Shorn Again Believers: Colors: The human eye is most sensitive to GREEN Colors: The human eye is most sensitive to GREEN IMPORTANCE OF COLOR Among the eyes of Nature, the human eye is designed to distinguish colors. Cats eyes, for example are much better at detecting movement, the condor can see "death", and the dog can "read" our faces better than we can. But for colors, we seem to have the leading specimen eye. And among the colors, which ones do we seem most keen to observe? More than any other color, we see green. Since color is a major quality factor to human beings, the fact that we are more sensitive to green than to other colors, must be significant. And what IS it about GREEN? In terms of dialectic, it is not associated with an "opposite" -- such as "black and white", or "blue and red". It is not a primary color, but is the mix of blue and yellow. So why does our eye especially read for green? THE "GREEN SENSITIVITY" Is WHAT is green the important thing?

-- the grass, the garden, the tree -- or is the prejudice merely the coincidence of the particular pigment or light wave to the shape of our eye? In all the perlustrations of color before us, in the parade of shade and light, is there an explanation for our green proclivity? THE EYE Because the sensitivity is indifferent to whether the green is pigmentation or wave length, solid or light, it is probable that our brain, not the naked physics of the peep-hole, the optic orb, is cue-ing the green. One test of this would be further experiments with creatures with eyes which are similar to human eyes -- such as squid, or even other primates. Do they have the same sensitivity, and can we test this differentiation? Squid live in an environment in which there is almost no ambient "green" light and few enough chorophylic plants, compared with the non-aquatic species. GREEN THINGS Assuming the brain is the organ which holds the sensitivity and is prejudiced in favor of the color green, is there anything about green things which explains the predisposition? PLANTS. We do seem to take comfort in Gardens.

It is almost an embarrassment how we putter in them -- and this is not simply explained as an association with our food sources. We rarely eat the foliage and flowers which populate our Gardens -- hanging and otherwise. GRASS. The word for the color Green stems etymologically from "growing grass" -- compare CGerm/Goth "gronjaz". It is the color of growing, the fresh, young, unripe, immature and living forms. It seems curious that no fruit gets green with age, and meats which green do so because of new growth of bacteria and mold upon them. The primordial grassland, or open veldt, may at least partially be reflected in the origin of the word and its cognates -- greenery, greening, greensward, greenhorn (new ox). EMERALD JEWEL. Emeralds are green, and are long and fondly regarded as the Queen of all precious gemstones. Emeralds adorned the Pharaohs of Egypt over 3,000 years ago, and Inka craftsmen worked it into their most prized jewelry. Of course, our attraction to a pretty rock would obtain no matter what its hue.

As Beryllium with trace chromium, vanadium, and occasionally iron, Emeralds are also extremely hard -- making them an excellent jewel impervious to wear. Depending on Color, Clarity, Cut and Carat weight, they are more valuable by weight than diamonds, or any other stone. With respect to clarity, the natural inclusions in emeralds are often called ‘gardens’. These gardens -- the inclusions -- make the stone green, of course, since beryl is colorless. We draw from these green shades of jewels and gardens the descriptions of "enchanting" or "mysterious". SHADES. The human eye detects an extraordinary range of hues and shades -- 100's of thousands. Eye detection of green chromaticity appears to be scaled on par with other colors. This seems odd in light of the Color Chart, your rainbow of primary and secondary colors, and the Green Sensitivity.

Consider the various virescent Chlorochrous hues: Bright or vivid green (emerald, smaragdine, day-glo, citrine); Light green (apple); Bluish green (teal, glaucous, some cerulean or sea-green); Yellow-green (chartreuse, avocado, Kelly, leek, hunter, Nile, absinthe, pistachio, verdegris, pea, grass, leaf, malachite, moss); Gold-green (chrysochlorous); Eye's Response to Light - The Human Eye's Response to Light The three curves in the figure above shows the normalized response of an average human eye to various amounts of ambient light. The shift in sensitivity occurs because two types of photoreceptors called cones and rods are responsible for the eye's response to light. The curve on the right shows the eye's response under normal lighting conditions and this is called the photopic response. The cones respond to light under these conditions. As mentioned previously, cones are composed of three different photo pigments that enable color perception. This curve peaks at 555 nanometers, which means that under normal lighting conditions, the eye is most sensitive to a yellowish-green color.

When the light levels drop to near total darkness, the response of the eye changes significantly as shown by the scotopic response curve on the left. At this level of light, the rods are most active and the human eye is more sensitive to the light present, and less sensitive to the range of color. Rods are highly sensitive to light but are comprised of a single photo pigment, which accounts for the loss in ability to discriminate color. At this very low light level, sensitivity to blue, violet, and ultraviolet is increased, but sensitivity to yellow and red is reduced. The heavier curve in the middle represents the eye's response at the ambient light level found in a typical inspection booth. This curve peaks at 550 nanometers, which means the eye is most sensitive to yellowish-green color at this light level. Fluorescent penetrant inspection materials are designed to fluoresce at around 550 nanometers to produce optimal sensitivity under dim lighting conditions. References: Robinson, S. J. and Schmidt, J. T., Fluorescent Penetrant Sensitivity and Removability - What the Eye Can See, a Fluorometer Can Measure, Materials Evaluation, Vol. 42, No.

8, July 1984, pp. 1029-1034 Is it yellow, green or yellow green color to which human eye is most sensitive? - Anatomy, Physiology and Neuroscience - Science Forums Is it yellow, green or yellow green color to which human eye is most sensitive? Started by answers , Apr 05, 2011 Please log in to reply 21 replies to this topic Posted 5 April 2011 - 07:45 AM Hello first post here! I can't find an answer to what hue of green/yellow exactly is the human eye most senstivite to (in RGB). These are the most informative articles I could find: [url="[url]#Cone_cells_in_the_human_eye"] [url="[url]"] This could be useful for stuff like enemy color in FPS games that allow brightskins for enemies, etc. #edit: [url="[url]"] Sensitivity in daylight vs. darkness Our eyes are most sensitive in daylight to green light around 550nm and slightly less sensitive to yellow light. They are approximately half as sensitive to orange light and only about a tenth as sensitive to red and violet light.

Therefore, for example, we can see a 100-Watt green light bulb three times farther away than we can a red or violet bulb of the same power. A 532nm Nd:YAG laser diode module will emit highly visible green output and is, therefore, a popular choice for outdoor pointing and alignment applications. In darkness, however, we are most sensitive to blues and least sensitive to reds. This is interesting. Does it mean that if I play quake in dark settings then I should set my enemies to blue color. But again -- which hue of blue? Edited by answers, 5 April 2011 - 07:49 AM. 0 5 posts Posted 5 April 2011 - 03:27 PM Luminous flux/intensity are defined at ~555 nm (540 THz), because that's the peak sensitivity of scotopic vision. Photopic vision shifts to the blue about 50 nm. [url="[url]"] What is the value in RGB? 0 255 0 ? Although I risk derailing the actual topic now: isn't contrast more important than the actual sensitivity of the eye?

Since we're talking about gaming, shouldn't this be about how enemies can stand out the most in different settings? My humble apologies if I take this discussion in the wrong direction. Could the answer be "optic yellow" that was concluded to be most visible tennis ball color for TV viewers in one scientific research? typical environments in the game: [url="[url] 197 posts Posted 5 April 2011 - 06:52 PM CaptainPanic is in the correct diretion for this topic. OP needs a better contrast setting in your game design, not better sensitivity to the visual object. Generally there are two kinds of contrast: the contrast of brightness and the contrast of color. The contrast of brightness depends on the ratio of brightness between visual object and background. For a light source with pulse wavelength, ~555 nm is the most sensitive to your eyes in brightness, which means that the object with this wavelength in dark background should have the highest contrast subjectively. The contrast of color depends on the wavelength difference between the object and background. The human eyes are more sensitive for the difference between RED and GREEN.

