COLORIMETRY

Friedrich GierlingerInstitut für Rundfunktechnik GmbH

Traditionally, the ITU-R BT.470 standardized RGB colour space has been used intelevision production. However, with the introduction of computer-based editingplatforms in the early 1990s, other colour spaces such as HCV and CIELab haveentered the television environment, along with colour space conversion techniquessuch as that developed by the ICC. This article presents an overview of the colourspaces and colour management techniques that are used today in the TV productionenvironment.

The first attempts at transmitting colour images were made on 4 September 1940, with the “FieldSequential” technique developed by P.C. Goldmark (CBS). A very helpful factor were the findings ofH. Hartridge, a British ophthalmologist, who proved that the eye’s resolving power was weaker whenprocessing colour information, compared to brightness information. Thus, the bandwidth for trans-mitting colour information could be significantly lower, which made the transmission of colour videosignals much easier. On 15 December 1953, the first colour television programme was broadcast inthe USA by stations belonging to NBC and CBS. The colour transmission system was NTSC(National Television System Committee).

Europe became interested in the technology only in November 1962 and founded the “EBU Ad HocColour Television Group”. This Ad Hoc Group met from 3 to 5 January 1963 at the NDR in Hanover,on the initiative of Dr Hans Rindfleisch. At this meeting, a comparison between NTSC and SECAMshould have led to a decision on which of the two systems should become the colour TV standardfor Europe. However, the PAL colour system was also demonstrated at this meeting (by Mr WalterBruch of Telefunken) and, in the event, PAL became the system of choice for most of Europe.

Colour space in conventional television broadcastingDuring the first 20 years of colour transmission, the standards for production and broadcasting werebased on the CCVS-PAL level space which, in turn, is based on the RGB colour space. Capturing ofthe image source and image processing were based on this standard. It should be noted that theE’R - E’G - E’B colour space was only defined for the phosphorus used for capturing and reproduc-tion (EBU phosphorus). Due to the technical requirements for a better image quality, video signalswere edited on the analogue E’Y - E’CB - E’CR level by the mid 1980s. Fig. 1 shows the correlationof the E’R - E’G - E’B level space within the E’Y - E’CB - E’CR cube.

Obviously, it is easy to recognize that processing of video signals within the E’Y - E’CB - E’CR levelspace (cube) can also result in signal levels that are outside of the smaller E’R - E’G - E’B cube.

Colour Spacesin modern multimedia environments

— Convergence after 40 years of colour television

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COLORIMETRY

These signal levels represent colours that areillegal for the E’R - E’G - E’B colour space andcannot be fed back into legal E’R - E’G - E’Bvalues without artefacts. The problem of illegalcolours surfaced relatively early, but only becamesignificant with digitalization at the E’Y - E’CB -E’CR component level. In the 1990s, more digitalgraphics systems were used to produce captionsand graphics in this domain. They were also ableto generate illegal colours. The relevant equip-ment at this time was usually developed by manu-facturers who had been in the televisionbroadcasting business for a long time, thus takinginto account special broadcasting requirements.However, illegal colours could be produced.Images were processed and generated at theE’Y - E’CB - E’CR component level, based on theRGB colour space. The quality of the image andthe colour were controlled on monitors using EBUphosphorus. Until the early 90s, television broad-

casting only used the E’R - E’G - E’B and E’Y - E’CB - E’CR colour spaces, which could be transferredinto each other by means of linear matrix equations.

Other colour spacesWith the introduction ofcomputer-based editing plat-forms, other colour spacesappeared, which could nolonger be imported intotelevision linearly. Graphics,images and sequences, whichwere generated on a computerplatform, could only achievecolour-correct reproduction ifthe colour spaces in questionwere converted. Fig. 2 showsthe points on the signal pathwhere adjustments to thecolour spaces may be needed.

