the way in which colour is seen

hi there it`s me again and i have another question to ask about the way
in which we see colour. As you well may know some people are colour
blind and so i was just wondering how they see colour and whats the
diffrence between wright and wrong.

bob wrote:

hi there it`s me again and i have another question to ask about the way
in which we see colour. As you well may know some people are colour
blind and so i was just wondering how they see colour and whats the
diffrence between wright and wrong.

Only one of them has a leading silent W.

Colour blind people typically have either red-green (most common) or
blue-yellow discrimination errors. One of the most famous tests being
the Ishihara colour blindness test - see for example:

http://www.toledo-bend.com/colorblind/Ishihara.html

Regards,
Martin Brown

"bob" wrote in message
ups.com...
hi there it`s me again and i have another question to ask about the way
in which we see colour. As you well may know some people are colour
blind and so i was just wondering how they see colour and whats the
diffrence between wright and wrong.
The human eye's vision, is based upon four sets of cells in the eye. One
set (the rods), is the most sensitive, but has a relatively 'broadband'
response centred on about 540nm. These also have the best response ot to
380nm at the 'blue' end of the spectrum. These are what are involved in
'night vision', giving no colour information. These are the commonest
sensor in the eye. The other three sets, have narrower responses, centred
on about 445, 535, and 575nm. These all have significant 'overlap'. These
are concentrated into the central part of the FOV in particular. The
nervous system behind the eye, processes these signals to give the
'colour' detail. Now if (for instance), you have a narrow band light at
560nm, this stimulates both the 'red', and 'green' cell responses, and we
percieve it as 'yellow'. However a red, and green light mixed together,
gives exactly the same response, yet has a completely different spectrum!.
This 'trick', is the basis of all normal colour 'reproduction' methods,
but shows that our 'perception' of colour, does not really represent what
is there...
The processing behind the eye, then plays a whole load more 'tricks'. The
worst is that the eye will adjust the responses it allocates to the
signals, based upon the brightest signal from each band, combined
together, being 'white'. If you walk into a shop with incandescent
lighting, and look at a piece of white paper, you see it as white, yet if
you go outside, and look in through the window (if you can see the paper
in the dim interior), it'll look quite red. Again you are not seeing the
real colour, but your brain is attempting to make good use of the signal
available to give a representation of colour, but the result is again that
you do not really see the colour that is actually 'there'. This is similar
to the 'white balance' control on a digital camera.
Even if two people look at an object, and agree that this is 'green', all
that is implied, is that both are getting the same signals triggered
inside their brains, that a 'green' part of the spectrum generates for
each of them. The actual perception does not have to be the same.
Colour 'blindness', covers various faults with both the sensing chemistry,
and the processing behind this. If (for instance), a person presented with
two cards, of matching brightness, with one coloured to just about match
the centre of the 'red' sensor, and the other coloured to match the centre
of the 'green' sensor, and cannot distinguish these, you have the classic
(most common) 'green - red' colour blindness. Failures in the 'blue'
system, are the rarest form. The commonest types of colour blindness are
genetic, and because of the way the gene combinations occur, these are
much more common in men (about 1 in 8 men, against 1 in 200+ women).
Failures do not have to be 'complete', with some people just lacking
sensitivity to a particular colour, but still being able to detect it.
If you are not colour blind, the following site, has example images,
showing how some forms affect what is 'seen' :
http://stuweb3.cmp.uea.ac.uk/laj-web...lindness.html#

Best Wishes

Colour blind people typically have either red-green (most common) or
blue-yellow discrimination errors. One of the most famous tests being
the Ishihara colour blindness test - see for example:

http://www.toledo-bend.com/colorblind/Ishihara.html

Regards,
Martin Brown

Thanks Martin - a most interesting site and sight which I seem, luckily, to pass with flying colours;-)

Nytecam

In article . com,
bob wrote:

hi there it`s me again and i have another question to ask about the way
in which we see colour. As you well may know some people are colour
blind and so i was just wondering how they see colour

We don't know .... each of us don't even know how anyone else (color-blind
or not) perceives color. All we can agree or disagree about is the *names*
we assign to various colors.

and whats the diffrence between wright and wrong.