If you have a mixed color on the background, the best contrast for the object should be some color between ~555 nm and red. That's why "optical yellow" is good color for tennis ball on TV. 0 1,608 posts Posted 7 April 2011 - 05:44 AM I can't find an answer to what hue of green/yellow exactly is the human eye most senstivite to (in RGB). Luminous flux/intensity are defined at ~555 nm (540 THz), because that's the peak sensitivity of photopic scotopic vision. Lime green (#BFFF00) corresponds roughly to 564 nm. Like the wiki page says "Lime (color#BFFF00) is a pure spectral color at approximately 564 nanometers on the visible spectrum when plotted on the CIE chromaticity diagram...

[it] is approximately the most visible color on the spectrum because it corresponds to a wavelength near the peak of the photopic luminosity function." Here is a splotch on a screen cap of one of the games you linked: because of jpg compression that probably won't be exactly #BFFF00, but it will be close. 1 197 posts Posted 7 April 2011 - 01:26 PM Lime green (#BFFF00) corresponds roughly to 564 nm. Like the wiki page says "Lime (color#BFFF00) is a pure spectral color at approximately 564 nanometers on the visible spectrum when plotted on the CIE chromaticity diagram... [it] is approximately the most visible color on the spectrum because it corresponds to a wavelength near the peak of the photopic luminosity function." This color is good for the background of the picture. But remember, it won't be always good for different backgrounds. 555 nm is the best color for perfect dark background, which has been written in the textbooks. If the background is a sunny beach, guess what? The color giving you the best contrast is RED!

0 1,608 posts Posted 7 April 2011 - 02:19 PM This color is good for the background of the picture. But remember, it won't be always good for different backgrounds. Respectfully, I think you might be missing the point. 550 nm yellow-green is the most visible color. This means that the human eye can distinguish more shades of that color than any other. We can, in other words, spot an enemy wearing some shade of yellow-green on a different shade yellow-green background better than we can spot an enemy wearing some shade of red on a different shade red background. This is why the military makes use of this color in a heads up display: it is the best color when the background could be anything. Edited by Iggy, 7 April 2011 - 02:22 PM. 0 Posted 7 April 2011 - 03:17 PM It will be the worst color for the background color at 550 nm! We need scientific understanding, not just searching. Ok, I'm assuming that you're being sincere.

The question in the OP is worded correctly, I can't find an answer to what hue of green/yellow exactly is the human eye most senstivite to (in RGB)... This could be useful for stuff like enemy color in FPS games He is looking for the best color of uniform in which to dress his enemy and he wants that color to be most sensitive to his sense of vision. The color of his opponent does not change as the background changes. It is constant. For this reason alone, it is pointless to say "well, for some backgrounds the color X provides the best contrast and for other backgrounds the color Y provides the best contrast". The question is rather: "what color is best in any background?" -- in other words, in a random background. Scientists have studied this question. For example, they have recommended that fire engines and other emergency vehicles should be colored lime green. They found that tennis balls are most visible when colored lime green. More than that, I looked at all three videos that the OP linked. They have predominantly dark-colored backgrounds. If you think some other specific color offers the OP a better answer then by all means...

otherwise, I don't see the point of your objection or your accusation that I'm somehow avoiding "scientific understanding". Edited by Iggy, 7 April 2011 - 03:21 PM. 0 Posted 7 April 2011 - 03:38 PM Ok, I'm assuming that you're being sincere. I am sincere as far as in this topic. At first I said that the color you picked is good for the background, which you specified at 564 nm. Then you left the question from OP and said the best color should be at 550 nm, that you claimed that "it is the best color when the background could be anything." If you do understand the contrast of color, how can you have two different results? Why is the second statement incorrect? Edited by thinker_jeff, 7 April 2011 - 03:44 PM. 0 1,608 posts Posted 7 April 2011 - 05:31 PM At first I said that the color you picked is good for the background, which you specified at 564 nm. You are still missing the point.

The question is not "what is the best color on a specific background?". The question is "what is the best color for many different backgrounds?". Then you left the question from OP and said the best color should be at 550 nm The OP itself said "550 nm". , that you claimed that "it is the best color when the background could be anything." Exactly. FPS games have a multitude of environments and backgrounds. The point is to find an RGB color that is most visible in all of those environments with all of those backgrounds -- taken as a whole. how can you have two different results? Most sources say "approximately 550 nm". The exact wavelength which is most visible will depend on the person. The RGB value that corresponds to that wavelength will vary widely between monitors. If a person were a serious gamer and wanted to fine tune the color of his opponent, I would recommend getting an "optic yellow" tennis ball (most people probably already have one). A person could then match the closest pure spectral color to match. A pure spectral color is either a single RGB primary color or a mix of red and green or green and blue. The color we're looking for is a mix of red and green.

So, Answers may put the color #BFFF00 (the one I quoted from wiki) on his game character and see if it matches his tennis ball. If it looks too yellow then he would make the "BF" part smaller. BF is a hex number. In decimal it is 191. To drop it down to 180 he would change it to "B4" and the new color would be #B4FF00. To make the color more yellow he would make the red value larger... maybe #C8FF00. Really, this would be the only way to find the exact RGB value that corresponds to the "approximately 550 nm" color that is being sought because monitors, depending on type, model, and age display very different colors. edited to add ---> Edited by Iggy, 7 April 2011 - 05:56 PM. 0 197 posts Posted 7 April 2011 - 06:06 PM A pure spectral color is either a single RGB primary color or a mix of red and green or green and blue. Wrong. Please look the following explanation. The C.I.E.

Color Space The CIE system characterizes colors by a luminance parameter Y and two color coordinates x and y which specify the point on the chromaticity diagram . This system offers more precision in color measurement than do the Munsell and Ostwald systems because the parameters are based on the spectral power distribution ( SPD ) of the light emitted from a colored object and are factored by sensitivity curves which have been measured for the human eye. Based on the fact that the human eye has three different types of color sensitive cones , the response of the eye is best described in terms of three " tristimulus values ". However, once this is accomplished, it is found that any color can be expressed in terms of the two color coordinates x and y. The colors which can be matched by combining a given set of three primary colors (such as the blue, green,a nd red of a color television screen) are represented on the chromaticity diagram by a triangle joining the coordinates for the three colors.

Visible Light and the Eye's Response Light Waves and Color - Lesson 2 - Color and Vision Visible Light and the Eye's Response Blue Skies and Red Sunsets As mentioned in the first section of Lesson 2 , our eyes are sensitive to a very narrow band of frequencies within the enormous range of frequencies of the electromagnetic spectrum. This narrow band of frequencies is referred to as the visible light spectrum. Visible light - that which is detectable by the human eye - consists of wavelengths ranging from approximately 780 nanometer (7.80 x 10-7 m) down to 390 nanometer (3.90 x 10-7 m). Specific wavelengths within the spectrum correspond to a specific color based upon how humans typically perceive light of that wavelength. The long wavelength end of the spectrum corresponds to light that is perceived by humans to be red and the short wavelength end of the spectrum corresponds to light that is perceived to be violet. Other colors within the spectrum include orange, yellow, green and blue. The graphic below depicts the approximate range of wavelengths that are associated with the various perceived colors within the spectrum.

