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Color MeasurementTechnology for thePrinting Industry.

TECHKONT h e C o l o r M a n a g e r s

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ContentWhat is Color ?Light and color

How do our eyes see ?

Color synthesisAdditive color synthesis

Subtractive color synthesis

Auto-typical color synthesis

Color systems and ColorimetryStandard chromatric values

CIE standard chromatic table

CIELAB and CIELUV

Color differences

Metamery

Color measurementMeasurement principle

Colorimetric method

Spectral method

Spectral density measurement

TECHKON measurement devicesand softwareSpectro-Densitometer SD 620

Scan-Spectrometer RS 800

Spectrophotometer SP-series

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What is Color ?Light and colorWe generally see colors only if the light is bright enough.Light is radiation composed of electro-magnetic waves.Any one wavelength corresponds to any one color.

Light is emitted by self-emitters such as the sun, a light bulb or amonitor. The radiation these objects emit often consists of a mixof different wavelengths. We can only see wavelengths at therange of 380 through 780 nm (nanometers) which falls betweenultraviolet and infrared.

White daylight contains all wavelengths in nearly equal shares.In blue light short wavelengths are the most intense, in red lightlong wavelengths are the most intense. A prism lets us see thevarious shares of color.

The monochromatic light of a laser is a special case because it consists ofonly one wavelength. The red light of a HeNe-laser is typical with a wave-length of 632 nm.

Most objects that we see in color are not self-emitters rather than objectswhich convert the self-emitting light into bodily light. These objects' coloris created by absorbing a share of the light and reflecting the remainder.

The colors of a rainbow are created by thespectral decomposition of white light.

At nights all cats are gray. Only light creates color.

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm 750 nm

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How do our eyes see ?

The retina in our eyes is made up of two kinds of light-sensitivecells:

• Approximately 120 million cells for perception of black, white and gray tones• Approximately 6 million cells for perception of colors.

There are three types of color-sensitive cells which react to threedifferent areas of wavelength: Red, green and blue. We create acolor blend from the red, green and blue signals. This enables usto distinguish several million colors.

The CIE (Commission International de l'Eclairage = InternationalLight Commission) investigated the spectral sensitivity of humansto red, green and blue in 1931. The results of the study are thestandard spectral value functions for the standard observerwhich represents our average capacity to perceive color. Thesecurves of sensitivity form the basis for any color measurement.

The CIE has defined the 2° and 10° standard observer becausewe perceive color in small objects, which we view at a smallerangle, somewhat differently than larger objects viewed at anangle of 10° or larger.

Color perception, however, is not an absolute sense rather than a subjective perception influenced by many factors.Whereas we can easily distinguish color differences between two adjacent color tones (hues), we find it difficult toremember color exactly and recognize it with certainty. This is why we measure colors to exactly define and reproducecolors as a numerical value independently of our visual impression.

This is how humans see color.The standard spectral functions for the 2° and10° standard observer describe our sensitivity tored, green and blue.

2,0

1,5

1,0

0,5

0700 nm500 nm400 nm 600 nm

10°2°

Sta

nd

ard

sp

ect

ral v

alu

es

z(λ)

y(λ)x(λ)

2° 10°

2° and 10° observation angle of the standard observer.We view small area of color areas somewhat differentlythan larger areas of color.

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Color synthesis

Additive color synthesis of light colors.This is how color TV images are created.

Subtractive color synthesis of bodilycolors. This is how, for example, printedimages are created.

To understand the perception of colors, it is important to understand theprinciples of color synthesis.

Additive color synthesis

Additive color synthesis occurs in any color image that is emanated by self-emitters and consists of a combination of the primary light colors red, greenand blue. Primary colors are basic colors that cannot be created by blendingother colors. In turn, however, primary colors can create all other colors.

Additive color synthesis results in the following:

red + green = yellowblue + green = cyanred + blue = magentared + green + blue = white

Blending the three basic colors of light in equal parts creates white light. Thevarying brightnesses of the light colors add up as they meet. The result of anadditive synthesis will correspondingly always be brighter than the brightnessof each individual color blended. Color TV is a typical example of additivecolor synthesis.