Neither of them is right or wrong - they just perceive colors differently.

People with so-called normal color vision (trichromats) naturally see
a wider range of colors than color-blind people (dichromats or
monochromats). So we tend to think that trichromats are "right" and
dichromats/monochromats are "wrong" in their perception of color. So
we call the di/monochromats "color-blind".

But there's another (very rare) anomaly in color vision: the
tetrachromats. They don't merely have three fundamental colors in
their retinae - they have four! And compared to them, we trichromats
are "color-blind"! The tetrachromats see all colors we see, plus a
lot of others we are unable to see. No, they don't see any
wavelengths of light we don't see, but they are able to perceive
finer differences in the wavelength composition of the light than we
and therefore they see more colors. But we really don't know how the
tetrachromats perceive the world since we have no way to communicate
that information from one individual to another. However we know
that the tetrachromats can distinguish more colors than the
trichromats can. And to the tetrachromats, color TV and color
slides/prints must appear quite inaccurate, since these things use
only three fundamental colors.


And it doesn't stop the some species of tropical fish have no less
than *five* fundamental colors in their retinae! What colors they
see we cannot even imagine - compared to them, even the tetrachromats
are "color-blind" -- and the trichromats are hoplelessly color-blind!


I hope you understand that there are no "right" or "wrong" about color
perception - there are merely different ways to perceive color. COlor
is not an inherent property of the objects e look at, it's a perception
created in our eye-brain system.



Finally, there's one kind of "color-blindness" which really is no
colorblindness, even from the perspective of a trichromat: these
people are trichromats too but with sensitivity curves of the
fundamental colors different from normal trichromats. So these
people are unable to distinguish colors we can distinguish - o.t.o.h.
we are unable to distinguish colors they can distinguish. So, as
seen from their perspective, we are somewhat colorblind too....

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

In article ,
Roger Hamlett wrote:

Now if (for instance), you have a narrow band light at 560nm, this stimulates
both the 'red', and 'green' cell responses, and we percieve it as 'yellow'.
However a red, and green light mixed together, gives exactly the same response,
yet has a completely different spectrum!.

Yellow is actually an interesting case.... artists frequently
considers four fundamental colors: red, green, blue, yellow.
Physicists and technicians consider only three: red, green, blue
(additive) or cyan, magenta, yellow (subtractive). So which ones are
right?

In a way, both are right!

If we (additively) mix green and blue, we see that as green-blue.

If we (additively) mix red and blue, we see that as "red-blue" or purple.

But if we (additively) mix red and green, we don't perceive that as
"red-green", but as a new color: yellow. How come?

Some of the most basic image processing in the eye is performed in
the retina, which can be considered part of the brain. The color
yellow is created in the retina. No, there are no "yellow-sensitive"
cones. However, adjacent red-sensitive and green-sensitive cones are
cross connected in such a way that if they are stimulated approximately
equally, a new color signal, "yellow", is created in our retina, and
then propagated to the brain.

That's why both the physicist/technician with their three fundamental
colors and the artists with their four fundamental colors are right: there
are three different kinds of cones in the retina: red-, green- and blue-
sensitive. But there are four different kinds of color signals which
reach the brain: red, green, blue, yellow.

Human color vision is a quite complex subject!


This 'trick', is the basis of all normal colour 'reproduction' methods,
but shows that our 'perception' of colour, does not really represent what
is there...

Indeed not .....

To represent a color, three numbers are needed for normal color vision:
red, green and blue intensity. For a dichromat, two numbers are needed
and for a monochromat (total color blindness) one number is needed. For
a tetrachromat (an extremely rare but nevertheless existent anomalous
color vision among humans) four numbers are needed, and for pentachromats
(nonexistent among humans but some species of tropical fish are known
to have this kind of vision) five numbers are needed.