Color Cones Color can be thought of as a psychological and physiological response to light waves of a specific frequency or set of frequencies impinging upon the eye. An understanding of the human response to color demands that one understand the biology of the eye. Light that enters the eye through the pupil ultimately strikes the inside surface of the eye known as the retina. The retina is lined with a variety of light sensing cells known as rods and cones. While the rods on the retina are sensitive to the intensity of light, they cannot distinguish between lights of different wavelengths. On the other hand, the cones are the color-sensing cells of the retina. When light of a given wavelength enters the eye and strikes the cones of the retina, a chemical reaction is activated that results in an electrical impulse being sent along nerves to the brain. It is believed that there are three kinds of cones, each sensitive to its own range of wavelengths within the visible light spectrum. These three kinds of cones are referred to as red cones, green cones, and blue cones because of their respective sensitivity to the wavelengths of light that are associated with red, green and blue.

Since the red cone is sensitive to a range of wavelengths, it is not only activated by wavelengths of red light, but also (to a lesser extent) by wavelengths of orange light, yellow light and even green light. In the same manner, the green cone is most sensitive to wavelengths of light associated with the color green. Yet the green cone can also be activated by wavelengths of light associated with the colors yellow and blue. The graphic below is a sensitivity curve that depicts the range of wavelengths and the sensitivity level for the three kinds of cones. The cone sensitivity curve shown above helps us to better understand our response to the light that is incident upon the retina. While the response is activated by the physics of light waves, the response itself is both physiological and psychological. Suppose that white light - i.e., light consisting of the full range of wavelengths within the visible light spectrum - is incident upon the retina. Upon striking the retina, the physiological occurs: photochemical reactions occur within the cones to produce electrical impulses that are sent along nerves to the brain.

The cones respond to the incident light by sending a message forward to brain, saying, "Light is hitting me." Upon reaching the brain, the psychological occurs: the brain detects the electrical messages being sent by the cones and interprets the meaning of the messages. The brain responds by saying "it is white." For the case of white light entering the eye and striking the retina, each of the three kinds of cones would be activated into sending the electrical messages along to the brain. And the brain recognizes that the messages are being sent by all three cones and somehow interprets this to mean that white light has entered the eye. Now suppose that light in the yellow range of wavelengths (approximately 577 nm to 597 nm) enters the eye and strikes the retina. Light with these wavelengths would activate both the green and the red cones of the retina. Upon striking the retina, the physiological occurs: electrical messages are sent by both the red and the green cones to the brain. Once received by the brain, the psychological occurs: the brain recognizes that the light has activated both the red and the green cones and somehow interprets this to mean that the object is yellow.

In this sense, the yellow appearance of objects is simply the result of yellow light from the object entering our eye and stimulating the red and the green cones simultaneously. If the appearance of yellow is perceived of an object when it activates the red and the green cones simultaneously, then what appearance would result if two overlapping red and green spotlights entered our eye? Using the same three-cone theory, we could make some predictions of the result. Red light entering our eye would mostly activate the red color cone; and green light entering our eye would mostly activate the green color cone. Each cone would send their usual electrical messages to the brain. If the brain has been psychologically trained to interpret these two signals to mean "yellow", then the brain would perceive the overlapping red and green spotlights to appear as yellow. To the eye-brain system, there is no difference in the physiological and psychological response to yellow light and a mixing of red and green light. The brain has no means of distinguishing between the two physical situations. In a technical sense, it is really not appropriate to refer to light as being colored. Light is simply a wave with a specific wavelength or a mixture of wavelengths; it has no color in and of itself.

An object that is emitting or reflecting light to our eye appears to have a specific color as the result of the eye-brain response to the wavelength. So technically, there is really no such thing as yellow light. Rather, there is light with a wavelength of about 590 nm that appears yellow. And there is also light with a mixture of wavelengths of about 700 nm and 530 nm that together appears yellow. The yellow appearance of these two clearly different light sources can be traced to the physiological and psychological response of the eye-brain system, and not to the light itself. So to be technically appropriate, a person would refer to "yellow light" as "light that creates a yellow appearance." Yet, to maintain a larger collection of friendships, a person would refer to "yellow light" as "yellow light." In the next several sections of Lesson 2, we will explore these concepts further by introducing three primary colors of light and generating some simple rules for predicting the color appearance of objects in terms of the three primary colors. Investigate! In this lesson we will spend a lot of time discussing three light colors - red, green, and blue. We will perceive light as consisting of red, green and blue qualities.

Use the RGB widget below to determine the ratios by which red, green and blue light combine to form other light colors. Enter the name of a color (maize, purple, orange, sky blue, etc.) into the field. Then click the Submit button to find out its red, green, and blue components. Molecular Expressions Microscopy Primer: Physics of Light and Color - Human Vision and Color Perception Movie Gallery Human Vision and Color Perception Human stereo color vision is a very complex process that is not completely understood, despite hundreds of years of intense study and modeling. Vision involves the nearly simultaneous interaction of the two eyes and the brain through a network of neurons, receptors, and other specialized cells. The first steps in this sensory process are the stimulation of light receptors in the eyes, conversion of the light stimuli or images into signals, and transmission of electrical signals containing the vision information from each eye to the brain through the optic nerves. This information is processed in several stages, ultimately reaching the visual cortices of the cerebrum. The human eye is equipped with a variety of optical components including the cornea, iris, pupil, aqueous and vitreous humors, a variable-focus lens, and the retina (as illustrated in Figure 1).

Together, these elements work to form images of the objects that fall into the field of view for each eye. When an object is observed, it is first focused through the convex cornea and lens elements, forming an inverted image on the surface of the retina, a multi-layered membrane that contains millions of light-sensitive cells. In order to reach the retina, light rays focused by the cornea must successively traverse the aqueous humor (in the anterior chamber), the crystalline lens, the gelatinous vitreous body, and the vascular and neuronal layers of the retina before they reach the photosensitive outer segments of the cone and rod cells. These photosensory cells detect the image and translate it into a series of electrical signals for transmission to the brain. Despite some misconceptions due to the wide spectrum of terminology employed for describing eye anatomy, it is the cornea, not the lens, which is responsible for the major part of the total refractive power of the eye. Being smooth and clear as glass, yet as flexible and durable as plastic, the anterior, strongly curved, transparent part of the exterior wall of the eyeball allows the image-forming light rays to pass through to the interior.

The cornea also protects the eye by providing a physical barrier that shields the inside of the eye from microorganisms, dust, fibers, chemical, and other harmful materials. Although much thinner in width than the crystalline lens, the cornea provides about 65 percent of the eye's refractive power. Most of the power to bend light resides near the center of the cornea, which is rounder and thinner than the peripheral portions of the tissue. As the window that controls the entry of light into the eye, the cornea (Figure 2) is essential to good vision and also acts as an ultraviolet light filter. The cornea removes some of the most damaging ultraviolet wavelengths present in sunlight, thereby further protecting the highly susceptible retina and crystalline lens from damage. If the cornea is curved too much, as in the case of nearsightedness, distant objects will appear as blurry images, because of imperfect light refraction to the retina. In a condition known as astigmatism, imperfections or irregularities in the cornea result in unequal refraction, which creates distortion of images projected onto the retina. Unlike most tissues of the body, the cornea does not contain blood vessels for nourishment or to protect it against infection.