Subtractive color synthesis

Subtractive color synthesis of bodily colors is the exact opposite of additivecolor synthesis of light colors. The secondary colors magenta, yellow and cyanmixed in subtractive color synthesis are the counterpart of the primary colorsmixed in additive color synthesis, as indicated by the table of blending results:

magenta + yellow = redcyan + yellow = greencyan + magenta = bluecyan + magenta + yellow = black

Blending two primary colors creates secondary colors. Blending two secondarycolors in turn creates a primary color. Blending secondary basic colors in equalparts creates the bodily color black. Color created from secondary colors willalways be darker than the colors from which they originate.

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Auto-typical color synthesis

Auto-typical color synthesis occurs during multi-color screenprinting. The screen dots of the colors cyan, magenta and yelloware partially printed next to each and partially over each other.Because the print colors are translucent, the colors red, green,black and blue are created in the areas of superimposed printingas a result of subtractive color synthesis.

If the individual screen dots are viewed with a magnifying glass,you can see the subtractively created colors. Without a magnifyingglass, our eyes do not resolve the screen dots and will see colorsthat are created by an additive synthesis of the colors reflected bythe screen dots. We call this change from subtractive to additivecolor synthesis an auto-typical color blending.

Auto-typical color synthesis is a special caseof multi-color screen printing. A magnifyingglass shows the screen structure's subtractivecolor components. Without a magnifyingglass, we see the additive image (E.L. Kirchner,Berlin Street Scene, 1913, detail)

Color systems and ColorimetryStandard chromatic values XYZ

Standard spectral value functions are the basis for any arithmetic eva-luation of colors through measurement. Using a colorimeter, they providethe standard chromatic values XYZ of a color. The standard chromaticvalues XYZ are used for calculating all other colorimetric indices such asL*a*b* and L*u*v*. The standard chromatic values XYZ lead to a definite,arithmetic description of a color. A certain type of orange, for example,features the standard chromatic values in terms of standard light D 65 anda 10° observation angle:

X = 49,13 for the red component Y = 34,51 for the green component Z = 2,67 for the blue component.

No other color will have the same values.At high intensity the values for XYZ = 100.

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0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 x

0,1

0

0,2

0,3

0,4

0,5

0,6

0,7

0,8

y

The CIE standard color table shows additively describedcolors made up of the primary colors red, green and blue.Due to technical processes, only a section of the theoreticalcolor range can be shown.Orange outline: Color range of four-color offset printing.Yellow outline: Color range of a monitor.

CIE standard color table

Very early on people recognized that the standardchromatic values XYZ weren't clear enough and didn'tprovide a better picture of the hue and brightness of acolor. The CIE first tried to circumvent this weakness bydeveloping a CIE standard color table as indicated bythe coordinates of the standard chromatic valuecomponent x and y.

The color coordinates x and y are complemented bythe standard chromatic value Y which describes thebrightness of a color. This creates a color space with thecoordinates Yxy in which the color spot of any one coloris fixed by three values, a characteristic also featured bythe newer color models CIELAB and CIELUV.

The chromatic components are calculated according tothe formulas

Xx =

X+Y+Z

and

Yy =

X+Y+Z

and will range from 0 to 1. Y will range from between 0for black and 100 for white.

Our perception of colors depends on the light con-ditions. The color impression of an object will varydepending on whether it is viewed in daylight orunder artificial light. The standard chromatic valuesXYZ therefore refer to standard light whose spectralcomposition has been determined by the CIE.

The important standardized types of light are:

• Type A for the light emitted by a light bulb• C, D 50 and D 65 for varying daylight

Orange, for example, has the following values:

Y = 34,51x = 0,569y = 0,400.

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+byellow

-agreen

+ared

-bblue

a

bC

h

Der CIELAB-circle shows a cross-section of the color space.

