# Color theory question ....

Discussion in 'Photoshop Tutorials' started by Wdflannery, Jan 9, 2004.

1. ### WdflanneryGuest

This is something that puzzles me ..... I understand sound theory pretty
well... and we know sound is vibrating air ....for pure tones the air pressures
varies as a sine wave.... and, color is a vibrating electromagnetic field
......again with pure colors represented by sine waves .....

Here is what I don't understand ...how is it that you can, as I understand,
choose 3 primary colors..... and mix them to get any color at all ????? It
seems quite unlikely .... for example.... when three sinewaves of different
frequencies are added, the result is not a pure sinewave at a different
frequency .... suggesting to me that you can't generate the pure colors, much
less all the colors.

And, no one would suggest that you can take 3 pure (sound) tones and generate
all the pure tones, much less all the sounds ..... so, what is it about light
that is different from sound .. ?????

Is there any easy explanation ???? Or a link I could check, to relieve my
bafflement????

Wdflannery, Jan 9, 2004

2. ### James GiffordGuest

In very simple terms, the frequency of the light reaching the retina is
perceived as a specific color. It isn't that colors are somehow being
mixed in the waves, it's that the eye is perceiving a very narrow range
of EM energy as a color. Shift the frequency slightly, and the perceived
color changes. The human eye is capable of distinguishing tens of
thousands of different frequencies (colors).

The "primary colors" are something of an abstraction used by various
human endeavors to make synthesis of the full spectrum of colors simple.
Instead of having to have a phosphor that reacts precisely to every
wavelength (color) of light, you measure the RGB values of the light at
that point, and use the values to synthesize an equivalent color value.
At output, instead of having a phosphor that can produce any color in the
spectrum, you use the simpler technique of combining the RGB components
to produce what the eye perceives as a dot of a particular color.

The notion of light being sine waves like sound is not quite accurate.
Remember that photons show the characteristics of both waves and
particles - and I'm not sure it's been established exactly how the retina
responds to these varying frequencies and perceives them with such fine
differentiation.

James Gifford, Jan 9, 2004

3. ### J CGuest

There is no one-to-one correspondence between sound and light. They
are not the same thing. Do not even consider them together. Sound at
its basic unit is simply particles of matter coliding against each
other. When we hear a violin, we are perceiving a rhythmic pattern of
the air hitting our ear drum. Sound cannot move through a vacuum.
Light can. Light is an actual physical entity (albeit one that is
still not fully understood by even top physicists -- read what string
theorists have to say about light if you really want to get freaked
out).

-- JC

J C, Jan 9, 2004
4. ### Warren SarleGuest

It's a matter of the neurophysiology of color perception. The retina
contains
three kinds of color receptors, which you can think of as outputting
something very roughly like an RGB color space. As the nerve impulses
travel towards the brain, they get transformed into something very roughly
like LAB space. Further on, many more complicated transformations are
performed that some people spend their whole lives trying to understand,
with limited success. But the fact that 3-dimensional color spaces like
RGB and LAB work is based on the the three kinds of color receptors.
There are other species with either more or fewer than three kinds of
color receptors; if they had invented printing and computers, they would
have used color spaces with more or fewer dimensions.

Sound receptors in the ear are totally different, since there are receptors
for many different wavelengths of sounds. Therefore, the perception of
sound cannot be reduced to a low-dimensional space.

Warren Sarle, Jan 9, 2004
5. ### Robert E. WilliamsGuest

Here is the way I see it.
A plucked string, for instance, causes a single frequency (say 1000 Hz) plus a few
overtones (2000, 3000.....N)at much reduced intensity. There are no intermediate
frequencies say, 1018 Hz or 1759 Hz. So basically we do no have a CONTINUOUS
spectrum of frequencies. Even if we pluck 3, 4, 5 strings we will get discrete
frequencies and their overtones, PLUS their sum and difference frequencies. But
still not a continuous spectrum of frequencies.
Perhaps if you plucked a thousand different strings simultaneously, you would get
enough fundamentals, overtones and sum / difference frequencies interacting that we
would get something akin to "WHITE NOISE"
With Light, however, when we refer to e.g., "RED" light, we mean we get a
CONTINUOUS spectrum of frequencies with a gaussian distribution from about
600-750 nm. "GREEN" may mean a similar continuous frequency distribution around
500-650 nm. Similarly "BLUE " means a continuous distribution around 400-550 nm.
So any color, say orange (610 nm) is present in what we call RED. It is also
present in what we call GREEN.
By mixing equal amounts of RGB we get a continuous spectrum that contains every
conceivable frequency in the Visible range. We call this "WHITE LIGHT". By mixing
various amounts of RGB we get different hues of color.
If we used equal amounts of pure MONOCHROMATIC laser light at say 680.00 nm
(Red), 525.00 nm (Green), and 470.00 nm (Blue), we would not get white, because the
combination does not contain ALL frequencies in the visible spectrum. I won't even
go into how the retina and the brain convert this information into the perception
of color.
Bob Williams

