Colors and the perception of colors
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Colors and the perception of colorsVisible light is only a small member of the family of electromagnetic(EM) waves. The wavelengths of EM waves that we can observe usingmany different devices span from tens of kilometers (long radio waves) to picometers (gamma rays, i.e., EM radiation produced by radioactivenuclei). The range of wavelengthswe can see by our eyes isrelatively narrow, spanningfrom about 700 nm (red co-lor to about 400 nm (violetlight). If light is a mixtureof all wavelengths from thisrange, we see it as white.
White light can be split intoconstituent wavelengths(or colors) using a prism ora grating.
We can see colors because the retina i.e.,the light-sensitive organ in our eyes con-tains photosensitive cell (called cones) ofthree types: one is most sensitive to red light, another to green light, and the thirdtype to blue light.Most of the conesare located in theretinas centralarea (the macula).In addition to thecones, in retina there are also cells called rods.Many rods arelocated in the ma-cula, and even more in the peripheral regions of the retina.
Rods and cones in the retina:
Rods are the night-vision cells. They are activated in low light conditions.However, thy are not sensitive to colors. Therefore, a landscape viewed inmoonlight seems gray. Actually, the colors are the same as in daylight. We cannot seethem, but cameras can!This picture was taken at10 pm in Febru-ary 2007, at full moonlight,using a verylong exposuretime. Tiny Specs In the sky are stars.
Another picture takes the same night as that in the preceding slide. Thepeak right from the center is Marys Peak. The spots in the lower part arebright windows of residential houses.
What causes the well-known red-eye effect in flash pictures?This is nothing else than the color of the retina! The retina needsmuch blood, which is supplied to it by a dense web of tiny bloodvessels. Therefore, the red color.
However, there is no red-eye effect in flash pictures of many animals.In contrast, their eyes seem to backreflect the flash. This is the same effects as the eyeshine you can see if you drive on a rural highway inthe night, and a cat or a dog caught in your headlights looks toward theapproaching car.
The eyeshine effect is caused by anextra layer of special tissue calledtapetum lucidum that is located behindthe retina of many animals especially,nocturnal animals. The tapetum lucidumact as a backreflecting mirror.
Humans and diurnal animals do not havetapetum lucidum in their eyes. Most birdspecies do not have it owls are an ex-ception. But nocturnal animals carnivo-res in particular -- need to see well theirprey in low light condition, and tapetumlucidum does enhance their nightimevision can you explain how? (if you missed the lecture at which we talked about that, you may find an explanationin this webpage). Most primates i.e., members of the biological order we belongto do not have tapetum lucidum. Lemurs small cat-like nocturnalprimates, unique to the island ofMadagascar, are an exception.
The sensitivity of the three types of human retina cones to light ofdifferent wavelengths from the visible region. Note that there isanother smaller maximum for the red cones in the violet region.It causes that violet light looks somewhat reddish to us.
From the curves one can read the relative strength of the signal passed to the brain for a given wavelength. A triad of such numbers is called thetristimulus values.
600 nm546 nm436 nmThe RGB color scheme fundamentals: Lets take threepure (monochromatic) colors corresponding to the maximum sensitivity of the three cone types: When projected on one screen, the area where allthree colors merge appears white. The fusion of redand blue produces magenta, blue and green cyan,and green and red yellow. One can also say:red + cyan = white, blue + yellow = white, green + magenta = white. Such pairs: red-cyan, blue-yellow,green-magenta are called complementary colors(they are not the only complementary color pairs).
Link to an on-line generator of RGB color schemesThe preceding slide showed the results of mixing (or ad-ding) primary colors, each of which had 100% intensity.
But one can add colors with any intensity proportions!(e.g., 80% red, 45% green, and 23% blue). This is theidea of a color scheme, known as RGB, widely used in computer graphics.
However, in the practical RGB scheme, instead of0 100% scale, one uses a 0-255 scale (correspondingto a Byte, i.e., a binary number of 8 Bits). So, for instance,80% translates to 255x(80/100) = 204. The color in the above example is then encoded as 204, 115, 59.
The best way of demonstrating how it works is to useone of the many available Web on-line RGB generators.
In the simple figure with the mixing of the three primary colors(a.k.a. the primes -- red, green and blue) there are only eightcolors altogether (the three primes, three complementary to theprimes, white, and black. In the RGB scheme white is 255, 255, 255,black is 0, 0, 0, cyan is 0, 255, 255, and so on. But since we can use the 0-255 scale, the total number of available colors is (256)3 = 16.7 million!
However, RGB is only one of the possible schemes of describingcolors. Another, no less popular, is the one called HSB or HSV,where H stands for Hue, S for Saturation, and B for brightness(or V for Value). The idea of the HSB (HSV) scheme is explainedin the next slide.
Begin with the familiar figure:Next, arrange the three primesin a circle. Then, insertthe complementarycolors in between.Next, merge theprimes with theadjacent comple-mentary colors toobtain more piechunks (red plusyellow yield orange,and so on) keepdoing that until youget a continuous distributionof colors. Such a figure is called the RGB Color Wheel. Now, in order to describe the colors, one has to intro-duce a numerical scale. Most often a 0 360 scale is used, with 0 at the top red, and the angle incrementingclockwise. So, e.g., yellow is 60, pure green 120, cyan180, and so on. And this angular value is called the hue of a given color.
Saturation: at the centerof the color wheel, all colors merge to form white. So, the closer to the center you go, the more white componentis admixed to the color or, one can say, the more The color is de-saturated.Colors on the rim have 100%saturation, white = 0% saturation. Brightnes (or Value): you can think of it asan admixture of grayto your color describedby H and S. 0% is totalblackness. Its an ana-log of the brightness ingrayscale graphics (black and white pictures) for a colorscale.Brightness, in the HSB color scheme:0%100%Brightness, in grayscale (black and white) scheme:0%100%
RGB to HSB on-line converterFancy on-line HSB color picker with convenient mouse-operatednumber inputs
RGB HSB generator/converter
A sophisticated HSB tool, with the names of colors created theangle changes counterclockwise