CIELAB and CIELUV

In 1976 CIE optimized color systems with two newstandardized color spaces:

• L*a*b* color space CIE 1976• L*u*v* color space CIE 1976

CIELAB and CIELUV are currently the most importantcolor spaces for analysis and description of bodilycolors. The formulas for calculating

L*a*b*L*u*v*

and their derived polar coordinates

L*C*h*

were defined in 1990 with a new version of standardDIN 5033-3. The standard chromatic values XYZ onceagain form the basis for calculation.

The color spot of a color is determined by the vertical brightnesscoordinate L and the color coordinates ±a and ±b in L*a*b* colorspace. Saturation C equals zero at the center of the L-axis andgains as it moves outward.

In L*a*b*-color space, chromatic values are defined by

L* for brightnessa* for red-green valuesb* for yellow-blue values.

C* describes saturationh* describes hue in the CIELAB circle

L = 100

L = 0

L

+a*

+b*-a*

-b*

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L* ranges from 0 for black and 100 for white. Thebrightness values are distributed along a vertical axisin the center of the color space.

Axis a describes the progression from green to red,axis b the progression from blue to yellow. Values ofa* are negative in the green section and positive in thered section. In an analogous manner, b* is negative inthe blue section and positive in the yellow section.A and b equal zero in the colorless center of the CIE-circle. Saturation value C is also zero at the center andgains in all directions moving outwardly.

In CIELUV color space the color coordinates a* and b*have been replaced by u* and v*. L* is the same for bothcolor spaces.

The described values for the example orange are:

L* = 65,37 L* = 65,37a* = 50,86 u* = 122,37b* = 21,92 v* = 60,22C*ab = 96,42 C*uv = 136,38h*ab = 58,17° h*uv = 26,20°

These values show the basic concept of the new colorspaces, for example, describing colors with measure-ment values for:

• brightness• hue and• saturation.

The new color spaces have gained in clarity throughnew color measurement values which are calculatedwith far less clear formulas, as illustrated by theexample of the CIELAB formulas.

L*=116 √(Y/Yn)-163

a*=500[ √(X/Xn)-√(Y/Yn)]3 3

b*=200[ √(Y/Yn)-√(Z/Zn)]3 3

C*ab= √(a*2+b*2)

hab= arctan(b*/a*)

In these formulas Xn , Yn , and Zn are the standardchromatic values of a completely offwhite body for acertain type of light. Xn , Yn , and Zn describe theachromatic spot in color space. This is the referencepoint for the color coordinates.

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The entire gap between two colors A and Bis expressed by ∆E. The value ∆E considersdifferences in hue, brightness and saturation.

Color differences

Color spaces L*a*b* and L*u*v* have the special advantageof displaying color differences equidistantly in terms of ourperception. This makes it easier to evaluate color differencesbetween rated and actual samples.

Determining color differences is the basis for judging colorquality. This applies to all areas of production and applicationof colors and, of course, for the graphic arts industry. The growingdemands placed on quality and conformity of color presentationsare being satisfied by special processes and modern measurementtechnology.

Color differences are quantified by ∆-values (∆ = delta). ∆-valuesare the difference between rated values and actual values. All colormeasurement values may be expressed as ∆-values.

For example:

∆L* for differences in brightness∆C* for differences in saturation∆a*, ∆b*, ∆u*, ∆v*, ∆h* for differences in color.

The ∆E-values which evaluate the entire color distances in termsof brightness, saturation and hue take on special significance inpractice. In L*a*b* color space ∆Eab expresses the result of ∆L*, ∆a*,∆b*. The value is calculated according to the following formula.

∆E*uv is calculated in the same way ( ) forL*u*v* color space.

Color differences less than ∆E*ab = 1 are practicaly invisible,divergences of ∆E*ab = 3 and larger are clearly visible.

Divergences of light or low saturated colors are more bothersome than for stronger colors.

Other known values for expressing the entire color distance are∆E*CMC and the newly defined, not yet standardized, ∆E*94.

A

A'B

∆E

-a* +a*+b*

-b*

L*

∆E*ab= √∆L*2+∆a*2++∆b*2

∆E*uv= √∆L*2+∆u*2++∆v*2

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Metamery

Metamery is the deviation of a color from two or moresamples resulting from changes in ambient light. Twosamples may look exactly the same in daylight, however,they may deviate significantly from each other underartificial light.

