OPTICS OF COLOUR TELEVISION

Transcription

1 1 OPTICS OF COLOUR TELEVISION INTRODUCTION For fully grasping the essentials of colour television it is necessary to be fully conversant with the basics and functions of monochrome (B & W) television. Also, in order to attain a clear concept of Colour Television (CTV), certain fundamental qualities of light must be fully understood, it is also necessary to grasp the technique of mixing colours to produce different hues (colours) and the sensitivity of human eye to perceive them. The purpose of this chapter is to briefly explain such aspects so that essentials of CTV are fully assimilated. 1.1 NATURE OF VISIBLE LIGHT When white light from the sun is examined, it is found that the radiation does not consist of a single wavelength but it comprises of a band of frequencies. In fact, white light is a very small part of the large spectrum of electromagnetic waves which extend from very low to beyond Hz. The visible spectrum extends over only an octave that centres around a frequency of the order of Hz. When radiation from the entire visible spectrum directly reaches the eye we see white light. If, however, part of the range is filtered out, and only the remainder of the visible spectrum reaches the eye, we see a colour. The entire electromagnetic spectrum is shown in Fig. 1.1 where the visible spectrum has been expanded and shown separately to demonstrate the range of colours it contains. Note that the various colours merge into one another with no precise boundaries. 1.2 PERCEPTION OF COLOURS All objects that we observe are focused sharply by the lens system of the eye on its retina. The retina which is located at the back side of eye has light sensitive organs which measure visual sensations. The retina is connected with the optic nerve which conducts light stimuli as sensed by the organs to the optical centre of the brain. 1

2 2 COLOUR TELEVISION According to the theory formulated by Helmholtz the light-sensitive organs are of two types rods and cones. The rods provide brightness sensation, and thus perceive objects only in various shades of grey from black to white. The cones that are sensitive to colour are broadly in three different groups. One set of cones detects the presence of blue colour in the object focused on the retina, the second set perceives red colour and the third is sensitive to the green range. Each group of cones can thus be thought of as a set tuned to only a small band of frequencies and so absorbs energy from a definite Visible spectrum f(hz) Radio waves Infra-red Ultra-violet X-rays Cosmic rays Spectrum of electromagnetic radiation l (meters) Red Orange Yellow Green Blue Indigo Violet ( l in nm) (1nm=10 m) Fig. 1.1 Region of sunlight in the electromagnetic spectrum. range of electromagnetic radiation to convey the sensation of corresponding colour or range of colours. The combined relative luminosity curve showing relative sensation of brightness produced by individual spectral colours radiated at a constant energy level is shown in Fig It will be seen from the plot that 1.0 Total response Relative response Blue cones Green cones Red cones nm Wavelength in nanometres Fig. 1.2 Approximate relative response of the eye to different colours.

3 OPTICS OF COLOUR TELEVISION 3 sensitivity of the human eye is greatest for green light, decreasing towards both the red and blue ends of the spectrum. In fact the maximum is located at about 550 nm, a yellow green, where the spectral energy maximum of sunlight is also located. 1.3 MIXING OF COLOURS CHAPTER 1 The radiation of each wavelength in the visible spectrum is perceived by the eye as a certain hue or colour in its pure and fully saturated form. These colours cannot be exactly reproduced by conventional methods like printing or painting. However, the infinite variety of colours that we see around can be produced either by subtractive mixing using various pigments or by additive mixing of light of different colours. Subtractive mixing. A sensation of colour is produced by opaque objects or materials when white light falls on them. It is so because any surface does not reflect all the wavelengths of the incident light uniformly. In fact, only some out of these are reflected and the rest get absorbed at the surface of the object. Thus, any object is seen in a colour corresponding to the spectrum of radiated light which is not absorbed but instead gets reflected. For example, a red apple absorbs light of all colours except red, which it reflects and thus looks red. An object that appears black absorbs all the visible light. Similarly, a surface looks grey when the entire light spectrum is reflected uniformly but less, i.e., a part of the incident light being absorbed by the object. Based on the above facts special chemicals called pigments are used for printing and painting. A yellow pigment absorbs violet, blue, green, orange and red but reflects the remaining yellow. Similarly, blue pigments reflect only blue and absorb all other colours. When pigments of two or more colours are mixed they reflect wavelengths of only those colours which are common to both and these combine to give the sensation of a new colour. However, when no wavelength of the light spectrum is common in the pigments which are mixed, the entire incident light is absorbed and the object looks black. Thus, pigments create a given colour by subtracting parts of the spectrum of incident white light and reflecting the remaining which gives the objects its characteristic colour. Making colours by mixing paint pigments is therefore described as Subtractive Mixing, since each added pigment subtracts more from white light and leaves less to be reflected to the eye. The formation of various colours including black by subtractive mixing is shown in Fig. 1.3 (a). Additive mixing. This is altogether a different method of producing various colours. When lights of two or more different colours are mixed and projected on a white screen, the eye perceives it as a combined colour. In effect, the light reflected from the screen seems to the eye as if it is coming from a new source of light which is different from any of the sources actually projected. As explained earlier, all light sensations to the eye are divided into three main groups. The optic nerve system then integrates different colour impressions in accordance with the curves shown in Fig. 1.2 to perceive actual colour of the object being seen. A yellow colour, for example, can be distinctly seen by the eye when red and green groups of cones are excited at the same time with corresponding intensity ratio. Similarly, any colour other than red, green and blue will excite different sets of cones to generate cumulative sensation of that colour. Thus, a white colour is perceived by additive mixing of sensations from all the three sets of cones.