Robert E. Williams, Jan 9, 2004
6. ### Warren SarleGuest

This may or may not be the case. What we perceive as red light may have
a very narrow or a moderately broad range of frequencies.
If the intensities are balanced properly, it will in fact be perceived as
white.

Warren Sarle, Jan 9, 2004
7. ### XalinaiGuest

Other than acoustic waves that (mechanically) interfere with each
other in the transport medium lightwaves do not interact.

The human eye responds to certain frequencies of light and most of the
color impression is created in your brain as a result of amount and
intensity of only three major areas of frequence in the range of red,
green and blue.

That the frequencies are still available separately can be checked
using different laser colors (with a very narrow bandwidth) on one end
and a prisma on the other end - there is no modulation of frequencies
and demodulation.

Michael

Xalinai, Jan 9, 2004
8. ### Peter WollenbergGuest

It's quite simple, you may learn about it at:

HTH
Peter

Peter Wollenberg, Jan 9, 2004
9. ### Mike RussellGuest

Yes, there are many useful analogies between sound and light. Both are wave
energy phenomena, and both have the concept of mixed frequencies.
Your intuition is accurate. It's not possible to generate all the colors
from three primaries, but you can generate a good approximation, and let the
eye, which is very good at this, put the pieces together into something
reasonable.
The difference is between the eye and ear, more than between sound and
light. Where the eye merges three frequencies of light into a single color,
the ear will hear a chord when three frequencies are present

You can fool the eye into thinking a particular frequency is present by
using the correct ratio of red, green, and blue, as well as create new
colors between red and blue that don't exist in the spectrum. The ear is
smarter in this regard in that it takes all the frequencies apart instead of
lumping them together, so you hear chords.

This is because the eye uses just three overlapping detectors that are
sensitive to red, green, and blue. The ear uses an array of thousands of
hair cells, each tuned to a slightly different frequency, to extract more
information from the perceptual mix than the three detectors in the eye are
able to.

The ear works like a spectrophotometer and the eye is more like a
colorimeter.

Of course, the ear has an easier job, since it doesn't have to create and
interpret an acoustical image based on the sound reflecting from the objects
around you. Experiments with acoustic holograms have been done, and in the
natural world, bats and owls, do some of this.

You could conceivably build a smarter eye in the form of a digital camera
that gathers complete frequency information at each and every pixel - maybe
this will happen some day. For now the technology isn't there, and our
cameras use mixed arrays of sensors that are filtered for a relatively small
number of colors, very similar to the way the eye works.
Everything you need is right here in c.g.a.p

Take care.

Mike Russell, Jan 9, 2004
10. ### jjsGuest

Rather than trying to explain it, may I suggest a rather enlightening and
fun read? "Vision and Art: The Biology of Seeing" by Margaret Livingstone.
Note the "biology" aspect. We perceive color in terms that the physics
doesn't address in the pure abstract.

jjs, Jan 9, 2004
11. ### jjsGuest

Recent literature which covers the physiology of the eye does a very good
job of describing how the normal human eye perceives color. How the eye
works is astonishing, and to learn how it cannot see certain colors is
equally surprising.

jjs, Jan 9, 2004
12. ### jjsGuest

Keep in mind that the normal human eye sees _certain_ colors which exist at
near-opposite ends of the true spectrum as the same color.

jjs, Jan 9, 2004
13. ### jjsGuest

Which is irrelevant to how the eye works. The eye infers color using rather
fuzzy logic which depends upon a kind of interference.
Most of the color is made from signals in the eye. The brain cannot image
what the eye does not first render.
The human eye does not see all the colors in the so-called visible spectrum.
There are areas of less and greater sensitivity. You might say the eye is

jjs, Jan 9, 2004
14. ### BobsGuest

And some animals have color perception that exceeds our own. Birds of
prey, for example, have 4-color receptors instead of 3, thus adding a
whole dimension of color discrimination (compared to human vision).

Bobs, Jan 9, 2004
15. ### WdflanneryGuest

Amazing. Thanks to all. Great info.

Wdflannery, Jan 9, 2004