Metamery has nothing to do with the wellknown dailyoccurrence whereby an object changes color under diffe-rent lighting. A white dress that appears red under a redparasol and yellow under a yellow parasol is not metamery.The phenomenon of metamery exists when a red hat isadded to the red dress such that both look the same underone parasol, but differ from each other under anotherparasol.Metamery never exits with one single sample rather thanrepresents the color deviation under differing lightbetween two or more samples.

Metamery exists when two samples' remission curves(see p. 13) feature slight differences. The difference inremission is such that there is no visible color differenceunder a certain light, however, there is a clear colordeviation under different lighting. We call such colorsconditionally equal colors as opposed to equal colorswhich look absolutely the same under any light due tocompletely identical remission curves.

The strongest differences in metameric colors occur duringchanges in very different types of light, for example, whengoing from daylight to artificial light. The metamery index,as defined in standard DIN 6172, is correspondinglycalculated with a spectrophotometer for changes fromdaylight types C, D50 and D65 to artificial light A.

Metamery is especially important in selecting color fortextiles. However, print colors too, especially special inks,should feature as little metamery as possible.

Metameric colors change their visible colordistance as the light conditions change.

Metamery is caused by slight differences inremission curves. Metameric colors are onlyconditionally equal. They only appear thesame under certain light.

Sample 1 Sample 2

Same appearance of color in daylight

Sample 1 Sample 2

Color deviation under artificial light due to metamery

Sample 1Sample 2

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Color measurement

Colorimetric method

The measuring light emitted by a lamp is reflected by thesample and received by three sensors. Filters producing spectralsensitivity in the three color channels corresponding to thestandard spectral functions, and as such simulate the spectralsensitivity of the retina, are fitted in front of the sensors.Evaluation of the sensors' signals derives the standard chromaticvalues XYZ for red, green and blue. These are then used for allother colorimetric calculations.

The simple measurement principle makes for inexpensive andreliable measuring devices. Despite constant improvements,however, these do not attain the absolute measurement accu-racy of spectrophotometers. They are, however, usefulfor comparative measurements.

Some of the limitations imposed by this system includeincomplete simulation of several types of light, the lack ofspectral remission values and measurement of metamery.

The colorimetric method works on the principle ofour eyes. The color shares of red, green and blue areregistered by three sensors.

Measuring principle

We measure color with the goal of objectively describing and quantifying ourvisual impression of a color with color measurement values. This lets us definecolors numerically and to transmit the color information without the sample justby numbers. Another important application is the measuring of color differencesbetween sample and printing proof for quality assurance in printing.Color measurement is also the basis for formulating special colors. Anotherimportant application area is color characterization of open desktop-publishing(DTP) systems as part of color management.There are two measuring methods for these tasks:

• Colorimetric method• Spectral method.

Both methods are defined in the standard DIN 5033.

Z Y X

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Spectral methods

Spectrophotometers measure the remission values of theentire visible spectrum. To do so, the spectrum is dividedinto sections with a bandwidth of 10 - 20 nm. Each sectionrepresents one remission value. In modern measuringdevices, grid (bzw. grated-array)-diode or filter-diodemodules cause the spectral sectioning of the measurementlight reflected by the sample.

The diffraction grid of the grid-diode module dissects thelight and is projected onto a diode array with preferably256 adjacently placed diodes. Electronic elements firstamplify, digitalize and then further evaluate the high-resolutions signals produced by the many diodes. The firstresult of spectral measurement is a series of remissionvalues which are graphically represented as remissioncurves.

Filter-diode modules consists of several diodes fitted withnarrow-width color filters. Each diode measures a certainbandwidth of the spectrum. An alternative way of obtai-ning remission values consists of irradiating the sample insequence with spectrally narrow-width light at differentwavelengths as emitted by colored light-emitting diodes.A spectrally broad-width sensor then registers the individualremission values.

