COLOR, VISION AND VIDEO

Transcription

1 COLOR, VISION AND VIDEO COLOR This section is devoted to describing color as a perception, the rules to combine colors and ways to obtain them. Color as a perception: Color is a human perception, not anything that characterizes light. When light of a given spectral composition shines on the retina of the eye, the light signal is transformed into other forms of energy by the light detectors in the eye. The information is then transmitted, processed and interpreted by our neurological system and the final result is our perception of color. The whole process of color perception is very complex and not fully understood due to our current limited knowledge about the connections between physical-chemical processes and psychology. It is important to emphasize again that light does not have color although common talk refers to red light, blue light, yellow light and so on. When we talk about a yellow light what is meant is light that produces the perception of yellow color. What light has is wavelength and frequency compositions, speeds, amplitudes and so on. By perceiving light with a certain color, we can not know the spectral composition of it. For example, if we perceive a certain light as yellow, the light could be just pure spectral yellow or also a combination of spectral green and red. The only way to find the spectral composition of a particular light is by separating its spectral components using dispersion with a prism or by other means. Our perception of color contains three attributes or qualities. They are hue, saturation and brightness. Hue is the adjective used to describe a color as blue, red, yellow, and so on. White, gray, and black lack any hue and are called achromatic. White and black are considered extreme cases of gray. Saturation is related to the amount of white that a color contains; for example, if a beam of pure spectral red light is combined with increasing amounts of white light, the perceived color will go from pure spectral red to pink and finally to white. Spectral red will be completely saturated red, pink can be described as unsaturated red and white is completely unsaturated. Brightness refers to the perceived intensity of light. The following example illustrates the concepts of hue, saturation, and brightness: The Sun is seen at noon to have a yellow hue, which is strongly unsaturated and very bright. At sunset the hue has shifted to red and the color is more saturated and dimmer. Color perception is much richer than mentioned here so far but they are beyond the scope of this introductory material and won t be explained. An example is the effect of contrast

2 in how colors are perceived ( the same color is perceived slightly differently in the presence of other colors ). Color mixing: Most people are somehow familiar with the process of mixing two colors to obtain a third color. The mixing can be done by mixing lights ( like in a computer monitor ) or by mixing paints ( like in paintings or printing ). When lights are mixed the process is called color addition. When paints are mixed the process is called color subtraction. The results obtained by color addition and subtraction are very different. Color addition: As mentioned above, when two light beams of different colors are added, a third color is perceived. The result depends on the color of the added beams and also on the relative intensities of each other. For example: If the relative brightness of added green and red lights is changed, the resulting color will range from pure green through olive green, yellow, orange, and finally red. If the relative brightness is approximately the same, then adding green and red lights produces yellow light. The range of colors obtained when a specific set of two or more colors are added is called the gamut of that set of colors. When two colors are added together and produce white they are called complementary colors. When three colors are added together and produce white, they are called a set of primary colors. There are several combinations of three colors that produce white when combined and therefore there are several sets of primary colors. The most widely used set of primary colors is composed of red ( R ), green ( G ), and blue ( B ). Additions of these RGB set of primary colors in approximately equal proportions, produces the results shown in the picture and schematic addition relations below. G + R Y ( yellow ) B + R M ( magenta ) B + G C ( cyan ) R + G + B --- W ( white ) Y + B ---- W ( white ) M + G ---- W ( white ) C + R ---- W ( white ) Y, M and C are called the secondary additive colors. The primary ( R, G, and B ) and secondary ( C, M, and Y ) colors for color addition can be conveniently represented in a color wheel in such a way that complementary colors are opposite to each other and secondary colors appear in between the primary colors that form them. This wheel is useful to very quickly remember the color addition relations and is represented on the right. C.I.E chromaticity diagram: In the previous section it was mentioned that many colors can be reproduced by the combination of a specific set of two or more colors. You might

3 wonder if the common set of primaries RGB ( or any other finite set of colors ) is able to reproduce all colors that humans can perceive. The answer is not. There is not any finite set of colors able to reproduce the full range of perceivable colors. The set RGB reproduces a large range of perceivable colors but not all. The C.I.E. chromaticity diagram ( below you can see a crude approximation of it ) is supposed to show all perceivable colors. Spectral colors are represented on the curved boundary with their wavelengths marked. Purples ( which are not spectral colors ) are represented on the lower straight segment joining red and violet. White is represented at point E inside the diagram. Spectral colors R, G and B are represented by the small circles on the curve. On the diagram, the color produced by the combination of two colors is located at a point on the segment joining the position of the two colors. The colors reproduced by the primaries RGB are bounded by the triangle formed by the R, G, and B points on the diagram. Therefore, many other colors fall outside the triangle, and we understand why the primary set RGB does not reproduce all possible colors. Color subtraction: There are many examples of situations where color subtraction takes place. For example, it happens when paints or inks are mixed by an artist or a color printer; or when light passes through a sequence of colored light filters. By convention, a color light filter is named by the color of the light that emerges from it when it is placed in a white beam of light. The process by which some colors are absorbed and others are not is called selective absorption. If the emerging light is described in terms of the additive primary colors RGB, filters would behave as follows:

4 Red filter: R light passes. G and B are absorbed. Green filter: G light passes. R and B are absorbed. Blue filter: B light passes. R and G are absorbed. Magenta filter: R and B pass. G is absorbed. Yellow filter: R and G pass. B is absorbed. Cyan filter: G and B pass. R is absorbed. Knowing what each filter does enables the prediction of the color of light emerging by a sequence of filters. For example: What is the color of light ( if any ) emerging from a sequence of magenta and yellow filters when white light is incident on the system? To find the answer, let us assume that the magenta filter is placed before the yellow filter. Under those conditions, white light reaches the magenta filter and only R and B primaries emerge and reach the yellow filter. However, the yellow filter allows the R component to pass but absorbs B. Therefore, only R light emerges from the combination of magenta and yellow filters. The procedure can be repeated with any combination of filters to find the outcome. In particular, if all three filters are used in sequence, no light will emerge ( this corresponds to black ). The outcome of mixing paints and inks can also be understood following the same procedure illustrated above with filters. The essence is that a paint of a certain color is composed of pigments or dyes which work in the same way as a filter because they absorb certain colors and not others ( the colors that are not absorbed are said to be reflected ). Therefore, the result of mixing a set of paints of different colors is equivalent to using a set of filters of the same colors. It can then be understood that the color of an illuminated object ( not a source of light ) is a result of selective light absorption on its surface. It must be mentioned that the outcome of mixing paints and inks can also be obtained using concepts such as subtractive primary colors and corresponding wheel or wheels and in fact this procedure is used by artists. This approach was not used here because the intent was to give a more basic science understanding to the topic. To finish, it is worth pointing the reason for the terms additive and subtractive color mixing. The point is that in additive color mixing, lights of different colors are added together. The more lights are added, the brighter the resulting colored light. With color subtraction on the other hand, as a colored paint is added, some brightness is lost because the paint acts as a filter, taking away part of the light. That is why the larger the number of paints mixed, the dimmer the light that comes from it.

5 HUMAN VISION: Once an image of an object is formed on the retina of the eye, the photoreceptors in the retina undergo photochemical changes that generate electrical pulses which are sent to the brain via the optic nerve. The brain then, makes the final processing and interpretation of the information and the visual perception is complete. The retina has two kinds of photoreceptors called rods and cones. Their names come from their shapes. They appear on the right side of next diagram ( showing 1 cone cell and 9 rod cells ). The rods are much more sensitive than the cones and are the ones most responsible for low illumination vision like at night time. They can only produce a black and white visual perception. The cones are found in three types. One type shows maximum sensitivity to red light, another type shows maximum sensitivity to green light and the last one shows maximum sensitivity to blue light. People that are color blind lack one or more of these cone types. Cones are much less sensitive than rods and for this reason, the color perception is possible only at sufficiently high levels of illumination. At low illumination levels, colored objects look gray. It is important to state again that the human eye has three color receptors with maximum sensitivity to R ( red ), G ( green ) and B ( blue ) which are a set or additive primary colors.

6 The diagram below shows the spectral sensitivity of the rods ( dashed line ) and the three kinds of cones as a function of the wavelength of the light. The final important point about human vision that needs consideration here is called visual persistence. It happens that when the eye is stimulated by a very brief flash of light, the neural activity does not die away immediately, but persists for about 0.1 seconds. Consequently, if one sees a series of flashes sufficiently closely spaced in time, the perception is of continuous light. If the series of flashes is not closely space enough, flicker is observed. The rate at which all trace of flicker disappears is about 40 flashes per second.

7 VIDEO SIGNAL AND TELEVISION: A black and white camera only needs to record the light intensity pattern. Similarly, a black and white TV only reproduces the light intensity pattern. In a color camera, the color information from the scene needs to be recorded. Simulating the RGB sensitivity of the cone receptors in the eye, cameras record the R ( red ), G ( green ) and B ( blue ) components of the light. The way three chip ( CCDs of CMOSs ) cameras separate the RGB components is using a beam splitter composed of dichroic prisms. A dichroic prism reflects light ( or colors ) of certain wavelengths and transmits the rest. The diagram below shows how each R, G, and B components are separated by the beam splitter. Single chip cameras use a different procedure that won t be explained here. Once light shines on a light detector CCD, it induces photo-physical processes that produce an electric signal. A megapixel CCD is composed of more than a million very tiny light-sensitive diodes ( an electronic component ) called photosites. When a photon of light collides and is absorbed by a photosite, it removes an electron from it. Electrons are then stored at a different place until a specialized circuit is able to read the amount of charge accumulated there. The larger the intensity of the incident light, the larger the amount of stored charge at each pixel. In this way, the CCD produces a pattern of light intensity that is later recorded in permanent memory. In order to provide compatibility of TV color programming with black and white television sets, rather than transmitting the R, G, and B signals separately, they are combined into a signal that contains three other components. The first component is called the luminance which is formed by adding the R, G, and B signals together in proper proportions. The luminance signals is the only one that a black and white TV needs and is able to interpret. The other two signals (collectively called the chrominance signal) contain the color information ( hue and saturation ). The luminance and chrominance signals arrive at a color TV set and the color TV set separates them into the original R, G, and B components again.

8 The CRT color TV produces three electron beams. Each beam is responsible for each primary color. When electrons hit a phosphor material on the TV screen, it excites the atoms to higher energy levels and when the atoms make transitions to lower energy levels, light is emitted. Different phosphors emit light of different colors. The phosphors are selected in order to emit Red, Green or Blue light. The TV screen is engineered in such a way that only the beam with the information for the R color is able to reach the R phosphors. The same thing is done with the other electron beams and phosphors. The phosphors on the screen are arranged in compact groups ( called pixels ) of three phosphors. The R, G, and B lights coming from each pixel reach the TV viewer and combine via additive color mixing. The final point to consider has to do with the refresh rate in TV screens. While explaining human vision it was mentioned that the process of visual persistence allows people to see a flashing light without flicker when the flashing rate is equal or larger than approximately 40 flashes per second. It is based on this fact that in the US, TVs show half a frame 60 times per second ( corresponding to 30 full frames per second ).