View source for Color notes - the eyes and the brain

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==The eyes==

SPDs may have scientific uses, to do reproduction not just for human eyes, 
and to keep the underlying mechanics in mind. 

...but for many practical uses for consumption of human vision, 
it helps to know what the eyes do to flatten/simplify this.


In this context the retina is most interesting. 
The retina is the back of the eye, the bit that's actually sensing the light,
with rods to sense just intensity, and cones to sense colors.


[[Image:eyeresponse-sml.png|thumb|200px|right|Spectrum response of the three cone types. Normalized to show varied shapes, in reality their sensitivity also differs.]]
The reception of '''cones'''' is called 'tri-stimulus', meaning they describe what they see in terms of the intensity of three signals - three colors.

At a physical level, cones are basically a colored filter in front in the form of a pigment, before the light that's then left over gets to the light-receptive area. 

There are effectively three types of cones, responsive to three different (and overlapping) ranges of wavelengths.

Quite usually this is referred to as L, M and S refer to '''l'''onger, '''m'''edium or '''s'''horter wavelengths (relative to each other).

Their peak sensitivity (approximately 610, 540 and 420nm, respectively) roughly corresponds to with red, green and blue, but this is somewhat misleading,
because these response curves both overlap considerably, and have different shapes.


'''Rods''' have no pigmentation filter so receive only brightness information.
Their response peak is around ~520nm, putting it between S and M at a slightly bluish green.




<!--
Light sensitivity in rods and cones works via rhodopsins, proteins and variations you could call red rhodopsin, green rhodopsin, and blue rhodopsin. Cones have one of these.
(rods have a somwhat different structure{{verify}}, but still work with rhodopsin{{verify}})

Rhodopsins are the reason you should eat some beta-carotene (Vitamin A)
-->


===Rod, cone, and cone type distribution; sensitivities; night vision===
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[[Image:Human_photoreceptor_distribution.png|thumb|right|300px]]

'''Rods versus cones'''

From the center of your retina (the [http://en.wikipedia.org/wiki/Fovea fovea])
* from zero to about two degrees: many more cones than rods
: meaning we see color best in the center of our vision.
* about two to 15&#xB0;: more rods than cones
* beyond 15&#xB0;: mostly rods
: meaning our peripheral vision is more light-sensitive, and explains the ability to see things slightly better in the dark by not looking at them directly. 

Apparently rods also resolve a little less detail, as cones connect to a single signal nerve but rods usually have to share a little.



'''Cones versus cones'''

In general, you have more red cones than green cones, and noticably fewer blue cones (~2%?). 
So we see red a little faster and a little more accurately. 

For blue, we are similarly sensitive to its intensity, but with fewer cones, we essentially have lower resolution so find it harder to see detail in it.



'''General notes on sensitivity'''

Sensitivity and discenrability varies with light levels, because of how cones or rods respond.
The technical terms:
* '''Photopic vision''': refers to vision under well-lit conditions, primarily with cones (1 to 1000000 cd/m^2) 
* '''Mesopic vision''': darkish vision at which you have limited color vision(at about .02 to 1 cd/m^2)
* '''Scotopic vision''': refers to the monochromatic vision in low light, when cones do not work (about .02 down to about 0.000001 cd/m^2)




====On night vision====
{{stub}}

{{warning|Warning: Popular fact is regularly myth here, and I'm not sure all this is right.}}

Night vision is mostly due to rods.

Since the center of our vision has a low amount of rods, relatively speaking our night vision is ''worse'' in the center few degrees of our vision, and our peripheral vision is actually better at seeing things in the dark, though that ends up meaning we see movement better than detail.



Night vision being usable depends on the amount of rhodopsin protein in your eyes, and its regeneration. 

Rhodopsin depletes in daylight-level intensities.
When it regenerates, your eyes can do more with the same amount of light.

A few minutes is enough for regeneration of ten, twenty percent (at which point you start noticing night vision),
fifteen minutes for the bulk of it, and over half an hour for almost all.


If you want to illuminate something in the dark -- instrumentation, a dark room, or such, the best color to use depends on 'what for'.

The main choice here is:
: do you want to see detail
: or do you want the least effect on your night vision?

