AP1_03.vp 1 Introduction The colorimetry of all present-day TV standards (analog as well as digital) emanates from the original NTSC TV stan- dard. When this TV standard was put forward the cathode ray tube (CRT) was the only available display device. The conven- tional scanning system (the RGB prism) is characterized by the reproductive lights of the display and by a comparative white light. The existence of negative parts of the colour matching functions � � � � � �r g b� � �, , causes complications by optical separation of partial pictures R, G, B in the classic scanning system. This leads to distortion in the reproduction of colour images. However, the present-day market offers many kinds of display devices, most notably LCD and plasma displays. These new displays have other, sometimes prefera- ble colorimetric features, but we cannot take advantage of these better features due to dependence on the conven- tional RGB system [1]. However, the TV camera, which scans in a colorimetric system of unreal lights X, Y, Z, is not predetermined by the colorimetric features of any display device. Theoretical spectral reflectances of partial filters of the XYZ prism correspond to the colour matching functions � � � � � �x y z� � �, , , which are only positive [2]. This solves a number of problems that are encountered when realizing the conventional scanning system (RGB prism). A camera working in the XYZ colorimetric system produces electrical analogs of trichromatic components X, Y, Z. So every kind of display device has a circuit of colorimetric transformation from the system of lights X, Y, Z into the system of primary (reproductive) lights in the given display device. Another advantage of the XYZ colorimetric system is that one of the scanning channels is directly channel Y. In this case, its noise is identified only by its own scanning device. The two remaining channels (X, Z) carry colour information and just by realizing such a camera we can anticipate lower resolution of details in the X and Z channels. 2 Colorimetric systems – a comparison The basis for each colorimetric space is created by three lights. For each colorimetric system, the colour matching functions are defined. These functions are determined by the basic lights and the comparative white light. Each colori- metric system has its chromaticity diagram. This is usually issued from the colorimetric system of unreal lights X, Y, Z. As the triangle (in the MKO chromaticity diagram, see Fig. 1), whose vertices create the lights X, Y, Z, overlays the whole gamut of existing colours, the colour matching functions � �x � , � �y � , � �z � , are only positive (Fig. 2). The XYZ colori- metric system is the only one with only positive colour match- ing functions. The RGBNTSC colorimetric system has been used in TV since 1953. The lights R, G, B create a triangle, which is inscribed in the gamut of all existing colours, hence the corresponding colour matching functions � �r � , � �g � , � �b � are bipolar (Fig. 3). Negative parts of these colour matching functions come up to partitions of the area of all existing colours, which lie in the second and the fourth quadrant of the RGBNTSC chromaticity diagram (Fig. 4). Mutual conver- sion among colorimetric systems is conducted by means of the general colorimetric transformation [3]. For correct scanning of colour information, the three channels of the TV camera scanning set must have sensitivi- ties equal to some colour matching functions. It is apparent that only positive sensitivities can be realized optically. The reason for this is fundamental. There is no negative radiation intensity, there is no negative medium transparency and the photoeffect is also a response of the output quantity (charge, current, voltage) only to the radiation intensity. Hence the real sensitivities of the RGB prism channels follow at the very most only the positive parts of the ideal sensitivities (Fig. 4). Colour information about the scanning scene is in this way knowingly neglected ahead of the optical–electrical conver- sion on the image sensors. The end effect of incorrect scan- ning is reduced fidelity of colour reproduction on a display unit and also on the CRT from which the channel sensitivities of the RGB prism are derived. The summing curve of any colour matching functions em- bodies the characteristic minimum at a wavelength of around 500 nm. The summing curve of colour matching functions � �x � , � �y � , � �z � and � �r � , � �g � , � �b � is shown in Fig. 5. It is interesting that, for evaluating colour, human vision does not use information that is contained at a wavelength of around 500 nm. 60 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 41 No. 3/2001 Colorimetry and TV Colour Splitting Systems J. Kaiser, E. Košt’ál The colorimetric standard of the present-day television system goes back to the American NTSC system from 1953. In this RGB colorimetric system it is not possible, for basic reasons, to produce a scanning device which will provide signals suitable for controlling any displayed unit. From the very beginning of the television system the scanning device has produced inevitable colour deformation. The range of reproductive colours is not fully utilized either by a contemporary Cathode Ray Tube display unit or by a Liquid Crystal Display. In addition, the range is not sufficient for true reproduction of colours. Specific technical and scientific applications in which colour bears a substantial part of the information (cosmic development, medicine) demand