If the same phosphorus, e.g.the EBU-defined phosphorus,is used for image capturing andreproduction, a colour conver-sion is not necessary. Scan-ners, digital still-picture cameras, flat screens and print media sometimes use different types ofphosphorus and thus use different colour spaces. In addition, additive colour mixing (the mixedcolour is produced by adding at least two light sources of different wavelengths) is used for repro-duction on conventional CRT monitors, while print media use subtractive colour mixing (the mixedcolour is produced by filtering different wavelengths from the colour spectrum) (see Fig. 3). Thetransfer of one into the other is not possible in a linear manner, and sometimes not without artefacts.

In the following sections, a few colour spaces that play an important part in modern production envi-ronments are presented.

Figure 1Correlation between the RGB and E’Y - E’CB - E’CR level spaces

Figure 2Colour space conversion

C

C C

C

C

C

C

C

Processing

Processing

CRT

Flat Panel

Printer

TV

Stills

Scanner

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COLORIMETRY

Perceptual colour spacesComputer-based processingplatforms often use perceptualcolour spaces. These colourspaces are adjusted to humanperception and are describedby Hue, Brightness andSaturation. In 1915, thepainter Albert Henry Munsell(1858 – 1918) developed acollection of colour samplesbased on the same perceptualdifference of colours (Fig. 4).

Based on this collection, anumber of perceptual colourspaces have been developed,which take Hue as an axis ofthe respective colour spacemodel; they are thus verysimilar. The display of colourin a 3D construction waschosen to arrange the coloursin a way that makes thecolours comprehensible to thehuman imagination.

The vertical axis of the twoupper models in Fig. 5 gives agrey scale and is called Value(V). The distance from the

Figure 4Munsell colour samplesAdditive colour mixture

Subtractive colour mixture

Figure 3Colour mixing

�

HCV colour space HSV colour space

HLS colour space HSB colour space

Value

ChromaHue

Green 120°

Cyan 180°

Blue 240° Magenta 300°

Red 0°

Yellow 60°

V

HS

Hue

0 - 300°

Saturation

0 - 100%

Lig

htn

ess 0

- 1

00

%

Saturation

Hue

Brig

htn

ess

Figure 5Perceptual colour spaces

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COLORIMETRY

axis equals the saturation and is called Chroma (C) in the top-left model (not to be confused with theterm “chroma” in television). The Hue reflects the actual spectral gradation (usually given indegrees). Saturation denotes the intensity of the hues (pale or vivid). Lower saturation places thecolour – depending on its brightness – towards white, grey or black. Luminance is a measure ofbrightness.

CIE colour spacesIn the 1920s, there was agrowing desire to define col-ours objectively, mathemati-cally. The Commission Inter-nationale d’Eclairage (CIE)was entrusted with develop-ing a method to make thispossible.

In 1928, the mineralogistSiegfried Rösch described abody (Fig. 6), whose surfaceis constructed of optimumcolours. Optimum coloursare those among the colours(trichromatic), which are thebrightest of the same chro-maticity and the most satu-rated of the same brightness.

The base of this body is thediagram published by the CIEin 1931. It was constructedas the result of expansivetests with emmetropes. Thisdiagram enables calculation

of the position of every colour in relation to its primarycolour. This two-dimensional diagram is only valid for onerespective colour temperature, and it is a section of afunnel-shaped body (Fig. 7).

By transforming the triangular face shown in the cube, theCIE diagram of 1931 is derived (Fig. 8).

The advantage of the 2D representation according to CIElies in the fact that instead of a colour description with threetristimulus values, the description can be made with astatement about hue and saturation i.e. with the two valuesx and y. The disadvantage is that the diagram is only validfor one spectral brightness and the colour coordinationdoes not correspond well with human colour perception –that means an unequal distribution of the colours within theperceptual colour space. This attribute was described byMacAdam and is called MacAdam ellipses in the CIE-1931diagram.

In order to achieve a more even distribution, the CIEdescribed the CIELUV colour space in 1976. It rectifies thedeficiency mentioned above in a way that enables the

Z

Y

X

xy

Figure 7Section from a CIE-1931 diagram

Figure 6Optimum colour body (Siegfried Rösch)

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COLORIMETRY

human eye to perceive equaldistances between colours onthe diagram as equal colourdifferences, independent oftheir position.