What "really is there" is a spectrum. And to accurately represent a
spectrum a lot of numbers are needed. In principle, an infinite
amount of numbers are needed, but in practice a finite amount of
numbers will do well - the more detail the spectrum contains, the more
numbers are needed. If we need to represent the entire visual range
from 4000 to 7000 Angstroms, with 1 Angstrom resolution, we need 3000
numbers. If 100 Angstrom resolution is enough, 30 numbers are
needed, and if 1000 Angstrom resolution is enough, 3 numbers are
needed - the last case vaguely mimics the three fundamental colors
of the eye.




The processing behind the eye, then plays a whole load more 'tricks'. The
worst is that the eye will adjust the responses it allocates to the
signals, based upon the brightest signal from each band, combined
together, being 'white'. If you walk into a shop with incandescent
lighting, and look at a piece of white paper, you see it as white, yet if
you go outside, and look in through the window (if you can see the paper
in the dim interior), it'll look quite red. Again you are not seeing the
real colour, but your brain is attempting to make good use of the signal
available to give a representation of colour, but the result is again that
you do not really see the colour that is actually 'there'.

That's because there is no colour "actually there". Colour is a perception
in our eyes, not an inherent property of an object. What is "actually there"
is a spectrum.

There are many examples of two (or more) objects perceived to have
exactly the same colour (as far as the human eye can perceive) in one
kind of illumination (e.g. daylight) but who get colors different
enough to be easily distinguishable from one another in another kind
of illumination (e.g. fluorescent light). This is a clear example
that colors of objects aren't inherent properties of the objects but
are dependend on how the objects are illuminated. If we take a
high-resolution spectrum of the light sources and of the reflectances
of the objects, we can easily determine why the two objects appear to
have the same color in one illumination but a different color in
another illumination.

One extreme example: bring a vividly colored object, having vivid
red, green, yellow, blue - all the "colors of the rainbow", into a
room or other location illuminated *only* by low pressure sodium
lights. Make sure no other light source illuminates that place.
Look at the vividly colored object there -- the first time you do
this, it's quite amazing to see that all the colors vanish and seem
to be replaced by shades of gray. Bring a flashlight too: now light
the flashlight and see the colors "magically" reappear.


This is similar to the 'white balance' control on a digital camera.
Even if two people look at an object, and agree that this is 'green',
all that is implied, is that both are getting the same signals triggered
inside their brains, that a 'green' part of the spectrum generates for
each of them.

Not even that is impled -- all that's implied is that these two people
have agreed to label what they see as "green".

The actual perception does not have to be the same.

No - it can be widely different. We have no way to communicate this
information reliably from one individual to another. All we can do
is to ask the person to give a name for the color he sees. Or we can
show him two or more colors and ask him if he sees any difference -
the latter method is commonly used to judge whether a person is
color-blind or not.


Colour 'blindness', covers various faults with both the sensing chemistry,
and the processing behind this. If (for instance), a person presented with
two cards, of matching brightness, with one coloured to just about match
the centre of the 'red' sensor, and the other coloured to match the centre
of the 'green' sensor, and cannot distinguish these, you have the classic
(most common) 'green - red' colour blindness. Failures in the 'blue'
system, are the rarest form. The commonest types of colour blindness are
genetic, and because of the way the gene combinations occur, these are
much more common in men (about 1 in 8 men, against 1 in 200+ women).
Failures do not have to be 'complete', with some people just lacking
sensitivity to a particular colour, but still being able to detect it.
If you are not colour blind, the following site, has example images,
showing how some forms affect what is 'seen' :
http://stuweb3.cmp.uea.ac.uk/laj-web...lindness.html#

Best Wishes
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

Paul Schlyter wrote:
Finally, there's one kind of "color-blindness" which really is no
colorblindness, even from the perspective of a trichromat: these
people are trichromats too but with sensitivity curves of the
fundamental colors different from normal trichromats. So these
people are unable to distinguish colors we can distinguish - o.t.o.h.
we are unable to distinguish colors they can distinguish. So, as
seen from their perspective, we are somewhat colorblind too....