Even the smallest capillaries would interfere with the precise refraction process. The cornea receives its nourishment from tears and the aqueous humor, which fills the chambers behind the structure. The outer epithelial layer of the cornea is packed with thousands of small nerve endings, making the cornea extremely sensitive to pain when rubbed or scratched. Comprising about 10 percent of the tissue's thickness, the epithelial layer of the cornea blocks foreign matter from entering the eye while providing a smooth surface for oxygen and nutrient absorption. The central layer of the cornea, known as the stroma, comprises about 90 percent of the tissue, and consists of a water-saturated fibrous protein network that provides strength, elasticity, and form to support the epithelium. Nourishing cells complete the remainder of the stroma layer. Because the stroma tends to absorb water, the endothelium tissue's primary task is to pump excess water from the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately turn the cornea opaque, rendering the eye blind. The partial or complete loss of transparency by the crystalline lens, or its capsule, results in a common condition known as cataracts.

Cataracts are the leading cause of blindness worldwide and represent an important cause of visual impairment in the United States. Development of cataracts in adults is related to normal aging, sunlight exposure, smoking, poor nutrition, eye trauma, systemic disease such as diabetes and glaucoma, and undesirable side effects from some pharmaceuticals, including steroids. In the early stages, an individual suffering from cataracts perceives the world as blurry or out of focus. Clear vision is prevented by a reduction in the amount of light that reaches the retina and by clouding of the image (through diffraction and light scattering) as though the individual were observing the environment through a fog or haze (see Figure 3). Removal of the opaque lens during cataract surgery, with subsequent replacement by a plastic lens (intraocular lens implants), often results in corrected vision for unrelated conditions such as nearsightedness or farsightedness. The function of the retina is similar to the combination of a digital image sensor (such as a charge-coupled device (CCD)) with an analog-to-digital converter, as featured in modern digital camera systems.

The image-capturing receptors of the eyes, known as rods and cones, are connected with the fibers of the optic nerve bundle through a series of specialized cells that coordinate the transmission of signals to the brain. The amount of light allowed to enter each eye is controlled by the iris, a circular diaphragm that opens wide at low light levels and closes to protect the pupil (the aperture) and retina at very high levels of illumination. As illumination changes, the diameter of the pupil (positioned in front of the crystalline lens) reflexively varies between a size of about 2 to 8 millimeters, modulating the amount of light that reaches the retina. When illumination is very bright, the pupil narrows and peripheral portions of the refractile elements are excluded from the optical pathway. The result is that fewer aberrations are encountered by image-forming light rays, and the image on the retina becomes sharper. A very narrow pupil (approximately 2 millimeters) produces diffraction artifacts that spread the image of a point source on the retina.

In the brain, the neural fibers of the optic nerves from each eye cross at the optic chiasma where visual information from both retinas traveling in parallel pathways is correlated, somewhat like the function of a time base correction generator in a digital video tape recorder. From there, the visual information travels through the optic tract to the knee-shaped lateral geniculate nuclei in the thalamus, where the signals are distributed via the optic radiations to the two visual cortices located on the lower rear section of each half of the cerebrum. In the lower layers of the cortex, the information from each eye is maintained as columnar ocular dominance stripes. As the visual signals are transmitted to the upper layers of the cortex, information from the two eyes is merged and binocular vision is formed. In abnormal ophthalmic conditions such as phorias (misalignments) of the eyes, including strabismus (better known as crossed-eyes), stereovision is disrupted as are the individual's bearings and depth perception. In cases where ophthalmic surgery is not warranted, prismatic lenses mounted in spectacles can correct some of these anomalies.

Causes of interruption to the binocular fusion may be head or birth trauma, neuromuscular disease, or congenital defects. The central fovea is located in an area near the center of the retina, and positioned directly along the optical axis of each eye. Known also as the "yellow spot", the fovea is small (less than 1 square millimeter), but very specialized. These areas contain exclusively high-density, tightly packed cone cells (greater than 200,000 cones per square millimeter in adult humans; see Figure 4). The central fovea is the area of sharpest vision, and produces the maximum resolution of space (spatial resolution), contrast, and color. Each eye is populated with approximately seven million cone cells, which are very thin (3 micrometers in diameter) and elongated. The density of cone cells decreases outside of the fovea as the ratio of rod cells to cone cells gradually increases (Figure 4). At the periphery of the retina, the total number of both types of light receptors decreases substantially, causing a dramatic loss of visual sensitivity at the retinal borders.

This is offset by the fact that humans constantly scan objects in the field of view (due to involuntary rapid eye movements), resulting in a perceived image that is uniformly sharp. In fact, when the image is prevented from moving relative to the retina (via an optical fixation device), the eye no longer senses an image after a few seconds. The arrangement of sensory receptors in the outer segments of the retina partially determine the limit of resolution in different regions of the eye. In order to resolve an image, a row of less-stimulated photoreceptors must be interposed between two rows of photoreceptors that are highly stimulated. Otherwise, it is impossible to distinguish whether the stimulation originated from two closely spaced images or from a single image that spans the two receptor rows. With a center-to-center spacing ranging between 1.5 and 2 micrometers for the cones in the central fovea, optical stimuli having a separation of approximately 3 to 4 micrometers should produce a resolvable set of intensities on the retina. For reference, the radius of the first minimum for a diffraction pattern formed on the retina is about 4.6 micrometers with 550-nanometer light and a pupil diameter of 2 millimeters.

Thus, the arrangement of sensory elements in the retina will determine the limiting resolution of the eye. Another factor, termed visual acuity (the ability of the eye to detect small objects and resolve their separation), varies with many parameters, including the definition of the term and the method by which acuity is measured. Over the retina, visual acuity is generally highest in the central fovea, which spans a visual field of about 1.4 degrees. The spatial arrangement of rod and cone cells and their connection to neurons within the retina is presented in Figure 5. Rod cells, containing only the photopigment rhodopsin, have a peak sensitivity to blue-green light (wavelength of about 500 nanometers), although they display a broad range of response throughout the visible spectrum. They are the most common visual receptor cells, with each eye containing about 125-130 million rod cells. The light sensitivity of rod cells is about 1,000 times that of cone cells. However, the images generated by rod stimulation alone are relatively unsharp and confined to shades of gray, similar to those found in a black and white soft-focus photographic image.

Rod vision is commonly referred to as scotopic or twilight vision because in low light conditions, shapes and the relative brightness of objects can be distinguished, but not their colors. This mechanism of dark adaptation enables the detection of potential prey and predators via shape and motion in a wide spectrum of vertebrates. The human visual system response is logarithmic, not linear, resulting in the ability to perceive an incredible brightness range (interscene dynamic range) of over 10 decades. In broad daylight, humans can visualize objects in the glaring light from the sun, while at night large objects can be detected by starlight when the moon is dark. At threshold sensitivity, the human eye can detect the presence of about 100-150 photons of blue-green light (500 nanometers) entering the pupil. For the upper seven decades of brightness, photopic vision predominates, and it is the retinal cones that are primarily responsible for photoreception. In contrast, the lower four decades of brightness, termed scotopic vision, are controlled by the rod cells. Adaptation of the eye enables vision to function under such extremes of brightness.

However, during the interval of time before adaptation occurs, individuals can sense a range of brightness covering only about three decades. Several mechanisms are responsible for the ability of the eye to adapt to a high range of brightness levels. Adaptation can occur in seconds (by initial pupillary reaction) or may take several minutes (for dark adaptation), depending upon the level of brightness change. Full cone sensitivity is reached in about 5 minutes, whereas it requires about 30 minutes to adapt from moderate photopic sensitivity to the full scoptic sensitivity produced by the rod cells. When fully light-adapted, the human eye features a wavelength response from around 400 to 700 nanometers, with a peak sensitivity at 555 nanometers (in the green region of the visible light spectrum). The dark-adapted eye responds to a lower range of wavelengths between 380 and 650 nanometers, with the peak occurring at 507 nanometers. For both photopic and scoptic vision, these wavelengths are not absolute, but vary with the intensity of light. The transmission of light through the eye becomes progressively lower at shorter wavelengths.