These are at odds, almost by definition, 
because that which makes us see better also depletes rhodopsin.

But it's a little more complex than that.

When you value night vision above all, then higher-frequency reds gives slower depletion of rhodopsin, than blue, green, and even lower-frequency reds give.

If you care more about detail (than about minimal effect on night vision), for example for quick-glance readability of relatively precise instrumentation, then various colors will do, and arguably (dim) white is better.

Also, you may care a to use the center of your eyes to read out these details - which primarily has cones.

You can argue that any color will do, and red slightly better as you have more red cones than other cones.
However, rods respond to lowish red frequencies, and (aside from depleting night vision a little) that means your brain will get a signal from both rods and cones.
In near-darkness (up to lowish mesopic), both signals are noticable, 
and your brain having to interpret the combination is less readable{{verify}} (and perhaps a little more tiring{{verify}}). 

Higher-frequency reds are useful in that you get more response from cones than rods that way{{verify}}


See also 
* [http://en.wikipedia.org/wiki/Trichromatic_color_vision Wikipedia: Trichomatic color vision]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html the density of rods and cones])
* a good book

===Color blindness===

Given the above rods-and-cones introduction, pretty much all possible malfunctions you can think of can happen {{comment|(A few more, in fact, given the other parts of processing)}}.


But of all of those, only two are common.

There are also two distinct ways to categorize the common types: 
: by their causes (e.g. low cone response), which is a more precise description
: and by the effect they have (which colors are harder to distinguish), which is a more pragmatic description


<!--
Lower cone response primarily means that specific differences are perceived much less,
but explaining exactly what you see to people without that deficiency is hard.

No, you don't see in black and white. 

You could say it's like lower saturation for specific colors, 
or that it's comparable to how it is harder to tell apart colors in low light.


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The only real effect is not being able to distinguishing certain colors, that other people can easily tell apart.
(As a side effect, some kinds of color blindness mean you may be ''better'' at focusing on brightness differences{{verify}}, and relatedly, ''some'' kinds of colorblindness correlate to better night vision).


''Unusual'' (often meaning ''limited''{{verify}}) response to one type of cone is one common type of problem.

Because of the opposite-processing of transmission, both 
protanomaly (proto referring to L cones) and 
deuteranomaly (referring to M cones) limit the red-green channel, 
while tritanomaly (S cones) limits blue-yellow.


''Missing'' response of the L, M, or S cones is called Protonopia, deuteronopia and tritanopia respectively, 
and means little response on one of the opposite-processing channels.


Deuteranomaly (implying red-green) is most common, affecting 5% of men, while the rest affects 1% or less of anyone.


The anomalies are hereditary, in mildly complex ways.
There are other ways of damaging your eyes, but since it's selective, this is a little harder.


See also:
* http://en.wikipedia.org/wiki/Color_blind
* http://www.w3.org/TR/AERT#color-contrast

Related:
* [http://ray.tomes.biz/b2/index.php/a/2007/06/21/p147 Tetrachromat vision]
* http://www.toledo-bend.com/colorblind/Ishihara.html

==Nerves and the brain==
{{stub}}


===Color mixing and opposite processing===

It was observed by various people that we don't see certain colors mixing. While 'greenish blue' and 'yellowish red' work as descriptors because we can imagine what that mixes to, 'greenish purple' or 'blueish yellow' doen't. 

This non-mixing isn't so much an effect of reception, not part of the cones, but instead caused by the way their signals are processed before they are sent to our brain. 


Before sending signals to the brain, our eyes convert to a system named ''' opponent processing, ''' that turns the three types of cones' signals into two channels.

Essentially, one channel is red stimulus minus green stimulus, the other yellow minus blue. Note there are no yellow cones; yellow comes from the combination of L and M (red and green) cones. 
In reality, effects like the overlap of conce response make this even more complex than described here -- get a good book if interested. 