Remission values and remission curves provide completeinformation on the measured color. The standard chromaticvalues XYZ are rendered with a special calculation, the so-called valence-metric evaluation. This process relates theremission curves and the standard spectral value functions.The remission curve at the right, for instance, was regis-tered with a TECHKON Spectrophotometer from the SP-series. The remission curve starts at the left in the blue areaat 380 nm and ends in the red area at 780 nm.

The diffraction grid on the grid-diode-moduleprojects the dissected spectrum onto a seriesof diodes.

Series of diodes

Diffractiongrid

Optic fiberinput

The remission curve R(λ) of the spectrophotometercontains all information on the measured color.

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Spectral density measurement

Remission curves not only provide colorimetric but densito-metric values as well. This fact has led to the developmentof spectrophotometers that provide the option of colorvalue as well as density value measurement. There are alsoso-called spectro-densitometers that are equipped withspectral measurement modules as opposed to the better-known filter-equipped densitometers.

The measurement principle of these spectro-densitometersconsists of deriving density curve D(λ) from the remissioncurve R(λ). The density curve is the proportional mirrorimage of the remission curve: high remission values corres-pond to low density values and vice versa. For any onewavelength, the density curve provides the density valuesand the index values derived therefrom such as dot area,dot gain and printing contrast.

The standardized process colors cyan, magenta and yellowon the euro-scale have a density maximum at 620, 540 and430 nm. Their density is measured at these wavelengthswith equally standardized filters. Special colors generallyfeature a density maximum at different wavelengths andtherefore cannot be satisfactorily measured by conventionaldensitometers fitted with filters. Spectro-densitometers,however, are capable of defining density at any point in thespectrum using matematically-defined filters chosen at will.This makes them suitable for use with any color.

The density curve D(λ) is calculated from theremission curve R(λ). Density maximum andremission minimum are at the same wavelength.

The remission curve R(λ) of a purple special colorwith the lowest remission at 570 nm.

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TECHKON Measurement devices and Software

TECHKON Spectro-Densitometer SD 620

The TECHKON Spectro-Densitometer SD 620 represents a newgeneration of color reflection densitometers. The spectral measure-ment principle expands applications beyond four-color print to multiand special color print. For the first time even special colors can bedensitometrically controlled and reproduced without limit in terms ofsolid density, screen values and printing contrast.

A high-resolution spectral measurement element forms the basis forthe multiple measurement functions. The measurement elementspectrally registers colors under standardized conditions and usesmathematical filter curves to exactly calculate key densitometricvalues. This permits the selection of any filter characteristic, such asDIN E or ISO T/I, for example.

The graphic display shows print index lines andspectral density curves. Using the density curve, thedevice selects the most appropriate measurementfilter for a special color.

The TECHKON QS Pro as well as TECHKON EXChangeprograms give users the value-added of a modern,computer-based evalution of measurement data.The spectro-densitometer's measurement values aredisplayed in tables and clear charts.

Spectral density curve of a special color with a densitymaximum at 560 nm.

The Spectro-Densitometer SD 620 can be used for anycolor thanks to freely selectable measurement filters.

Print index line for a special color with a dot gain of26,5% at a reference value of 40%.

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TECHKON Scan-Spectrometer RS 800

The Scan-Spectrometer RS 800 is another spectrally-measuringdensitometer in the TECHKON product line. The device auto-matically measures all common key densitometric values onany print control strip, step wedge and test chart. To measure,the RS 800 is quickly passed over the print control strip. Thedevice automatically recognizes the different measurementpatches and their position on the print control strip. Guide-wheels on the bottom of the device ensure a straight run. TheRS 800 measures individual measurement control patches like aregular densitometer.

For measurements on images, the RS 800 is guided from adefined starting position with high accuracy of repeatability tothe selected spot on the image. This lets user measure printgray and color balance at critical spots in the image. Anotherinteresting application is the ability to measure density profilesof color gradations.

Thanks to its spectral-measurement principle, the TECHKONScan-Spectrometer RS 800 can be used beyond four-color printfor all special colors.

The Scan-Spectrometer RS 800 always works in conjunctionwith a PC and the TECHKON ExPresso Pro software. Thesoftware shows the current density deviations in the individualcolor zones.