In the blue-green region (500 nanometers), only about 50 percent of light entering the eye reaches the image point on the retina. At 400 nanometers, this value is reduced to a scant 10 percent, even in a young eye. Light scattering and absorption by elements in the crystalline lens contributes to a further loss of sensitivity in the far blue. Cones consist of three cell types, each "tuned" to a distinct wavelength response maximum centered at either 430, 535, or 590 nanometers. The basis for the individual maxima is the utilization of three different photopigments, each with a characteristic visible light absorption spectrum. The photopigments alter their conformation when a photon is detected, enabling them to react with transducin to initiate a cascade of visual events. Transducin is a protein that resides in the retina and is able to effectively convert light energy into an electrical signal. The population of cone cells is much smaller than rod cells, with each eye containing between 5 and 7 million of these color receptors. True color vision is induced by the stimulation of cone cells.

The relative intensity and wavelength distribution of light impacting on each of the three cone receptor types determines the color that is imaged (as a mosaic), in a manner comparable to an additive RGB video monitor or CCD color camera. A beam of light that contains mostly short-wavelength blue radiation stimulates the cone cells that respond to 430-nanometer light to a far greater extent than the other two cone types. This beam will activate the blue color pigment in specific cones, and that light is perceived as blue. Light with a majority of wavelengths centered around 550 nanometers is seen as green, and a beam containing mostly 600 nanometer wavelengths or longer is visualized as red. As mentioned above, pure cone vision is referred to as photopic vision and is dominant at normal light levels, both indoors and out. Most mammals are dichromats, usually able to only distinguish between bluish and greenish color components. In contrast, some primates (most notably humans) exhibit trichromatic color vision, with significant response to red, green and blue light stimuli.

Illustrated in Figure 6 are the absorption spectra of the four human visual pigments, which display maxima in the expected red, green, and blue regions of the visible light spectrum. When all three types of cone cell are stimulated equally, the light is perceived as being achromatic or white. For example, noon sunlight appears as white light to humans, because it contains approximately equal amounts of red, green, and blue light. An excellent demonstration of the color spectrum from sunlight is the interception of the light by a glass prism, which refracts (or bends) different wavelengths to varying degrees, spreading out the light into its component colors. Human color perception is dependent upon the interaction of all receptor cells with light, and this combination results in nearly trichromic stimulation. There are shifts in color sensitivity with variations in light levels, so that blue colors look relatively brighter in dim light and red colors look brighter in bright light. This effect can be observed by pointing a flashlight onto a color print, which will result in the reds suddenly appearing much brighter and more saturated. In recent years, consideration of human color visual sensitivity has led to changes in the long-standing practice of painting emergency vehicles, such as fire trucks and ambulances, entirely red.

Although the color is intended for the vehicles to be easily seen and responded to, the wavelength distribution is not highly visible at low light levels and appears nearly black at night. The human eye is much more sensitive to yellow-green or similar hues, particularly at night, and now most new emergency vehicles are at least partially painted a vivid yellowish green or white, often retaining some red highlights in the interest of tradition. When only one or two types of cone cells are stimulated, the range of perceived colors is limited. For example, if a narrow band of green light (540 to 550 nanometers) is used to stimulate all of the cone cells, only the ones containing green photoreceptors will respond to produce a sensation of seeing the color green. Human visual perception of primary subtractive colors, such as yellow, can arise in one of two ways. If the red and green cone cells are simultaneously stimulated with monochromatic yellow light having a wavelength of 580 nanometers, the cone cell receptors each respond almost equally because their absorption spectral overlap is approximately the same in this region of the visible light spectrum.

The same color sensation can be achieved by stimulating the red and green cone cells individually with a mixture of distinct red and green wavelengths selected from regions of the receptor absorption spectra that do not have significant overlap. The result, in both cases, is simultaneous stimulation of red and green cone cells to produce a sensation of yellow color, even though the end result is achieved by two different mechanisms. The ability to perceive other colors requires the stimulation of one, two, or all three types of cone cells, to various degrees, with the appropriate wavelength palette. Although the human visual system features three types of cones cells with their respective color pigments plus light-receptive rod cells for scotopic vision, it is the human brain that compensates for variations of light wavelengths and light sources in its perception of color. Metamers are pairs of different light spectra perceived as the same color by the human brain. Interestingly, colors that are interpreted as the same or similar by a human are sometimes readily distinguishable by other animals, most notably birds. Intermediary neurons that ferry visual information between the retina and the brain are not simply connected one-to-one with the sensory cells.

Each cone and rod cell in the fovea sends signals to at least three bipolar cells, whereas in the more peripheral regions of the retina, signals from large numbers of rod cells converge to a single ganglion cell. Spatial resolution in the outer portions of the retina is compromised by having a large number of rod cells feeding a single channel, but having many sensory cells participate in capturing weak signals significantly improves the threshold sensitivity of the eye. This feature of the human eye is somewhat analogous to the consequence of binning in slow-scan CCD digital camera systems. The sensory, bipolar cells, and ganglion cells of the retina are also interconnected to other neurons, providing a complex network of inhibitory and excitatory pathways. As a result, the signals from the 5 to 7 million cones and 125 million rods in the human retina are processed and transported to the visual cortex by only about 1 million myelinated optical nerve fibers. The eye muscles are stimulated and controlled by ganglion cells in the lateral geniculate body, which acts as a feedback control between the retina and the visual cortex.

The complex network of excitatory and inhibitory pathways in the retina are arranged in three layers of neuronal cells that arise from a specific region of the brain during embryonic development. These circuits and feedback loops result in a combination of effects that produce edge sharpening, contrast enhancement, spatial summation, noise averaging, and other forms of signal processing, perhaps including some that have not yet been discovered. In human vision, a significant degree of image processing takes place in the brain, but the retina itself also is involved in a wide range of processing tasks. In another aspect of human vision known as color invariance, the color or gray value of an object does not appear to change over a wide range of luminance. In 1672, Sir Isaac Newton demonstrated color invariance in human visual sensation and provided clues for the classical theory of color perception and the nervous system. Edwin H. Land, founder of the Polaroid Corporation, proposed the Retinex theory of color vision, based on his observations of color invariance. As long as color (or a gray value) is viewed under adequate lighting, a color patch does not change its color even when the luminance of the scene is changed.

In this case, a gradient of illumination across the scene does not alter the perceived color or gray-level tone of a patch. If the luminance level reaches the threshold for scotopic or twilight vision, the sensation of color vanishes. In Land's algorithm, the lightness values of colored areas are computed, and the energy at a particular area in the scene is compared with all the other areas in the scene for that waveband. The calculations are performed three times, one for each waveband (long wave, short wave, and middle wave), and the resulting triplet of lightness values determines a position for the area in the three-dimensional color space defined by the Retinex theory. The term color blindness is something of a misnomer, being widely used in colloquial conversation to refer to any difficulty in distinguishing between colors. True color blindness, or the inability to see any color, is extremely rare, although as many as 8 percent of men and 0.5 percent of women are born with some form of color vision defect (see Table 1).

