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What is the weakest color blind?

Rods and Cones

The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye’s color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the » fovea centralis «, a 0.3 mm diameter rod-free area with very thin, densely packed cones.

The experimental evidence suggests that among the cones there are three different types of color reception. Response curves for the three types of cones have been determined. Since the perception of color depends on the firing of these three types of nerve cells, it follows that visible color can be mapped in terms of three numbers called tristimulus values. Color perception has been successfully modeled in terms of tristimulus values and mapped on the CIE chromaticity diagram.

Rod and Cone Density on Retina

Cones are concentrated in the fovea centralis. Rods are absent there but dense elsewhere.

Measured density curves for the rods and cones on the retina show an enormous density of cones in the fovea centralis. To them is attributed both color vision and the highest visual acuity. Visual examination of small detail involves focusing light from that detail onto the fovea centralis. On the other hand, the rods are absent from the fovea. At a few degrees away from it their density rises to a high value and spreads over a large area of the retina. These rods are responsible for night vision, our most sensitive motion detection, and our peripheral vision.

The above illustration does make it appear that there are no cones outside the fovea centralis, but that is not true. The blue cones in particular do extend out beyond the fovea.

Cone details Rod details

Cone Details

Current understanding is that the 6 to 7 million cones can be divided into «red» cones (64%), «green» cones (32%), and «blue» cones (2%) based on measured response curves. They provide the eye’s color sensitivity. The green and red cones are concentrated in the fovea centralis . The «blue» cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye’s blue perception.

The cones are less sensitive to light than the rods, as shown a typical day-night comparison. The daylight vision (cone vision) adapts much more rapidly to changing light levels, adjusting to a change like coming indoors out of sunlight in a few seconds. Like all neurons, the cones fire to produce an electrical impulse on the nerve fiber and then must reset to fire again. The light adaption is thought to occur by adjusting this reset time.

The cones are responsible for all high resolution vision. The eye moves continually to keep the light from the object of interest falling on the fovea centralis where the bulk of the cones reside.

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«Blue» Cone Distinctions

The «blue» cones are identified by the peak of their light response curve at about 445 nm. They are unique among the cones in that they constitute only about 2% of the total number and are found outside the fovea centralis where the green and red cones are concentrated. Although they are much more light sensitive than the green and red cones, it is not enough to overcome their disadvantage in numbers. However, the blue sensitivity of our final visual perception is comparable to that of red and green, suggesting that there is a somewhat selective «blue amplifier» somewhere in the visual processing in the brain.

The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration). For an «off the wall» example of this defocusing effect on blue light, try viewing a hologram with a mercury vapor lamp. You will get three images with the dominant green, orange and blue lines of mercury, but the blue image looks less focused than the other two.

Rod Details

The rods are the most numerous of the photoreceptors, some 120 million, and are the more sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark-adapted, or scotopic, vision. The rods are incredibly efficient photoreceptors. More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions. The optimum dark-adapted vision is obtained only after a considerable period of darkness, say 30 minutes or longer, because the rod adaption process is much slower than that of the cones.

The rod sensitivity is shifted toward shorter wavelengths compared to daylight vision, accounting for the growing apparent brightness of green leaves in twilight.

While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision. If you see a dim star in your peripheral vision, it may disappear when you look at it directly since you are then moving the image onto the cone-rich fovea region which is less light sensitive. You can detect motion better with your peripheral vision, since it is primarily rod vision.

The rods employ a sensitive photopigment called rhodopsin.

Rods Do Not See Red!

The light response of the rods peaks sharply in the blue; they respond very little to red light. This leads to some interesting phenomena:

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Red rose at twilight: In bright light, the color-sensitive cones are predominant and we see a brilliant red rose with somewhat more subdued green leaves. But at twilight, the less-sensitive cones begin to shut down for the night, and most of the vision comes from the rods. The rods pick up the green from the leaves much more strongly than the red from the petals, so the green leaves become brighter than the red petals!

The ship captain has red instrument lights. Since the rods do not respond to red, the captain can gain full dark-adapted vision with the rods with which to watch for icebergs and other obstacles outside. It would be undesirable to examine anything with white light even for a moment, because the attainment of optimum night-vision may take up to a half-hour. Red lights do not spoil it.