Fig. 9 shows a CIE u’v’ colourspace with an even distributionof colour perception. Due to its2D representation, this colourspace is valid only for oneparticular spectral brightness.

The CIE u’v’ colour space is agood way to display colourmetric connections in a percep-

tual way – for evaluating TV monitors, projectors and the colour quality of image-capturing equip-ment.

In 1976, the CIE introduced the CIELab colour space (Fig. 10), which combines equal colourperception with a 3D representation.

CIELab is often used as a device-independent colour space for the conversion of different colourspaces (Profile Connection Space; PCS).

Standard and correlation of different colour spacesThe RGB colour space is based on an additive colour mixture with the real primary colours – red,green and blue. In a three-dimensional Cartesian coordinate system, the RGB colour space can berepresented as a cube. Fig. 11 shows the correlation of the CIE-1931 diagram and the cubic RGBcolour space.

The position of the triangle in the CIE-1931 diagram depends on the phosphorus used in thedevices. Thus the RGB colour space is a device-dependent colour space. A metrically correct

Figure 9CIE u’v’ diagram

Figure 8CIE xy diagram

White

L*

Black

Green

-a*

Red

+a*

Luminance

Hue

Saturation

Figure 10CIELab colour space

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COLORIMETRY

colour representation of videosignals is only possible bydefining the different types ofphosphorus on an exact wave-length spectrum. For the colourtelevision systems used inEurope, the EBU phosphoruswas made the standard.Fig. 12 shows the most signifi-cant attributes of the mostimportant colour televisionsystems. The yellow row of thetable highlights the attributes ofthe EBU phosphorus, asdefined by ITU-R BT.470.

The EBU phosphorus is alsothe basis for the sRGB colourspace, developed by Microsoftand HP and standardized underIEC 61966-2-1. In addition tothe EBU phosphorus character-istics, the sRGB colour spacealso takes viewing conditionsinto consideration, especiallyexternal light sources with D50and 20% reflection.

Common printing presses,colour printers and film tech-nology all work with subtractivecolour mixing. For film techno-logy, the three primary colours– cyan, magenta and yellow –are sufficient. For printing tech-nology, on the other hand,black is usually added; thecolour space is then calledCMY(K). The CMY(K) colour space can also be represented as a cube.

Just like the RGB colour space, the CMY(K) colour space is device-dependent, because it dependson the colour pigments of the printing products or the colouring agents in the film technology.

Due to the scattering effects of the colour pigments used, the conversion between the colour spacesinvolved is a non-linear process. In the sphere of the primary colours, the colours of the sourcecolour space cannot be displayed in the target colour space and must be replaced by similar colours.There are different conversion methods, which will be explained in the following section.

Colour space conversionFor a colour space conversion, it is necessary to describe the source colour space and the targetcolour space as accurately as possible. In the “profiles”, the attributes of a colour space are definedin relation to another, either by a characteristic, a matrix or by a so-called “Look-up table”. In order toavoid having to work out a profile for every possible combination of source and target colour space,the target or source colour space of choice is a device-independent “connection space” or ProfileConnection Space (PCS). So, only one profile is needed – with the PCS acting as a source or as atarget space, as required.

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.150

y = 0.060

x = 0.300

y = 0.600

x = 0.640

y = 0.330

IEC 61966-2-1

sRGBThe difference to ITU-R BT 709 :

Additional definition of the viewing

condition: external light sources with

D50 and 20% reflection

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.15

y = 0.06

x = 0.29

y = 0.60

x = 0.64

y = 0.33

ITU-R BT.470-6

Conventional TV-System PAL G

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.155

y = 0.070

x = 0.310

y = 0.595

x = 0.630

y = 0.340

SMPTE-C

SMPTE-RP 145-1999

SMPTE-P22- Phosphore

(NTSC since 1979)

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.310

y = 0.316

x = 0.140

y = 0.080

x = 0.218

y = 0.712

x = 0.674

y = 0.326

FCC 1953

(NTSC until 1979)