Inherited deficiencies in color vision are usually the result of defects in the photoreceptor cells in the retina, a neuro-membrane that functions as the imaging surface at the rear of the eye. Color vision defects can also be acquired, as a result of disease, side effects of certain medications, or through normal aging processes, and these deficiencies may affect parts of the eye other than the photoreceptors. Normal cones and pigment sensitivity enable an individual to distinguish all the different colors as well as subtle mixtures of hues. This type of normal color vision is known as trichromacy and relies upon the mutual interaction from the overlapping sensitivity ranges of all three types of photoreceptor cone. A mild color vision deficiency occurs when the pigment in one of the three cone types has a defect, and its peak sensitivity is shifted to another wavelength, producing a visual deficiency termed anomalous trichromacy, one of three broad categories of color vision defect. Dichromacy, a more severe form of color blindness, or color deficiency, occurs when one of the pigments is seriously deviant in its absorption characteristics, or the particular pigment has not been produced at all.

The complete absence of color sensation, or monochromacy, is extremely rare, but individuals with total color blindness (rod monochromats) see only varying degrees of brightness, and the world appears in black, white, and shades of gray. This condition occurs only in individuals who inherit a gene for the disorder from both parents. Dichromats can distinguish some colors, and are therefore less affected in their daily lives than monochromats, but they are usually aware that they have a problem with their color vision. Dichromacy is subdivided into three types: protanopia, deuteranopia, and tritanopia (see Figure 7). Approximately two percent of the male population inherits one of the first two types, with the third occurring much more rarely. Interactive Java Tutorial Ishihara Color Blindness Test Color blindness, a disruption in the normal functioning of human photopic vision, can be caused by host of conditions, including those derived from genetics, biochemistry, physical damage, and diseases. This interactive tutorial explores and simulates how full-color images appear to colorblind individuals, and compares these images to the Ishihara diagnostic colorblind test.

Protanopia is a red-green defect, resulting from loss of red sensitivity, which causes a lack of perceptible difference between red, orange, yellow, and green. In addition, the brightness of red, orange, and yellow colors is dramatically reduced in comparison to normal levels. The reduced intensity effect can result in red traffic lights appearing dark (unlit), and red hues (in general), appearing as black or dark gray. Protanopes often learn to correctly distinguish between red and green, and red from yellow, primarily based on their apparent brightness, rather than on any perceptible hue difference. Green generally appears lighter than red to these individuals. Because red light occurs at one end of the visible spectrum, there is little overlap in sensitivity with the other two cone types, and people with protanopia have a pronounced loss of sensitivity to light at the long-wavelength (red) end of the spectrum. Individuals with this color vision defect can discriminate between blues and yellows, but lavender, violet, and purple cannot be distinguished from various shades of blue, due to the attenuation of the red component in these hues.

Individuals with deuteranopia, which is a loss of green sensitivity, have many of the same problems with hue discrimination as do protanopes, but have a fairly normal level of sensitivity across the visible spectrum. Because of the location of green light in the center of the visible light spectrum, and the overlapping sensitivity curves of the cone receptors, there is some response of the red and blue photoreceptors to green wavelengths. Although deuteranopia is associated with at least a brightness response to green light (and little abnormal intensity reduction), the names red, orange, yellow, and green seem to the deuteranope to be too many terms for colors that appear the same. In a similar fashion, blues, violets, purples, and lavenders are not distinguishable to individuals with this color vision defect. Color Blindness Incidence and Causes Classification < 0.0001 Table 1 Tritanopia is the absence of blue sensitivity, and functionally produces a blue-yellow defect in color vision. Individuals with this deficiency cannot distinguish blues and yellows, but do register a difference between red and green. The condition is quite rare, and occurs about equally in both sexes.

Tritanopes usually do not have as much difficulty in performing everyday tasks as do individuals with either of the red-green variants of dichromacy. Because blue wavelengths occur only at one end of the spectrum, and there is little overlap in sensitivity with the other two cone types, total loss of sensitivity across the spectrum can be quite severe with this condition. When there is a loss of sensitivity by a cone receptor, but the cones are still functional, resulting color vision deficiencies are considered anomalous trichromacy, and they are categorized in a similar manner to the dichromacy types. Confusion often arises because these conditions are named similarly, but appended with a suffix derived from the term anomaly. Thus, protanomaly, and deuteranomaly produce hue recognition problems that are similar to the red-green dichromacy defects, though not as pronounced. Protanomaly is considered a "red weakness" of color vision, with red (or any color having a red component) being visualized as lighter than normal, and hues shifted toward green. A deuteranomalous individual exhibits "green weakness", and has similar difficulties in discriminating between small variations in hues falling in the red, orange, yellow, and green region of the visible spectrum.

This occurs because the hues appear to be shifted toward red. In contrast, deuteranomalous individuals do not have the brightness loss defect that accompanies protanomaly. Many people with these anomalous trichromacy variants have little difficulty performing tasks that require normal color vision, and some may not even be aware that their color vision is impaired. Tritanomaly, or blue weakness, has not been reported as an inherited defect. In the few cases in which the deficiency has been identified, it is thought to have been acquired rather than inherited. Several eye diseases (such as glaucoma, which attacks the blue cones) can result in tritanomaly. Peripheral blue cone loss is most common in these diseases. In spite of the limitations, there are some visual acuity advantages to color blindness, such as the increased ability to discriminate camouflaged objects. Outlines, rather than colors, are responsible for pattern recognition, and improvements in night vision may occur due to certain color vision deficiencies. In the military, colorblind snipers and spotters are highly valued for these reasons. During the early 1900s, in an effort to evaluate abnormal human color vision, the Nagel anomaloscope was developed.

Utilizing this instrument, the observer manipulates control knobs to match two colored fields for color and brightness. Another evaluation method, the Ishihara pseudoisochromatic plate test for color blindness, named for Dr. Shinobu Ishihara, discriminates between normal color vision and red-green color blindness (as presented in the tutorial and Figure 7). A test subject with normal color vision can detect the hue difference between the figure and background. To an observer with red-green deficiency, the plates appear isochromatic with no discrimination between the figures and the design pattern. As a natural part of the aging process, the human eye begins to perceive colors differently in later years, but does not become "colorblind" in the true sense of the term. Aging results in the yellowing and darkening of the crystalline lens and cornea, degenerative effects that are also accompanied by a shrinking of the pupil size. With yellowing, shorter wavelengths of visible light are absorbed, so blue hues appear darker. As a consequence, elderly individuals often experience difficulty discriminating between colors that differ primarily in their blue content, such as blue and gray or red and purple.

At age 60, when compared to the visual efficiency of a 20-year old, only 33 percent of the light incident on the cornea reaches the photoreceptors in the retina. This value drops to around 12.5 percent by the mid-70s. Interactive Java Tutorial Human Eye Accommodation Accommodation of the eye refers to the physiological act of adjusting crystalline lens elements to alter the refractive power and bring objects that are closer to the eye into sharp focus. This tutorial explores changes in the lens structure as objects are relocated with respect to the eye. Accommodation of the eye refers to the act of physiologically adjusting the crystalline lens element to alter the refractive power and bring objects that are closer to the eye into sharp focus. Light rays initially refracted at the surface of the cornea are further converged after passing through the lens. During accommodation, contraction of the ciliary muscles relaxes tension on the lens, resulting in changes to the shape of the transparent and elastic tissue, while also moving it slightly forward. The net effect of the lens alterations is to adjust the focal length of the eye to bring the image exactly into focus onto the photosensitive layer of cells residing in the retina.