4 4 COLOUR TELEVISION Magenta Red (white green) (white blue green) Blue (white green red) Yellow (white blue) Black Cyan (white red) Green (white blue red) Fig. 1.3 (a) Subtractive colour mixing: The diagram shows the effect of mixing colour pigments under white light. Hence, in additive mixing which forms the basis of colour television, light from two or more colours obtained either from independent sources or through filters can create a combined sensation of a different colour. Note that different colours are created by mixing pure colours and not by subtracting parts from white. The additive colours mixing is illustrated in Fig. 1.3 (b). Red Magenta (red + blue) Blue Yellow (red + green) 30% 41% 11% 100% White 89% 59% 70% Cyan (green + blue) Green Fig. 1.3 (b) Additive colour mixing. The diagram shows the effect of projecting green, red and blue light beams on a white screen in such a way that they overlap. 1.4 THREE COLOUR THEORY The number of different colours that can be formed by additive mixing depends on the number of coloured lights used for this purpose. The more the number of such lights employed in the mixing scheme, the wider can be the reproducible range of colours. However, the number of different lights has to be limited to a few to avoid system complexity and cost. Experimental results have established that lights of red, green and blue colours when combined with each other in different proportions, produce a wide range of gamut of colours. The perception of

5 OPTICS OF COLOUR TELEVISION 5 colours thus produced is enhanced by the fact that the three sets of cones that are associated with the optic nerve have greatest sensitivity to red, green and blue. Therefore, red, green and blue have been, standardized as basic colours for colour television and are known as Primary Colours. By pair-wise additive mixing of primary colours, the following complementary colours can be produced: Red + Green = Yellow Red + Blue = Magenta (purplish red shade) Blue + Green = Cyan (greenish blue shade) Colour Plate 1 depicts the location of primary and complementary colours on the colour circle. If a complementary colour is added in an appropriate and fixed proportion to the primary which it itself does not contain, white is produced. Colour plate 2 shows formation of complementary colours by mixing of primary colours. Note that additive mixing of the three primary colours in defined proportions produces white. Grass Man s Law As already explained the eye is not able to distinguish each of the colours that mix to form a new colour but instead perceives only the resultant colour. Thus, the eye behaves as though the output of three types of cones are additive. The subjective impression which is gained when green, blue and red lights reach the eye simultaneously, may be matched by a single light source having the same colour. In addition to this, the brightness (luminance) impression created by the combined light source is numerically equal to the sum of brightnesses (luminances) of the three primaries that constitute the single light. This property of the eye of producing a response which depends on the algebraic sum of red, green and blue inputs is known as Grass Man s law. White has been seen to be reproduced by adding red, green and blue lights. The intensity of each colour may be varied. This enables simple rules of addition and subtraction. CHAPTER TRISTIMULUS VALUES AND CHROMATICITY OF SPECTRAL COLOURS Equal quantities of primary colours do not appear to be equally bright nor do they involve equal amounts of light energy. As an illustration, if inputs to a colour picture are adjusted to make the raster white and later primary colours are viewed separately in turn, the green phosphor on excitation by the green signal will appear to be brightest while the blue will appear to be least bright. This is expected because of the nature of response of human eye to different colours as explained earlier. Therefore, in order to obtain quantitative values of various colours which must be mixed together, all such factors have to be taken into account. Based on these considerations and extensive tests with a large number of observers, the primary spectral colours and their intensities required to produce different colours by mixing have been standardized. The red, green and blue have been fixed at wavelengths of 700 nm, nm and nm, respectively. The component values (or fluxes) of the three primary colours to produce various other colours have also been standardized and are called tristimulus values of different spectral colours. The reference white for CTV has been chosen to be a mixture of 30% red, 59% green and 11% blue. These percentages of light fluxes are based on the sensitivity of eye to different colours.