These phenomena arise from the nature of the rod-dominated dark-adapted vision, called scotopic vision.

Can I bee Slightly Colorblind?

If you are colorblind it is not just about being colorblind or not. There are many different types and characteristics of color vision deficiency which you can suffer from. Color blindness is an umbrella term for all those different forms—and not even a good one.

I’m 13 years old and have been told I am red-green colour blind but only slightly. I have done research and taken MANY colour blind tests. But I’m still confused because how can I be slightly colour blind? Either I am or I am not. I have failed most of the tests I have taken and my optician has told me I am slightly colour blind towards red-green (the most common) but want to know how I am only slightly colour blind and what the scientific word for being slightly red-green colour blind.

First of all, you shouldn’t rely on the color blindness tests available online. These tests are just used to give you a feeling what kind of color blindness you are suffering from and how severe it is. But because every computer display can have different color adjustments they are not 100% reliable.

This young reader says, that either you are colorblind or you are not. This is true and false. If you go to your eye specialist and take a color blindness test, he will tell you if you have normal color vision or not. So yes, either you are colorblind or not.

But he can also tell you, what type of color blindness you are suffering from and the approximate severity of it. There is a whole terminology of color vision deficiency which I don’t wont to list here on the whole. Just the most important facts.

  • Types of color blindness:
    • Red-green colorblind: This is the most common form and according to its name, causes the biggest problems with red and green. There are two different subtypes: Red-blind (or -weak) and green-blind (or -weak).
    • Blue-yellow colorblind: This type of color vision deficiency is not so often seen and actually the colors blue and green (not yellow) are the problem area.
    • Completely colorblind: A very uncommon type which makes you see only in shades of gray. Unfortunately a lot of people think that every colorblind person can only see in shades of gray.
    • Anomalous trichromat: You have still three different types of color receptors as someone with normal color vision. But one of them is slightly shifted in its peak of sensitivity and therefore you can’t see the same broad color spectrum as others.
    • Dichromat: Here only two of three color receptors (cones) are working. You have to mix your perceived color just with two signals compared to three with normal color vision. The color spectrum is strongly reduced.
    • Monochromat: Either no color receptors are working (rod-monochromacy) or just one of them (blue-cone monochromcy). Anyway you will have either just grayscale vision or a very very restricted color vision.

    This means, that is possible to suffer from any possible severity of color blindness. This because the three different color receptors have a highest peak of sensitivity. With an anomalous trichromacy, one of those peaks is shifted towards another one; usually the red peak towards the green or vice versa.

    Because of that shift and the resulting peaks which are closer to each other, less information is available to mix up the final color in your brain. And this makes you either slightly colorblind (peaks still far apart) or more severely colorblind (peaks coming closer).

    The academic term for a slight red-green color blindness is either protanomly (red-weakness) or deuteranomaly (green-weakness). Both types are a subtype of red-green color blindness and a form of anomalous trichromacy.

    11 responses on “ Can I bee Slightly Colorblind? ”

    1. Amanda parra October 3, 2007 at 5:46 pm im not sure what kind of color blind i am..i have trouble telling the difrence between blue and green..i always say that orange is green but i can see orange but not really just want to know what my deficiency is called.
      amanda parra

    What is the weakest color blind?

    HISTORY MATTERS - The U.S. Survey Course on the Web

    “A Heritage of Scorn”: Harper Urges A Color-Blind Cause

    The struggle for woman suffrage lasted almost a century, beginning with the 1848 Woman’s Rights Convention at Seneca Falls, New York, and including the 1890 union of two competing suffrage organizations to form the National American Woman Suffrage Association (NAWSA). NAWSA and other organizations campaigned diligently for the vote in a variety of ways, but did not achieve success until the ratification of the Nineteenth Amendment to the U.S. Constitution in 1920. This prolonged struggle entangled female activists in other important political and moral issues that divided the nation along racial, ethnic, and class lines, and debates over the vote for women often took a divisive tone. In this 1891 speech to the National Council of Women, African-American abolitionist, lecturer, and writer Frances Ellen Watkins Harper answered the racist charges of white suffragists who saw the vote for (white) women as a way to maintain white supremacy. The vote for African-Americans, both men and women, Harper argued, was a matter of “justice, simple justice.”