Accommodation also relaxes the tension applied to the lens by the zonule fibers, and allows the anterior surface of the lens to increase its curvature. The increased degree of refraction, coupled with a slight forward shift in the position of the lens, brings objects that are closer to the eye into focus. Focus in the eye is controlled by a combination of elements including the iris, lens, cornea, and muscle tissue, which can alter the shape of the lens so the eye can focus on both nearby and distant objects. However, in some instances these muscles do not work properly or the eye is slightly altered in shape, and the focal point does not intersect with the retina (a condition termed convergent vision). As individuals age, the lens becomes harder and cannot be properly focused, leading to poor vision. If the point of focus falls short of the retina, the condition is referred to as nearsightedness or myopia, and individuals with this affliction cannot focus on distant objects. In cases where the focal point is behind the retina, the eye will have trouble focusing on nearby objects, creating a condition known as farsightedness or hypermetropia.

These malfunctions of the eye can usually be corrected with eyeglasses (Figure 8) using a concave lens to treat myopia and a convex lens to treat hypermetropia. Convergent vision is not totally physiological and can be influenced by training, if the eyes are not defective. Repetitive procedures can be utilized to develop strong convergent vision. Athletes, such as baseball shortstops, have well-developed convergent vision. In every movement, the two eyes have to translate in unison to preserve binocular vision, with an accurate and responsive neuromuscular apparatus that is not usually subject to fatigue, controlling their motility and coordination. Changes in ocular convergence or head motion are considered in the calculations made by the complex ocular system to produce the proper neural inputs to the eye muscles. An eye movement of 10 degrees may be completed in about 40 milliseconds, with the calculations occurring faster than the eye can reach its intended target. Small eye movements are known as saccades and the larger movements from one point to another are termed versions. The human visual system must not only detect light and color, but as an optical system, must be able to discern differences among objects, or an object and its background.

Known as physiological contrast or contrast discrimination, the relationship between the apparent brightness of two objects that are seen either at the same time (simultaneous contrast) or sequentially (successive contrast) against a background, may or may not be the same. In the human visual system, contrast is reduced in environmental darkness and with individuals suffering from color visual deficiencies such as red-green color blindness. Contrast is dependent on binocular vision, visual acuity, and image processing by the visual cortex of the brain. An object with low contrast, which cannot be distinguished from the background unless it is moving, is considered camouflaged. However, colorblind individuals are often able to detect camouflaged objects because of increased rod vision and loss of misleading color cues. Increasing contrast translates into increased visibility, and a quantitative numerical value for contrast is usually expressed as a percentage or ratio. Under optimal conditions, the human eye can barely detect the presence of two percent contrast. With human vision, an apparent increase in contrast is perceived in a narrow zone on each side of the boundary between two areas of different brightness and/or chromaticity. At the end of the nineteenth century, French physicist Michel Eug�ne Chevreul discovered simultaneous contrast.

As a special function of human visual perception, the edges or contour of an object are highlighted, setting the object away from its background and easing spatial orientation. When positioned over a bright background, the region at the edge of a dark object appears lighter than the rest of the background (in effect, the contrast is enhanced). With this perception phenomenon, the color with the strongest contrast, the complementary color, is created (by the brain) at the edge. Because the color and its complement are perceived simultaneously, the effect is known as simultaneous contrast. Borders and other lines of demarcation that separate the contrasting areas tend to lessen the effect (or optical illusion) by eliminating marginal contrast. Many forms of optical microscopy, most notably phase contrast illumination, take advantage of these features of the human visual system. By increasing the physical contrast of an image without having to change the object via staining or other technique, the phase contrast specimen is protected from damage or death (in the case of living specimens). The spatial frequency response of the human eye can be evaluated by determining the ability to detect a series of strips in a modulated sinusoidal grating.

Test gratings feature alternating regions (strips) of light and dark, which increases linearly from higher to lower frequencies along the horizontal axis while contrast decreases logarithmically from top to bottom. The boundary of stripes that can just be distinguished by individuals with normal vision is between 7 and 10 cycles per degree. For achromatic vision, when the spatial frequency is very low (broad line spacing), high contrast is required to detect the sinusoidally varying intensity. As the spatial frequency rises, humans can detect periods with less contrast, reaching a peak of about 8 cycles per degree in the visual field. Beyond that point, higher contrast is again required to detect the finer sinusoidal stripes. Examination of the modulation transfer function (MTF) of the human visual system reveals that the contrast necessary to detect the luminance variation in standardized sinusoidal gratings increases at both higher and lower spatial frequencies. In this regard, the eye behaves quite differently from a simple imaging device (such as a film camera or CCD sensor). The modulation transfer function of a simple, focused camera system displays a maximum modulation at zero spatial frequency, with the degree of modulation dropping more or less monotonically to zero at the camera's cutoff frequency.

When the luminance of a scene fluctuates periodically several times a second (as it does with television and computer monitor screens), humans perceive an irritating sensation, as though the sequential scenes were disjoined. When the fluctuation frequency increases, irritation increases and reaches a maximum at around 10 hertz, especially when bright flashes of illumination alternate with darkness. At higher frequencies, the scene no longer appears disjointed, and objects displaced from one scene to the next are now perceived to be moving smoothly. Commonly referred to as flicker, the annoying light fluttering sensation can persist up to 50-60 hertz. Beyond a certain frequency and luminance, known as the critical flicker frequency (CFF), screen flicker is no longer perceived. This is the primary reason why increasing the refresh rate of a computer monitor from 60 to 85-100 hertz produces a stable, flicker-free display. Advances in semiconductor fabrication technology, especially complementary metal oxide semiconductors (CMOS) and bipolar CMOS (BiCMOS) techniques, has led to a new generation of miniature photosensors that feature extraordinary dynamic range and fast response.

Recently, arrays of CMOS sensor chips have been arranged to model the operation of the human retina. These so-called eye chips, by combining optics, human vision, and microprocessors, are advancing ophthalmology through the new field of optobionics. Damaged retinas resulting from debilitating visual diseases, such as retinitis pigmentosa and macular degeneration, as well as aging and injuries to the retina, which rob vision, are being corrected with the implanted eye chips. The silicon eye chips contain approximately 3,500 miniature light detectors attached to metal electrodes that mimic the function of the human rods and cones. The light detectors absorb incident light refracted by the cornea and lens and produce a small quantity of electrical charge that stimulates the retinal neurons. Featuring a diameter of two millimeters (see Figure 9), the replacement retina is half as thick as a typical piece of paper and is implanted into a pocket under the damaged retina. As an alternative to the eye chip, a retinal prosthesis using a digital signal processor and a camera mounted on a pair of glasses, captures and transmits an image of an object or scene.

Wirelessly, the image is sent to an embedded receiver chip near the retinal layers where nerve impulses are sent to the brain. Artificial retinas, however, will not treat glaucoma or vision deficiencies that damage the nerve fibers leading to the optic nerve. As optobionics advances, so does science's understanding of the complex human visual system. Contributing Authors Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657. Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310. Molecular Expressions: Science, Optics, and You: Light and Color - Human Vision and Color Home Human Vision and Color Perception Human vision is a complex process that is not yet completely understood, despite hundreds of years of study and research. The complex physical process of visualizing something involves the nearly simultaneous interaction of the eyes and the brain through a network of neurons, receptors, and other specialized cells.