6 6 COLOUR TELEVISION Thus, for example, 0.3 lumen of red lumen of green lumen of blue will produce one lumen of white light. Furthermore, in accordance with the law of colour additive mixing, one lumen of white light (see Fig (b)) will also be produced by 0.89 lm of yellow lm of blue or 0.70 lm of cyan lm of red or 0.41 lm of magenta lm of green. It may be noted that if the concentration of luminous flux is reduced by a common factor from all the constituent colours, the resultant colour will still be white, though its level of brightness will decrease. The brightness of different spectral colours is associated with that of white. Yellow for example, (see Fig. 1.3 (b)) appears 89% as bright as the reference white, reflecting the addition of 59% brightness of green and 30% brightness of red. Similarly, any other combination of primary colours will produce a different colour with a different relative brightness reference to white which has been taken as 100 percent. Chromaticity Diagram The factors stated above are summed up in the chromaticity diagram shown in Fig As seen, it is a convenient space-coordination representation of all the spectral colours and their mixtures based on the tristimulus values of the primary colours contained by them. 520 Green T.V. Primary 550 Yellow-green Green Yellow 500 Yellow Illuminant D White 600 Orange Red Red T.V. Primary 700 Cyan Blue-green Pink Blue T.V. Primary 470 Blue 400 Purple Magenta Fig. 1.4 The chromaticity diagram. The inner triangle rests on the TV primaries and the area enclosed by the triangle represents the range of colours in a TV display 1.6 HUE, SATURATION AND LUMINANCE Any colour has three characteristics to specify its visual information. These are: (i) Hue or Tint, (ii) Saturation and (iii) Luminance.

7 OPTICS OF COLOUR TELEVISION 7 (i) Hue or Tint. This is the predominant spectral colour of the received light. Thus, the colour of any object is distinguished by its hue or tint. The green leaves have green hue and red tomatoes have red hue. Different hues result from different wavelengths of spectral radiation and are perceived as such by the sets of cones in the retina. (ii) Saturation. This is the spectral purity of the colour light. Thus, saturation may be taken as an indication of how little the colour is diluted by white. A fully saturated colour has no white. As an example, vivid green is fully saturated and when diluted by white it becomes light green. The hue and saturation of a colour put together is known as Chrominance. Note that it does not contain the brightness information. Chrominance is also called Chroma. (iii) Luminance. Brightness and luminance are often taken to mean the same information but actually it is somewhat different. It is difficult to define brightness for technical purposes because it only gives an objective assessment of sensation perceived by the eye due to the effect of illumination of surroundings. Thus, to overcome such a difficulty, luminance is taken to mean the subjective visual sensation of brightness of light or colour. To be more specific, luminance is a measure of the light emitted from a surface. Light may be emitted by direct radiation from a source at a surface or it may be reflected by a surface which is bathed in incident light. As an illustration, in monochrome TV camera, the video signal developed describes the amount of light falling on its target plate. Since this in turn depends upon the amount of light being reflected from the scene, the camera output represents luminance of the scene element by element. For a given colour, luminance is the fundamental brightness of the colour impression and is the amount of light intensity as perceived by the eye regardless of the colour. In black and white pictures, better lighted parts have more luminance than the dark areas. Different colours also have shades of luminance in the sense that though equally illuminated they appear differently bright as indicated by the relative brightness response curve of Fig Thus, on a monochrome TV screen, dark red colour will appear as near black, yellow, close to white and light blue colour as light grey. CHAPTER PERCEPTION OF COLOURS VERSUS BLACK AND WHITE The fundamental characteristics of a CTV system are based on perception of colours by the human eye. Black and White vision is more acute than colour vision. An important aspect of this distinction is the inability of human eye to perceive very small coloured areas in a scene. Detailed studies have shown that perception of colours produced by the three primaries is limited to relatively large areas. Further, for medium size objects or areas, only two primary colours are needed. This is so, because for finer details the eye fails to distinguish purple (magenta) and green-yellow hues from greys. As the coloured area becomes very small, the red and cyan hues also become indistinguishable from greys. Thus, for very fine colour details all persons with normal vision are colour blind and see only changes in brightness even for coloured areas. Its technical importance is that while the TV system should be capable of reproducing fine details of black, white and grey, coloured areas need not be sharply defined. The eye is also more critical of some hues than others. For example, the viewer tends to be very critical of variations in flesh tones whereas similar changes in brown and green are likely to be accepted. In other words, orange tints need to be reproduced with minimum departure from the original scene

8 8 COLOUR TELEVISION whereas green hues may vary somewhat without recognition by the viewer. Technically, this means that orange hues and flesh tones must be picked up, transmitted and reproduced with the minimum possible error. Special circuits are often included in colour receivers to achieve such a performance. REVIEW QUESTIONS 1. What is the ratio of visible electromagnetic spectrum to the total electromagnetic spectrum? 2. Explain how the human eye perceives brightness and colour sensations. Comment on the spectral response of the human eye. 3. What do you understand by additive and subtractive mixing of colours? Why is additive mixing suitable for colour television? 4. Name the primary and complementary colours used in colour television. What is Grass-Mans Law? Explain with an example how the primary and complementary colours can be combined to produce white? 5. What are tristimulus values of spectral colours? What is their significance in colour television? 6. Explain how hue, saturation and luminance describe fully any colour scene. What do you understand by the chrominance of a colour? 7. Explain limitations of the human eye to perception of colours when compared with black, grey and white? Is black the complement of white?