I sometimes wonder if i have an unusual sensitivity curve under
low-light conditions. At night, i often perceive objects as blue where
written references or other observers describe the object as white or
green. The Orion Nebula, for example, appears turquoise to me, and
mercury vapor lamps at a distance appear blue-white.

The canonical "white" star, Vega, appears a strong blue to me, although
i personally think that it's the person who divised the B-V system who
had the anomalous color vision!


Clear skies!

--
------------------- Richard Callwood III --------------------
~ U.S. Virgin Islands ~ USDA zone 11 ~ 18.3N, 64.9W ~
~ eastern Massachusetts ~ USDA zone 6 (1992-95) ~
--------------- http://cac.uvi.edu/staff/rc3/ ---------------

Cousin Ricky wrote:
Paul Schlyter wrote:

Finally, there's one kind of "color-blindness" which really is no
colorblindness, even from the perspective of a trichromat: these
people are trichromats too but with sensitivity curves of the
fundamental colors different from normal trichromats. So these
people are unable to distinguish colors we can distinguish - o.t.o.h.
we are unable to distinguish colors they can distinguish. So, as
seen from their perspective, we are somewhat colorblind too....


I sometimes wonder if i have an unusual sensitivity curve under
low-light conditions. At night, i often perceive objects as blue where
written references or other observers describe the object as white or
green. The Orion Nebula, for example, appears turquoise to me, and
mercury vapor lamps at a distance appear blue-white.

I agree. For me the OIII nebula emission (possibly mixed with some
H-beta) is right on the edge of the dark green/turquoise dividing line.
I tend to see it as turquoise though a few bright planetaries look apple
green to me.

The canonical "white" star, Vega, appears a strong blue to me, although
i personally think that it's the person who divised the B-V system who
had the anomalous color vision!

I just see Vega as painfully bright with no obvious colour.

Regards,
Martin Brown

In article ,
Martin Brown wrote:

I just see Vega as painfully bright with no obvious colour.

Not even white? :-)



--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

In article om,
Cousin Ricky wrote:

Paul Schlyter wrote:
Finally, there's one kind of "color-blindness" which really is no
colorblindness, even from the perspective of a trichromat: these
people are trichromats too but with sensitivity curves of the
fundamental colors different from normal trichromats. So these
people are unable to distinguish colors we can distinguish - o.t.o.h.
we are unable to distinguish colors they can distinguish. So, as
seen from their perspective, we are somewhat colorblind too....

I sometimes wonder if i have an unusual sensitivity curve under
low-light conditions. At night, i often perceive objects as blue where
written references or other observers describe the object as white or
green. The Orion Nebula, for example, appears turquoise to me, and
mercury vapor lamps at a distance appear blue-white.

The canonical "white" star, Vega, appears a strong blue to me, although
i personally think that it's the person who divised the B-V system who
had the anomalous color vision!

Clear skies!

You could very well have blue-sensitive cones which are more
sensitive to low light levels than most people. We have individual
variations in most of our physical properties, and most likely also
in our color vision.

If you have, or some time will get, access to a spectrometer or
a monochromator, you could check out the wavelength limits of your
vision and compare with the normal wavelength limits - particularly
the blue end of the spectrum ought to be interesting to you.


One special case of more blue-sensitive vision than normal occurs
among people who've had their eye lens removed in cataract surgery.
The eye lens (particularly the yellowish aging eye lens) is the prime
absorber in our eye at the blue end of the visible spectrum. Some of
these people have been able to see a bit into the near ultra-violet.
So what's the color of near ultraviolet light? They report the color
as being indistinguishable from very deep blue.


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/