Colors and the perception of colors

1. Colors and the perception of colors Visible light is only a “small member” of the “family” of electromagnetic (EM) waves. The wavelengths of EM waves that we can observe using many different devices span from tens of kilometers (long radio waves) to picometers (gamma rays, i.e., EM radiation produced by radioactive nuclei). The range of wavelengths we can see by our eyes is relatively narrow, spanning from about 700 nm (red co- lor to about 400 nm (violet light). If light is a “mixture” of all wavelengths from this range, we see it as white. White light can be split into constituent wavelengths (or colors) using a prism or a grating.

2. We can see colors because the retina – i.e., the light-sensitive organ in our eyes – con- tains photosensitive cell (called “cones”) of three types: one is most sensitive to red light, another to green light, and the third type to blue light. Most of the cones are located in the retina’s central area (the macula). In addition to the cones, in retina there are also cells called “rods”. Many rods are located in the ma- cula, and even more in the peripheral regions of the retina.

3. Rods and cones in the retina:

4. Rods are the “night-vision” cells. They are activated in low light conditions. However, thy are not sensitive to colors. Therefore, a landscape viewed in moonlight seems gray. Actually, the colors are the same as in daylight. We cannot see them, but cameras can! This picture was taken at 10 pm in Febru- ary 2007, at full moonlight, using a very long exposure time. Tiny Specs In the sky are stars.

5. Another picture takes the same night as that in the preceding slide. The peak right from the center is Mary’s Peak. The spots in the lower part are bright windows of residential houses.

6. What causes the well-known “red-eye effect” in flash pictures? This is nothing else than the color of the retina! The retina needs much blood, which is supplied to it by a dense web of tiny blood vessels. Therefore, the red color.

7. 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 in the night, and a cat or a dog caught in your headlights looks toward the approaching car.

8. The “eyeshine effect” is caused by an extra layer of special tissue called tapetum lucidum that is located behind the retina of many animals – especially, nocturnal animals. The tapetum lucidum act as a backreflecting mirror. Humans and diurnal animals do not have tapetum lucidum in their eyes. Most bird species do not have it – owls are an ex- ception. But nocturnal animals – carnivo- res in particular -- need to see well their prey in low light condition, and tapetum lucidum does enhance their nightime vision – can you explain how? (if you missed the lecture at which we talked about that, you may find an explanation in this webpage). Most primates – i.e., members of the biological order we belong to – do not have tapetum lucidum. Lemurs – small “cat-like” nocturnal primates, unique to the island of Madagascar, are an exception.

9. The sensitivity of the three types of human retina cones to light of different wavelengths from the visible region. Note that there is another 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 the tristimulus values.

10. The RGB color scheme – fundamentals: Let’s take three pure (monochromatic) colors corresponding to the maximum sensitivity of the three cone types: 600 nm 546 nm 436 nm When projected on one screen, the area where all three colors merge appears white. The fusion of red and 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).

11. The 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 the idea of a color scheme, known as RGB, widely used in computer graphics. However, in the practical RGB scheme, instead of 0 – 100% scale, one uses a 0-255 scale (corresponding to 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 use one of the many available Web on-line RGB generators. Link to an on-line generator of RGB color schemes

12. In the simple figure with the mixing of the three primary colors (a.k.a. the “primes” -- red, green and blue) there are only eight colors altogether (the three primes, three complementary to the primes, 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 describing colors. 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 explained in the next slide.

13. Begin with the familiar figure: Next, arrange the three “primes” in a circle. Then, insert the complementary colors in between. Next, merge the primes with the adjacent comple- mentary colors to obtain more “pie chunks” (red plus yellow yield orange, and so on) – keep doing that until you get a continuous distribution of 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 incrementing clockwise. So, e.g., yellow is 60°, pure green 120°, cyan 180°, and so on. And this angular value is called “the hue” of a given color.

14. Saturation: at the center of the color wheel, all colors merge to form white. So, the closer to the center you go, the more white component is “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 as an “admixture of gray” to your color described by H and S. 0% is total blackness. It’s an ana- log of the brightness in grayscale graphics (“black and white pictures”) for a color scale. Brightness, in the HSB color scheme: 0% 100% Brightness, in grayscale (“black and white”) scheme: 100% 0%

15. RGB to HSB on-line converter Fancy on-line HSB “color picker” with convenient mouse-operated number inputs RGB  HSB generator/converter A sophisticated HSB tool, with the names of colors created – the angle changes counterclockwise