Y-matrixreference

white

BGR

Primary coloursStandards

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.150

y = 0.060

x = 0.300

y = 0.600

x = 0.640

y = 0.330

IEC 61966-2-1

sRGBThe difference to ITU-R BT 709 :

Additional definition of the viewing

condition: external light sources with

D50 and 20% reflection

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.15

y = 0.06

x = 0.29

y = 0.60

x = 0.64

y = 0.33

ITU-R BT.470-6

Conventional TV-System PAL G

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.313

y = 0.329

x = 0.155

y = 0.070

x = 0.310

y = 0.595

x = 0.630

y = 0.340

SMPTE-C

SMPTE-RP 145-1999

SMPTE-P22- Phosphore

(NTSC since 1979)

E’Y

= 0.299 · E’R

+ 0.587 · E’G

+ 0.114 · E’B

x = 0.310

y = 0.316

x = 0.140

y = 0.080

x = 0.218

y = 0.712

x = 0.674

y = 0.326

FCC 1953

(NTSC until 1979)

Y-matrixreference

whiteBGR

Primary coloursStandards

Figure 12Primary colour for different TV standards

Human eye

Monitor

E'R

E'G

E'B

E'R

E'G

E'B

Figure 11Correlation between RGB cube and CIE xy diagram

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COLORIMETRY

If the target colour space is smaller than the source colour space, the colour space has to becompressed, which can be relevant to the entire colour space or just to parts of it. If the target colourspace is larger than the source colour space, the choice is either to keep the colours of the sourcecolour space, or to expand them. Depending on the desired result of the colour space conversion,different rendering intents have been defined, which get their own chart entries in the profiles.

The rendering intent defines the desired effect of a colour space conversion. Since colour spaceconversions cannot be done without artefacts if the target colour space is smaller than the sourcecolour space, a specific rendering intent must be defined. Usually, we can choose between thefollowing three rendering intents.

The perception-oriented rendering intent optimizes the overall colour impression of the targetcolour space, so that hue changes are accepted in favour of the overall impression. The absolutecolour metric rendering intent accurately (i.e. exactly) converts colours that are inside the targetcolour space. Colours that are outside the target colour space are moved to the next representablecolour of the target colour space. This does not change the white reference. In contrast to this,however, the relative colour metric rendering intent may possibly change the white reference ofthe source colour space into the white reference of the target colour space.

The ICC (International Colour Consortium) developed a method for describing colour spaces andcolour space transformations, which is finding increasing acceptance. Fig. 13 shows ICC colourmanagement as a block diagram.

As the diagram shows, theincoming content is transferredto the PCS via a colour spacetransformation. For reproduc-tion with printers or monitorsusing the PCS, another colourspace transformation must beconducted. Every colour spacetransformation has its respec-tive profile and rendering intent.

In order to compare colourspaces, the ICC developed aprogram for comparing twocolour spaces in three dimen-sions (http://www.iccview.de).This program offers the oppor-tunity to visualise in advancethe colours that cannot bedisplayed in another colourspace and which must there-fore be brought to the targetcolour space according to therendering intent.

Colour space conversion

Colour space conversion

Colour space conversion

Embedded Profile

Colour data Profile data

Pattern

Display Profile

Output Profile

Input Profile

Reproduction

CMM = Colour Management Module with

colour space conversion and Profile

Connection Space (PCS)

Figure 13ICCview colour management system

AbbreviationsCCVS Composite Colour Video Signal

CIE Commission Internationale d’Eclairage

CMYK Cyan-Magenta-Yellow-blacK (colour space)

HCV Hue-Chroma-Value (colour space)

HLS Hue-Lightness-Saturation (colour space)

HSB Hue-Saturation-Brightness (colour space)

HSV Hue-Saturation-Value (colour space)ICC International Colour ConsortiumNTSC National Television System Committee (USA)PAL Phase Alternation LinePCS Profile Connection SpaceRGB Red-Green-Blue (colour space)SECAM Séquentiel couleur à mémoire

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COLORIMETRY

Practical colour managementAs mentioned before, the tradi-tional field of television broad-casting has no use for colourspace transformations. Onlywhen devices with “foreign”colour spaces are used, is acolour space conversion neces-sary. Fig. 14 takes this fact intoaccount.