The human eye is equipped with a variety of optical elements including the cornea, iris, pupil, a variable-focus lens, and the retina, as illustrated above in Figure 1. Together, these elements work to form images of the objects in a person's field of view. When an object is observed, it is first focused through the cornea and lens onto the retina, a multilayered membrane that contains millions of light-sensitive cells that detect the image and translate it into a series of electrical signals. These image capturing receptors of the retina are termed rods and cones, and are connected with the fibers of the optic nerve bundle through a series of specialized cells that coordinate the transmission of the electrical signals to the brain. In the brain, the optic nerves from both eyes join at the optic chiasma where information from their retinas is correlated. The visual information is then processed through several steps, eventually arriving at the visual cortex, which is located on the lower rear section of each half of the cerebrum. Interactive Tutorial Human Vision Explore how images are produced on the retina of the human eye as an object is moved closer or farther away.

A particularly specialized component of the eye is the fovea centralis, which is located on the optical axis of the eye in an area near the center of the retina. This area exclusively contains high-density tightly packed cone cells and is the area of sharpest vision. The density level of cone cells decreases outside of the fovea centralis and the ratio of rod cells to cone cells gradually increases. At the periphery of the retina, the total number of both types of light receptors decreases substantially, causing a dramatic loss of visual sensitivity at the retinal borders. This is offset, however, by the fact that humans constantly scan objects in their field of view, usually resulting in a perceived image that is uniformly sharp. A graphical illustration of the spatial arrangement of rod and cone cells and their connection to neurons within the retina can be seen below in Figure 2. Rod cells are most sensitive to green wavelengths of light (about 550-555 nanometers), although they display a broad range of response throughout the visible spectrum. They are the most populous visual receptor cells in humans, each eye containing about 130 million rods. Extremely responsive, the light sensitivity of rod cells is about 1000 times that of cone cells.

However, the images generated by rod stimulation alone are relatively unsharp and confined to shades of gray, similar to those found in a black and white soft-focus photographic image. Rod vision is commonly referred to as scotopic or twilight vision because in low light levels it enables individuals to distinguish shapes and the relative brightness of objects, but not their colors. Cones, on the other hand, consist of three different types of cells, each "tuned" to a distinct wavelength peak of response centered at either 430, 535, or 590 nanometers. Often referred to as photopic vision, cone vision is dominant at normal light levels, both indoors and out. The quantity of cone cells possessed by humans, however, is much smaller than the number of rod cells, each eye only containing about 7 million cones. Stimulation of these visual receptors results in what is known as true color vision. The relative intensity of the stimulation incurred by each of the three types of cone receptors is what largely determines which color is imaged.

For example, a beam of light that contains mostly blue short-wavelength radiation stimulates the cone cells that respond to 430-nanometer light far more than the other two cone types and, therefore, that light is seen as blue. Correspondingly, light with a majority of wavelengths centered around 550 nanometers appears green, and a beam containing mostly 600 nanometer wavelengths or longer is seen as red. When all three types of cone cells are stimulated equally, light is perceived as being achromatic or white. For instance, noon sunlight appears to humans as white light because it contains approximately equal amounts of red, green, and blue light, uniformly stimulating all types of cone receptors. Normal cones and pigment sensitivity enable humans to distinguish all of the different colors as well as subtle mixtures of hues. This type of color vision is known as trichromacy and relies upon the mutual interaction of all three types of photoreceptor cones. Yet, human color perception is also dependent upon illumination levels, shifts in color sensitivity occurring whenever lighting is varied. For example, blue colors look relatively brighter in dim light, while red colors appear more vivid in bright light.

This effect can be simply observed by pointing a flashlight onto a color print, which results in the reds suddenly appearing much brighter and more saturated. Focus in the eye is controlled by a combination of elements, including the iris, lens, cornea, and muscle tissue. Properly functioning together, these components can alter the shape of the lens so that the eye can focus on both nearby and distant objects. However, in some instances the components do not work correctly or the eye is slightly altered in shape and the focal point does not intersect with the retina. As people age, for instance, the lenses of their eyes become harder and cannot be focused properly, which results in poor vision. If the point of an eye's focus is short of the retina, as illustrated in the lower part of Figure 3, the condition is called nearsightedness or myopia. People with this affliction are unable to focus on distant objects. In cases where the eye's focal point is behind the retina, as illustrated in the upper part of Figure 3, people have trouble focusing on nearby objects, which is a condition called hypermetropia, commonly known as farsightedness.

These malfunctions of the eye can usually be corrected through the use of glasses, with concave lenses correcting myopia and convex lenses rectifying hypermetropia. Contributing Authors Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747. Shannon H. Neaves and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310. The Color-Sensitive Cones The Color-Sensitive Cones In 1965 came experimental confirmation of a long expected result - there are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors. Painstaking experiments have yielded response curves for three different kind of cones in the retina of the human eye. The "green" and "red" cones are mostly packed into the fovea centralis . By population, about 64% of the cones are red-sensitive, about 32% green sensitive, and about 2% are blue sensitive.

The "blue" cones have the highest sensitivity and are mostly found outside the fovea. The shapes of the curves are obtained by measurement of the absorption by the cones, but the relative heights for the three types are set equal for lack of detailed data. There are fewer blue cones, but the blue sensitivity is comparable to the others, so there must be some boosting mechanism. In the final visual perception, the three types seem to be comparable, but the detailed process of achieving this is not known. When light strikes a cone, it interacts with a visual pigment which consists of a protein called opsin and a small molecule called a chromophore which in humans is a derivative of vitamin A. Three different kinds of opsins respond to short, medium and long wavelengths of light and lead to the three response curves shown above. For a person to see an object in color, at least two kinds of cones must be triggered, and the perceived color is based on the relative level of excitation of the different cones. A fourth pigment, rhodopsin, is employed in rod vision . Sensitivity of the human eye Sensitivity of the human eye This page summarizes the basic characteristics of sensitivity of the human eye.

This is useful when converting light intensities from luminous units like Lumens (lm) that take into account eye perception into radiometric units like Watts (W) that only take into account physical aspects. A light source radiating 1 W of green light will appear much brighter than another source radiating the same amount of power of red light because the eye is more sensitive in the green region. The human eye is sensible to light wave which wavelength is roughly between 400 nm (violet) and 700 nm (red). Wavelengths shorter than 400 nm (ultraviolet, UV) or longer than 700 nm (infrared, IR) are not visible. The eye behaves differently in high or low light conditions: in daylight, for brightness levels above 3 cd/m2 the vision is mainly done by the centre of the retina, we can see colors and the maximum sensitivity is at 555 nm (in the green region). This type of vision is called photopic vision.

In low light conditions, for brightness levels below 30 μcd/m2, the vision is mainly done by the peripheral region of the retina which is color-blind, while the centre region is not sensitive enough to see any color. This type of vision is called scotopic vision. Maximum sensitivity is at 507 nm (in the blue-green region) and red light is almost invisible. The vision in-between photopic and scotopic vision is called mesopic. The nice thing about scotopic vision and its reduced sensitivity to red light is that you can use a red flashlight to illuminate an object (to read a map, for example) without disturbing your nigh vision. It takes several minutes to get your vision used to the darkness; if you used a white flashlight instead, the eyes will switch back to photopic vision and you'll have to wait a few minutes again. Red light can directly be picked up by the centre of the retina without affecting night vision. For this reason, astronomers often use red flashlights. The standard eye response is visible in the two figures below in both linear and logarithmic scale.

This standard eye sensitivity is also called standard luminosity function V(λ) and is used, for photopic vision, to define a conversion between the radiated energy (in Watts) and the luminous flux (in Lumen). The standard luminosity function V'(λ) refers to scotopic vision, but it shouldn't be used to convert to and from photometric units. A tabulated form of the two standard luminosity functions V(λ) and V'(λ) are represented hereafter: Wavelength

Question: To what colour is the human eye most sensitive?
Answer: Green