high fidelity colour reproduction. The colour splitting system, working in the RGB colorimetric system, continues to be universally used. This article submits the results of a design for a colour splitting system working in the XYZ colorimetric system (hereafter referred to as the XYZ prism). A way to obtain theoretical spectral reflectances of partial XYZ prism filters is briefly described. These filters are then approximated by real optical interference filters and the geometry of the XYZ prism is established. Keywords: TV colorimetry, colour splitting system, interferential filters, TV reproduction, colour gamut. © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 61 Acta Polytechnica Vol. 41 No. 3/2001 [- ] wavelenght [nm] z y x Fig. 2: Colour matching functions � �x � , � �y � , � �z � y x Fig. 1: MKO chromaticity diagram with lights X,Y,Z and (R,G,B)NTSC g r Fig. 3: RGBNTSC chromaticity diagram with lights X, Y, Z and (R,G,B)NTSC [- ] wavelenght [nm] b g r Fig. 4: Colour matching functions � �r � , � �g � , � �b � A) B) wavelenght [nm] wavelenght [nm] [- ] [- ] Fig. 5: Summing curve of colour matching functions � �x � , � �y � , � �z � (A) and � �r � , � �g � , � �b � (B) 3 Spectral reflectances of partial filters of the XYZ prism On the basis of Fig. 6, a system of three equations for spec- tral sensitivities of partial channels � ��x � , � ��y � , � ��z � of the proposed scanning system can be compiled. The spectral reflectances of partial filters Ay(�), Bx(�), and Cz(�) are unknown [4]. � � � �� �y � �Ay (1) � � � � � �� �� � �x � � �Bx Ay1 (2) � � � � � �� � � �� �� � � �z � � � �Cz Ay Bx1 1 (3) The spectral sensitivities of partial channels � ��x � , � ��y � , � ��z � of the proposed scanning system are the colour match- ing functions � �x � , � �y � , � �z � (CIE 1931, 2-deg), which are corrected for maximum efficiency of transmission of light flux through the colour splitting system, also for maximum transparence of the splitting system, and for the spectral sen- sitivities of image sensor CCD. With the solution of equations (1), (2), (3), we acquire spec- tral reflectances of partial filters Ay (�), Bx (�) and Cz (�). The solutions show that the colour splitting system is not orthogo- nal. It turns out that the curves of the spectral sensitivities of the scanning system overlap one another above the wave- length axis. By separation, the light energy is sucked into two and in some places even into three paths. The ideal spectral reflectances of the partial filters of the XYZ prism are � � � �Ay � �� �y (4) � � � � � �� �Bx � � �� � � �x y1 (5) � � � � � � � �� �Cz � � � �� � � � � �z y x1 . (6) The approximations of ideal spectral reflectances Ay (�), Bx (�), Cz (�) by real optical interference filters (see Fig. 7) were made using the Synopsys programme [5]. The technical solution of real optical interference filters involves producing dichroic thicknesses [6], [7], [8]. These form a coating of a shiny pellucid medium (e.g., boro-silicate glass BK7) with thicknesses comparable with the wavelength of light. The thicknesses are sorted step by step with the alternating higher and lower refractive index. Filter Ay (�) is built up from eight layers of three materials (MgF2, CeF3, CeO2), filter Bx (�) is built up from twelve layers of four materials (MgF2, CeF3, CeO2, SiO), and filter Cz (�) has twelve layers of three materi- als (MgF2, CeF3, ZrO2) [11]. 62 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 41 No. 3/2001 �y� �x � �z � Incident light Ay Bx Cz Fig. 6: The sequence of separation of partial components (im- ages) wavelenght [nm] wavelenght [nm] Ay [- ] Bx [- ] wavelenght [nm] Cz [- ] Fig. 7: Ideal spectral reflectances Ay(�), Bx(�), Cz(�) and their approximation byreal optical interference filters (approx- imation by real filters – full line) 4 Geometry of the XYZ colour splitting system The XYZ colour splitting system (see Fig. 8) consists of four prisms and three interference filters. The colour splitting system constitutes a three-band frequency selective switch of light pencils and a three-band amplitude switch. The pencils generate the partial images X, Y, Z on the outputs of the switch. The images are scanned via three sensors, e.g., CCD sensors, and video signals EX, EY, EZ are obtained as electrical analogs of the trichromatic components X, Y, Z. The prism is made of BK7 glass. The third prism functions only as an ad- justing shim to provide sufficient room for image sensor Z. Each of the prisms is proposed and sorted in set so that the trace lengths of the partial light tubes will be identical and the proportions of the prisms will enable trouble-free transit of light tubes of the required diameter. All three filters are reflective-interferential. The rear surfaces of the first, second and fourth prism are coated with these filters, i.e., in the direction in which the beams are going. The third prism creates an adjusting shim. The filters are built up of dielec- tric multilayers of the following materials: SiO, MgF2, CeF3, ZrO2, CeO2. The spectral reflectances of partial filters Ay (�), Bx (�), Cz (�) are illustrated in Fig. 2. Each partial light tube executes two reflections in the XYZ prism. The first reflections of the partial light tubes occur on filters Ay(�), Bx(�) and Cz(�). These reflections are frequency and amplitude selec- tive. The second reflections of the light tubes are total and oc- cur on the front walls (on the glass-air passages) of the first, second and fourth prism. After the second reflections, the partial