If non-broadcast sources andsinks have to be integrated intoa television studio environment,then also their colour spacesmust be converted. The sameholds true if content from thetelevision studios is to be proc-essed with PC-based graphicssystems. As already explained,a PCS can achieve this taskeconomically and it can only bereached through the use ofprofiles. Unfortunately, the nec-essary profiles are not alwaysprovided by the manufac-turers. Usually, the manufacturers provide standard profiles but they are not sufficient for a correctconversion. If profiles are not available, they have to be produced by means of calibration andsubsequent profiling. Commercial equipment that is currently available for profile production islimited to test charts and profiling software for digital cameras, scanners, computer monitors andprinters. Software for the profiling of video equipment (film scanners, exposure systems, videocameras etc.) is still scarce.

For input devices (scanners, digital cameras), manufacturers use a standard pattern (slides or posi-tives, depending on the device), in order to determine deviations from a reference database. Withthese deviations, a colour profile can be verified.

Printers can be profiled by printing out test image files, measuring the printouts photometrically andcomparing them with reference values. This profile takes the printing technique, paper quality, inksand application into account.

The most precise way of profiling TV monitors can be achieved with a spectral photometer andprofiling software. The profiling software shows a number of different colours that have been meas-ured with a spectral photometer and compared with the reference data. The software then createsthe monitor profile – also taking into account the features of the graphics card.

ConclusionsAs Fig. 14 shows, extensive colour management is only necessary when the production environ-ment uses devices which were not specifically developed for television broadcasting and which usecolour spaces other than the ITU-R BT.470 standardized RGB colour space. If the basic principlesoutlined in this article are not followed, it can lead to significant colour errors which will be difficult tocorrect at a subsequent stage in the broadcast chain.

Although the end user cannot normally recognize small errors in colour reproduction (because areference colour source is not available to him/her), it is absolutely necessary to deliver the colours

BearbeitungVideo-Grafic

C

C

C

CScan

PC-Grafic

Digital Print

PCS

C

C

C

C

Storage

BearbeitungProcessingBearbeitungProcessingBearbeitungProcessing

BearbeitungVideo-GraficBearbeitungVideo-GraficBearbeitungVideo

Graphics

C

C

C

C

C

C

CC

CCScanScan

PC-Grafic

Digital Digital

Camera

PrintPrint

PCS

C

C

C

C

CC

CC

Storage

PCS

PC Graphics

Figure 14Colour management system recommended by an expert group out of the public broadcaster in Germany

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COLORIMETRY

as accurately as possible – with minimal variation from the well thought-out colour schemesdesigned by the production staff. Commercials in particular must be transmitted in the exact coloursintended – to enable product recognition and advertising effects – because the consumers alreadyhave the colour references in the product itself.

Practical work has shown that, with correct colour management, even critical colours – such asterracotta gold – can be printed with a colour-managed printer, can be used as a studio backgroundand can appear as terracotta gold again on the consumer’s television set.

Friedrich Gierlinger graduated in Telecommunications from the advanced technicalcollege of Munich in 1979. Since then he has worked for the IRT – the centralresearch laboratory of the German, Austrian and Swiss public broadcasters – in thedevelopment of measurement techniques for analogue and digital SDTV, the devel-opment of HDTV A/D and D/A converters, matrix and clock generators. He was alsoinvolved in the development of the EBU measurement guidelines for SDI.

Mr Gierlinger chairs different working groups dealing with acceptance guidelines fordifferent frequently-used broadcast products such as VTRs and Non-Linear Editingsystems. He is member of a working group comprised of measurement and servicedepartment leaders from the public broadcasters of Germany. He is also a memberof the SMPTE and is particularly engaged in picture quality evaluation.

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