R&D Report 1950-11

"RESEARCH DEl? ARTMENT CHPJlACTERISTICS OF COLOUR FILTERS FOR I A THREE-COLOUR TELEVISION SYSTm~ J Report No. T.027 Serial No. 1950/11 Investigation by: W.C. West Report written by: W.C. \t1est RESEARCH DEPARTMENT REPORT NO. T.027 CHARACTERISTICS OF COLOUR FILTERS FOR A THREE-COLOUR TELEVISION SYSTEM Section Title ~ INTRODUCTION ------ - - - - - - - - - - 1 1 THE OBROMATIOITY DIAGRAM: - - - - - - - - - - 1 2. THE CATHODE RAY TUBE AT THE RECEIVING MONITOR ---------------- 3 ~ 3 THE SELECTION OF TEE PRD:IfARY COLOURS - 4- 4 SPECTRAL RESPONSE AT THE TRANSMITTING POINT ~ - ~ -------- ... ------ 6 5 ILLUMINATION -.- ---- ... --------- 8 6 INFRA-RED RADIATION SCREEN -- - -- -- -- 8 1 EFFECT OF THE ELEOTRICAL OIDUlAOTERISTICS OF THE INTERMEDIATE LINK -------- 9 8 COLOUR RESPONSE OF CliMERA ..; - :... ------ 9 9 TRANSMISSION OOIDUR SEPAR.ATION FILTERS --- ·10 . 10 OHOICE OF OOLOUR SEQUENCE -------- - 10 TilaLE 1 - - - - - - - - ... --- _ - ... -- - 12 Tl~ 2 - - - - - - ,- ---------- - 13 TABLE 3 - ~ - - ----------- - - ~ 1~ PRIVATE AND CONFIDENTIAL REPORT NO. T.027 Research Department April, 1950 Serial No. 1950/11 ' , Fig. Nos,. 1 to 15. CHARACTERISTICS OF COLOUR FILTERS FOR A THREE -COLOUR TELEVISION SYSTEM INTRODUCTION. The experimental colour system now being constructed is an addi tive three -colour sys tem in which the colour separation at, the camera and the synthesis at the receiving monitor are both perfonned by 'filters carried on sy.achronised :r:otating discs • . This note is intended to explain how the choice of the colour standards and conventions was made. ' -, Since the principles of trichromatic colorimetry underly the whole system,' the note commences "vith some details of them. Virgil may have had something similar in mind when, in Eclogue VIII, he made the luckless wife admonish herself (accordyng to Rieu's translation) with the words, "Twine the thrl3e colours, Amaryllis, in three knots; 11 - "perhaps :the firs t reference to addi tive trico1orimetry. 1. THE CHRm~TICITY DIAGRAM Colour sensations differ one from another in any of three ways. For present purposes the three characteristics may be called hue, saturation and apparent brightness. Hue is the essential property of being coloured; red, orange, blue and so on. If, however, a given hue (red, to take a definite example) is modified by the addition of white, becoming a less intense red, "pink", and finally indistinguish able from white, we say that its saturation is being altered. (It is' being reduced in this particular case.) 'By adopting a convontion'that all colour sensations which differ only in apparent brightness (i.e. haVing the same hue and saturation) are regarded as being the same colour, it becomes ppssib1e to represent every colour by a separ~te point ina plane diagram. It is preferable, however, to. choose more convenient axes than representations of hue and saturation. In Fig. 1, the point R represents a certain red colour and ever-:! other point in the diagram represents. some colour differing from this red colour either in hue or saturation or, of course, in both these properties. The' poin t G represents' a' certain unique green colour and we shall represent every colour c;>btained by mixing these red and green stimuli additive1Y,at, t,heeye by some point in the line RG. The point Y represents onc such colour - 2 .;. Dnd the ratio RY to (:.y is::1D.dc :i.nversel y proportional to the ra.tio of the 1118.sni tudes of the triO sti:::uli R end G.. In the S~lJ::.e fisure the point 'B represents Mother stimulus, this tirr,e of a blue colour. ,.: iUong the line BR are points such as P representing the various pUrple colour sensations obtained by allovving the blue and red stimuli, Band R, to affect the eye', simultaneously' or, asm the present system of television, in a sufficiently rapidly repeated sequenoe. So far mixtures of tvro colours cnly have been oonsidered. Mixtures of three oolours lie inside the triangle RGB and it could be found experimentally that white is one suoh colour. It would be found to be representable bya point such as W lying on the line BY, where Y (as already mentioned) represel1ts a co~our formed by mixing the red and green colours in a suitable r'atio. Now consider the line Ri,I' produced. R represents our original red Colour. As v~ add to it a colour stimulus represented by C (a .certain mixture of blue and gr:-etm) the point representing the result mOves along the line RW until W is reached. Points further along the line represent the result formed b,y removing from the white some of the original red. Such "minus oolours" are also .lalO\'ffi as complementary colours. A pair of complementary colours is thus seen to be such that its additive mixturc \'fill give white,. The complementary colour of red is cyan (minus red). The c~mp1ementary colour of green is magenta (minus green). The complementary eol.our of blUe is yellow .