light tubes with spectral sensitivities � ��y � , � ��x � and � ��z � , respectively, come to the image sensors. Due to the transmissivity of the XYZ prism, and due to the summing curve of the spectral sensitivities � � � � � �� � � � �x y z� � � , only a part of the incident light spec- trum is used to obtain the trichromatic components X, Y, Z (partial images X, Y, Z). The unused light spectrum, mainly the section around wavelength 500 nm, passes through filter Cz(�) and leaves the XYZ prism. This light must be absorbed in the camera (e.g., absorption with velvet) to prevent it being reflected back into the prism. Otherwise this light would cause spurious artefacts in the picture during reproduction. In order to create glass-air passages, i.e., total reflections also for components X and Z, there has to be a slim air interspace 0.1–0.2 mm in thickness between each two prisms. This air interspace is also needed between the second and the third prism. It does not engender total reflection, but the air interspace has a favourable effect on the number of layers and on the kinds of filter material Bx (�). In other words, the im- pedance match of the filters will be less demanding if there is a substance with different impedance on all sides of the filter. 5 Conclusion This paper aims to show how the colorimetry of the TV scanning set could proceed to full exploitation of the new range of colorimetric display devices. The XYZ colour split- ting system encounters no insuperable difficulties during optical separation of partial components (which are found in the classic RGB scanning system). Its ideal spectral sensitivi- ties are, in contrast to the ideal spectral sensitivities of the clas- sic scanning system, only positive. Hence, there is no longer any need to introduce additional corrections for areas with negative spectral sensitivities of partial channels. All three fil- ters in the XYZ prism are of the reflective-interferential type, unlike the green filter of the RGB prism, which is coloured and therefore absorptive. The colorimetric system of primary lights X, Y, Z overlays the whole gamut of existing colours. For light of any colour, the trichromatic components X, Y, Z are only positive. The end effect is that the colour gamut of repro- duction will not be reduced [10], [11]. Footnote The XYZ colour splitting system for TV cameras was submitted by doc. Ing. Emil Koš�ál, CSc., Ing. Jan Kaiser and Ing. Jiří Slavík as a utility model and patent application [12]. A registration certificate for the utility model was granted on 29. 5. 2000. The number of the utility model is 10026. The certificate of patent registration was granted on 19. 4. 2001. Number of the patent is 288456. References [1] Košt’ál, E.: Obrazová a televizní technika II – Televize (Image and television technology II – TV). Učební text ČVUT 1998, p. 135 [2] http://cvision.ucsd.edu/index.html, CIE Standards, Co- lor spectra databases [3] Ptáček, M.: Přenosové soustavy barevné a digitální televize (Transmission systems of colour and digital TV). 2. vydání, Nadas Praha 1981, p. 488 [4] Slavík, J.: Návrh světlodělící soustavy pro kameru pracující v kolorimetrickém systému X,Y,Z (Design of colour splitting sys- tem for TV camera working in the X,Y,Z colorimetric system). Unpublished manuscript, 1999 [5] http://www.GWI.net/OSD, Synopsys program [6] Dobrowolski, J. A.: Completely Automatic Synthesis of Optical Thin Film Systems. Applied Optics, Vol. 4, No. 8, Aug. 1965, p. 937 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 63 Acta Polytechnica Vol. 41 No. 3/2001 Fig. 8: The XYZ colour splitting system (1, 2, 3, 4 – glass prisms; 5, 6, 7 – dielectric multilayers (filters); 8, 9, 10 – image sen- sors; 11, 12, 13 – air interspaces) [7] Ditchburn, R. W.: Light. 3rd Edition, Acad. Press, 1976 [8] Novák, Z.: Optické soustavy snímacích zařízení (Optical set of scanning devices). Učební text pro postgraduální studium, ČVUT 1971 [9] Pazderák, J.: Kolorimetrie snímacích soustav barevné televize a elektronické kolorimetrické korekce (Colorimetry of scanning systems of colour TV and electronic colorimetric corrections). Edice ČS. TELEVIZE, řada II, svazek 16, Praha 1974, p. 150 [10] Svoboda, V.: Kolorimetrie a zdokonalené televizní soustavy (Colorimetry and improved TV systems). In: Televize 94 č.1, IVP ČT Praha 1994, pp. 65–114 [11] Kaiser, J.: Kolorimetrie zdokonalených TV soustav (Colori- metry of improved TV systems). Diploma project, ČVUT 2001 [12] Košt’ál, E., Kaiser, J., Slavík, J.: Hranolová světlodělící soustava pro televizní kamery (The colour splitting system for TV cameras). Přihláška vynálezu (patent application) č. PV 2000-1167, 30. 3. 2000 Ing. Jan Kaiser e-mail: xkaiserj@feld.cvut.cz Department of Radioelectronics Czech Technical University in Prague Faculty of Electrical Engineering Technická 2, 16627 Praha 6, Czech Republic Doc. Ing. Emil Košt’ál, CSc. e-mail: kostalem@worldonline.dk Ryvangs Allé 14 2100 Copenhagen 0 Czech Embassy, Dánsko 64 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 41 No. 3/2001 << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Error /CompatibilityLevel 1.4 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends false /DetectCurves 0.0000 /ColorConversionStrategy /CMYK /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments true /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 300 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.50000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. 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