{{comment|(It also helps explains the two typical types of [[color blindness]] - if one of the types of cone pigment is missing, one of the described color signals has little or no range)}}


After processing, less signal on the red-green means you see green, more signal means you see red. Little on yellow-blue means yellow, more means blue. (See e.g. [http://web.archive.org/web/20050209035707/http://driesen.com/opponent_processing.htm this] for a graphic explanation and [http://www.pigeon.psy.tufts.edu/avc/husband/avc4eye.htm this] for a textual one) 

Sensing both colors on a channel at once is neurally impossible, making the eye a bad tool for measuring the spectrum. This signaling is one of the main reasons for '''metameters''', non-identical spectra (SPD's) which we see as the same color, (depending also on the environment light - which is a problem, as color differentiation is different under different light sources). 
The existance of metameters sounds like a problem, but without them we would have needed a ''lot'' more color names and crayons in a kit - pretty much one for every noticably different SPD.

The fact that the neurons and the mind adapt in short and long terms is the reason for effects like 
* after-images of signals that oppose according to the opposite processing model (stare at red and you see greener for a few seconds, see eg. [http://www.yorku.ca/eye/afterima.htm this example]), 
* slowly adapting night vision, as the rods take over from the cones - also why we're mostly colorblind in the dark, 
...and a few more complex effects.


===How fast are our eyes===
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That depends on what you're ''really'' asking.



There is a difference between rate and delay.

We might be able use image information that changes in 5ms intervals {{comment|(not that our eyes have a singular framerate - and with lack of shutter, things blur instead)}}, even if a conscious response to it can come in no faster than 100-400ms (depending on the task).

And then, the best way to lower than response time is to ignore information.

If we want to interpret much of the ''content'' of an image, we are limited to 60fps at best,
but we can e.g. tell flashing speeds up to 150Hz{{verify}} well enough.


That is, put someone in front of a faster monitor and it turns out they can tell ''that'' a framerate is higher, up to maybe 150fps.
Our eyes seem to be faster than that, but but it seems the only thing we can respond to at even ~150fps is "yes, that sure is blinking quickly".

...but make them do reaction time experiments and the differences are very small indeed.



The chemical reaction in rods and cones is faster than you might think[https://www.cerc.gc.ca/news_room-salle_de_presse/spotlight-pleins_feux/ernst_vision-eng.aspx] (question: does it also recover that fast).

Myelinated nerves are good for at least a few hundred fires per second.


Our brain processing a full image in a useful way? ''Much'' slower than any of that.
: It takes at least 100ms before information even arrives at the occipital cortex [citation needed].
: The processing that then happens has a few different parts - some things can be responded to sooner.


Behavioural experiments illustrate this varies with complexity. 
Responsing ''at all'' to the very simplest stimuli may be possible in order of 200ms,
but when there is any sort of complexity, or any sort of choice, it quickly rises to and above 300ms. Easily over 400ms when memory needs to be involved{{verify}}.

Even if the task allows us to skip total-image responses and lets us focus only on what we need (and we absolutely ''will'' miss things - there are some classical experiments demonstrating this), we can't push it below the speed of image formation.


(Side note: visual reaction time is the slowest of our senses if you include the 'what do I do with this information', compared to e.g. sound and touch. Those aren't ''much'' better, mind, but it seems they're just easier to process.)


Behavioural experiments also point out it depends on how it's measured. Say, if they need to press a button there is some time involved in getting a motor response down to your fingers and muscles. Experiments that use EEG will often be able to measure that response a little earlier, and the measured button press may not even be used as a result{{verify}}.



Such details are why why people say that in many practical ways we see at 30 to 60fps (with a difference with lighting, and with center/peripheral vision).

Can we perceive things that happen in a shorter timespan than that, in lab conditions? Absolutely.  

With any detail? No.

With the ability to tell it apart from other things in that same timespan? Not really.


The test people cite here is usually a blank screen and flashing an image for a fraction for a second.
People start being able to tell what it is (roughly, shape-wise) when it persists longer than order of 10ms{{verify}} or so. 
But this is in complete absence of other stimuli. 
Flash a different image at them every 10ms and they will be able to tell you nothing.



When the question is asked for "how fast does my monitor need to be" - 60fps is mostly fine, but there are some extra effects here,
like that we do seem to perceive the difference between 'blurred because moving quickly' and 'rendered perfectly sharply half a screen apart' [citation needed]


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==See also==
http://blog.asmartbear.com/color-wheels.html


[[Category:Audio, video, images]]