(minus blue). In the present instance the limiting colour obtained by the process described would be represented by the intersection of RN with BG, i.e., the point C. ' However, it would be possible to f~nd (for example in the spectrum) a colour vvith even less red in its compOSition, i.e., lying on EN b~t outside the triangle at, say, a. Expressed another way, G and a are ofsirnilar hue, (in general it will not be quite i,dentioa1) but a is a more saturated colour than O. In fact, it could be shovvn experimentally that· all spectrum colours (monochromatic radiations) lie outside any such triangle as RGB.: This is shown in ,Fig. 2, where, if the spectrum colours '\i'lere indicated on a diagram similar to that already oonsidered, , they would be represented by points on the outer curved line (the spectrum lobus).. Here the poi;nt 400 represents the colour. with the shortest wavelength (400 m!-1) usually visible to the normal eye. It is a violet o.olour. Proceeding clockwiserbund the spectrum locus we pass through blue oolours before 500 JnI.L, greens up to approximately 565m1l, then yellows, oranges, and'finally red colours above, say, 625 mIl. " ... 3 ... The two ends of' this ourve may be joined by a straight line, on whioh would lie the representations of the (non-spectral) purples obtained by mixing the spectral red and violet. rt is then found that all possible colours can be represented only by points lying within the area so Circumscribed. It remains only to fit the diagram into suitable axes. It will be remembered that in the preoeding discussion we have ignored the effect of brightness upon colour sen$ation. It may noW be seen that this is tantamount to saying that it is the proportions of the three stimuli which govern the ohromaticity. This may then'be specified by quoting the ratio of the magnitudes of each stimulus to the SlUU. of all three and, of' course, in such a Case the specif'j,cation is complete when any two such ratios are given. The nature of the stimuli was standardised by the OOITmission In~ernationale de l'E0lairage in 19)1, and they will .be referred to as X, Y and Z. The ratios x = X/eX + Y + z) and y = Y/(X + Y -i- z) will be referred to as chroma tici ty co-ordinates, and these will be shown in a diagram of' the kind illu::>trated in Fig. 3. It will be noted that it is unnecessary' to use z as a separate co-ordinate as z = I ... (x + y), for all values of x and y. On this diagram is marked the point We which represents the colour sensation produced by the equal-energy stimulus (energy in 'equal wavelength intervals remaining constant at least throughout the visible spectrum)., ~here is also an additional curve, which indicates the colours that can be assumed by heating a full radiator (ideal black body). It passes close to the point We and at the equivalent temperature (approximately 5000 - 65000 K) such a radiator would be called white. At lower tempera tures the radiation appears more yellow or 'in' the extreme, red; at higher temperatures it would appear more or less blue. 2. THE CATHODE RAY TUBE AT THE REOEIV!NG MONITOR It is desired to produ~e at the vi81ring device three oolours \ whioh oan be used as the stimuli R, G and B in a system based on . the principles discussed. 'White (or near white) receiving tubes for television have phosphors nonnally giving a pair of complementary colours. These aregengrally obtained by a mixture of (blue) zinc sulphide and (yellow) zino cadmium SUlphide. Thespeotral emission curve of such a phosphor will have a general shape similar to that shovm in fig. 4, in which the peak at approximately 450 mM is due to the zinc sulphide, and the secondary peak is, the contribution of' the zinc cadmium sulphide. For the present use a rather more aocurate lmowledge of the emission charaoter is required and although spectrophotometrio methods vvere not then available, it is thought that the following method cf - 4- ~ assessing it has prbvided sufficiently accurate information • . It Was assumed that the phosphor was composed of a mixture in some proportion of the above two components. In attempts to determine its percentage composition the colour point for each component on the chromaticity diagram was first determined by colorimetric computation. If these two stimuli affect the . eye s imultnne.ou sly, the -resulting colour will be capabie of representa~ionby some point lying OD the straight line joining the ~vo points so determined. The characteristics of the two components itvere so. chosen that this straight line would in fact pass through the point representing the colour measured by a .

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