The RS 800 measures any kind of print controlstrip, step wedges and test charts.

The TECHKON ExPresso Pro software clearlydisplays measurement results of print controlstrips in the form of density profiles.The program is designed for special-color printof up to eight colors.

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TECHKON SpectrophotometerSP 810 SP 820 SP 830

The SP-series spectrophotometers with a resolution of 10 nm (internally = 3 nm) satisfy the highest demands interms of precision and measurement accuracy. Their performance characteristics make for universal applicationwherever colors need to be measured exactly. All elements in the grid-diode module are firmly connected andthus never need adjusting. There are no movable parts in the devices. The compact measurement head is posi-tioned directly over the measurement object, thus enabling quick and easy measurement. The three devices inthe SP-series feature the same high-value, spectral measurement module, but are equipped differently for theirspecial applications.

SP 810

The SP 810 Spectrophotometer is designed for inte-gration with computer programs. Accordingly, thedevice has no display and is especially compact.

The SP 810 is integrated into the color managementprograms of different manufacturers. The TECHKONEXChange software even transfers measurement datainto any programs.

TECHKON SP 820

SP 820

The SP 820 Spectrophotometer is a universallyapplicable measurement device for all areas of thegraphic arts industry. It features all typical colorimetricfunctions that are shown on a menu-driven, largegraphic display. The most important application isthe comparison of color samples with printing proofs,colorimetric control of master images and proofs aswell as the fine-tuning of special colors.

TECHKON SP 810

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SP 830

The SP 830 Spectrophotometer features the colorimetricfunctions, but in addition provides the user all densito-metric measurement processes including the display ofspectral density curves. The free choice of measurementfilters enables density measurements on any specialcolor.

Additionally selectable polarization filters guarantee highinter-instrument agreement with other densitometers.Whereas color measurements are usually taken withoutpolarization filters, polarization filters are typically usedin density measurements. The easy switching of polari-zation filters on the SP 830 allows for both measurementprocedures under standardized conditions. Just as theSP 810 and SP 820 Spectrophotometers, the SP 830communicates with the PhotoLab program and otherTECHKON software solutions.

Measurement data can be sent to a computer via theserial port and, in reverse, entire color files can be re-ceived from applications programs. TECHKON offers anumber of software solutions for different applications.The TECHKON PhotoLab program was developedespecially for print control and clearly displays the colorspot and any deviations of the printed color in L*a*b*color space.

Measurement head for the SP 830.Color measurements can be takenwithout and density measurement withpolarization filters.

The TECHKON PhotoLab programcontrols and documents the specialcolors of a print job.

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IndexCIE ...................................................................................................... 4, 7, 8

CIELAB ................................................................................................ 7, 8, 9

CIELUV ................................................................................................ 7, 8, 9

CIE standard color table ...................................................................... 7

Colorimetric method........................................................................... 12

Color management ............................................................................. 12, 17

Color synthesis .................................................................................... 4, 5, 6

∆E color differences ............................................................................. 10

Density ................................................................................................ 14

Density curve ...................................................................................... 14, 15, 18

L*a*b* ................................................................................................. 6, 8, 9, 10

Laser ................................................................................................... 3

L*u*v* ................................................................................................. 6, 8, 9, 10

Metamery ........................................................................................... 11

Primary colors ..................................................................................... 5, 7

Print charactreristic line ....................................................................... 15

Remission ............................................................................................ 11

Remissions curve ................................................................................. 11, 13, 14

Secondary colors ................................................................................. 5

Spectro-Densitometer ......................................................................... 14, 15

Spectral density measurement ............................................................ 14, 18

Spectral methods ................................................................................ 13, 15, 16, 17

Spectrophotometer ............................................................................. 11, 12, 13, 14, 17, 18

Standard chromatic values .................................................................. 6, 7, 8, 9, 12, 13

Standard light ..................................................................................... 6, 7

Standard observer ............................................................................... 4

Standard spectral value functions ........................................................ 4, 6, 12, 13

TECHKON measurement devices ........................................................ 13, 15, 16, 17, 18

TECHKON software ............................................................................. 15, 16, 17, 18

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