    I deem it a privilege to present the negro, not as a mere dependent asking for Northern sympathy or Southern compassion, but as a member of the body politic who has a claim upon the nation for justice, simple justice, which is the right of every race, upon the government for protection, which is the rightful claim of every citizen, and upon our common Christianity for the best influences which can be exerted for peace on earth and good-will to man.

    Our first claim upon the nation and government is the claim for protection to human life. That claim should lie at the basis of our civilization, not simply in theory but in fact. Outside of America, I know of no other civilized country, Catholic, Protestant, or even Mahometan, where men are still lynched, murdered, and even burned for real or supposed crimes. . . . A government which has power to tax a man in peace, and draft him in war, should have power to defend his life in the hour of peril. A government which can protect and defend its citizens from wrong and outrage and does not is vicious. A government which would do it and cannot is weak; and where human life is insecure through either weakness or viciousness in the administration of law, there must be a lack of justice, and where this is wanting nothing can make up the deficiency.

    The strongest nation on earth cannot afford to deal unjustly towards its weakest and feeblest members. . . . I claim for the negro protection in every right with which the government has invested him. Whether it was wise or unwise, the government has exchanged the fetters on his wrist for the ballot in his right hand, and men cannot vitiate his vote by fraud, or intimidate the voter by violence, without being untrue to the genius and spirit of our government, and bringing demoralization into their own political life and ranks. Am I here met with the objection that the negro is poor and ignorant, and the greatest amount of land, capital, and intelligence is possessed by the white race, and that in a number of States negro suffrage means negro supremacy? . . .

    It is said the negro is ignorant. But why is he ignorant? It comes with ill grace from a man who has put out my eyes to make a parade of my blindness,—to reproach me for my poverty when he has wronged me of my money. If the negro is ignorant, he has lived under the shadow of an institution which, at least in part of the country, made it a crime to teach him to read the name of the ever-blessed Christ. If he is poor, what has become of the money he has been earning for the last two hundred and fifty years? Years ago it was said cotton fights and cotton conquers for American slavery. The negro helped build up that great cotton power in the South, and in the North his sigh was in the whir of its machinery, and his blood and tears upon the warp and woof of its manufactures.

    But there are some rights more precious than the rights of property or the claims of superior intelligence: they are the rights of life and liberty, and to these the poorest and humblest man has just as much right as the richest and most influential man in the country. Ignorance and poverty are conditions which men outgrow. Since the sealed volume was opened by the crimson hand of war, in spite of entailed ignorance, poverty, opposition, and a heritage of scorn, schools have sprung like wells in the desert dust. It has been estimated that about two millions have learned to read. Colored men and women have gone into journalism. Some of the first magazines in the country have received contributions from them. Learned professions have given them diplomas. Universities have granted them professorships. Colored women have combined to shelter orphaned children. Tens of thousands have been contributed by colored persons for the care of the aged and infirm. . . . Millions of dollars have flowed into the pockets of the race, and freed people have not only been able to provide for themselves, but reach out their hands to impoverished owners.

    Instead of taking the ballot from his hands, teach him how to use it, and to add his quota to the progress, strength, and durability of the nation. . . .

    Underlying this racial question, if I understand it aright, is one controlling idea, not simply that the negro is ignorant; that he is outgrowing; not that he is incapable of valor in war or adaptation in peace. On fields all drenched with blood he made his record in war, abstained from lawless violence when left on the plantation, and received his freedom in peace with moderation. But he holds in this Republic the position of an alien race among a people impatient of a rival. And in the eyes of some it seems that no valor redeems him, no social advancement nor individual development wipes off the ban which clings to him. It is the pride of Caste which opposed the spirit of Christ, and the great work to which American Christianity is called is a work of Christly reconciliation. . . .

    Source: Frances Ellen Watkins Harper, speech, in Rachel F. Avery, ed., Transactions of the National Council of Women of the United States, assembled in Washington, D.C. on Feb. 22–25, 1891 (Philadelphia: J. B. Lippincott, 1891), 86–91. Reprinted in Gerda Lerner, The Female Experience: An American Documentary (Indianapolis: The Bobbs-Merrill Company, Inc., 1977), 355–357.

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