same as full spectrum white light.
Pigments come in 2 varieties, opaque and transparent. These combine
differently. Opaque pigments tend to absorb all colors except the one
they reflect. These are the ones we learn about in kindergarden, red,
yellow, and blue are the opaque primaries. Blue and yellow will make
green if mixed ( see Paul's answer about the cyan nature of that blue),
but if layered, only the top layer will be seen.
Transparent pigments are the ones used in printing. The primaries in
this system are cyan, magenta, and yellow (these colors are the
opposites of the RGB primaries in the color wheel). These are
so-called subtractive primaries. Transparent pigments tend to absorb
one color and reflect or transmit all the others. A yellow pigment
will absorb blue light and allow red and green through. Cyan absorbs
red and allows green and blue through. Combine or layer cyan and
yellow and only the green is not absorbed and is therefore visible.
> 2) Assuming the above description is correct: When I see a green leaf
> in nature am I seeing two frequencies at once (blue and yellow) or is
> the leaf reflecting a single frequency?
> The different chlorophyll pigments all tend to have absorption minima
> around the green part of the spectrum. They absorb most colors of
> light and reflect or transmit green.
> The different chlorophyll pigments all tend to have absorption minima
> around the green part of the spectrum. They absorb most colors of
> light and reflect or transmit green.
>
> 3) An acquantaince of mine is talking a lot about complimentary colors
> and the color wheel. What is the a physical basis of the fact that
> two color compliments will make "neutral grey"?
It sound like your friend is combining opaque pigments. If the pigments
were 100% effective at absorbing all colors but the one they reflect,
combining any 2 opposites would produce black. The grey is simply less
than 100% black.
>
> Burt Kessler
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