Color order system

COLOUR-ORDER SYSTEMS

• Introduction

• Munsell colour-order system: Munsell

colour-order system: realisation

• Visual assessments of colour

INTRO

A technical committee of the International Organisation for Standardisation, ISO/ TC187 (Colour Notations), has defined a colour-order system as a set of principles For the ordering and denotation of colours, usually according to defined scales.

colour-order system is a set of principles that defines:

(a) an arrangement of colours according to attributes such that the more similar their attributes, the closer are the colours located in the arrangement, and(b) a method of denoting the locations in the arrangement, and hence of the colours at these locations.

INTRO

The purpose of a colour-order system determines the number of attributes that must be considered, each attribute defining one dimension of the system.

For example, a one dimensional system may be adequate in the design of lighting systems, where it is sometimes sufficient to consider only the single attribute of CIE luminance factor (Y),

which is a function of the total reflectance of each surface within the volume to be lit.

INTRO

Colour is three-dimensional, however, and for a complete colour specification a colour-order system such as that given by CIE x, y and Y is necessary.

x, y and Y are attributes of a colour and each is used to define a dimension of the system.

The dimensions are arranged by means of three mutually perpendicular axes. The three attributes are fundamental to the system because they define it, and they are orthogonal (that is, each may be varied without having to change any other).

INTRO

We may however define a colour-order system by means of The orthogonal attributes λd (or λc), pe and Y, with x and y then being derived attributes.

Whether the system be defined by means of x, y and Y, or λd (or λc), pe and Y, the relationships between colours in it are the same.

Each defines the same colour space, that is, the geometric representation of colours in three dimensions .any three-dimensional colour-order system necessarily defines a colour space and any colour space allows colours to be ordered.

DIFERENCE b/w color order system and color space

a colour-order system is primarily defined by a set of material colour standards,

whereas a colour space is essentially a conceptual arrangement.

colour-order systems

Over the years, more than 400 colour-order systems have been compiled.

The first to be recorded was devised by Aristotle about 350 BC. It was vaguely three-dimensional and white was placed opposite black; red, however, was placed between black and white, red being the colour of the sky between the states of night and day.

Leonardo da Vinci (1452–1519) is said to have painted sequences in which closely related colours were placed near each other.

Newton (1642–1727), whose discovery of the nature of white light may be regarded as having begun the science of colour physics, arranged all the hues in a circle, with complementary hues opposite and white at its centre. These arrangements were two-dimensional, however, and could not therefore include all colours.

Munsell colour-order system: concept

• Munsell apparently first described his system in a lecture given in 1905 in which he listed its advantages, including:

• (a) colour names based on natural objects (which often vary in colour) are replaced by a definite notation;

• (b) each colour is named by its notation and can be recorded and transmitted by it, enabling contracts for coloration to be closely specified;

• (c) the system can be expanded to accommodate new colours.


• A model of the Munsell colour-order system is shown in Plate 2.

• The three fundamental orthogonal attributes defining the system are called

• Munsell value V,• Munsell chroma C• and Munsell hue H.


• A model of the Munsell colour-order system is shown in Plate 2.

• Each is scaled with the aim of perceptual uniformity, so that equal changes in any one of the attributes represent the same perceived difference in colour.

• Unlike the CIE xyY system, it uses cylindrical coordinates;

• for readers meeting this coordinate system for the first time, a brief explanation follows. (In the following three paragraphs, x and y are general variables,

• not CIE x and y.)


• We are all familiar with two-dimensional plotting of y against x,

with the axes of x and y perpendicular to each other (Figure 4.1).

In this so-called rectangular coordinate system,

• the point A = [2,2] • (meaning x = y = 2) is located, • relative to the origin (O =[0,0]), by measuring two units along the x-

axis to the point XA, and then from XA two units parallel

to the y-axis.


• The location of A may alternatively be specified using

Polar coordinates, i.e. in terms of a distance and an angle.

• Taking the positive part of the x-axis as a starting line,

• we may specify the location of A unambiguously in terms of the

• distance OA • and the angle in degrees between the

x-axis and OA, • Conventionally measured

anticlockwise.


• By Pythagoras’s theorem, the distance

• OA = (22 + 22) = 81/2 units • and the angle is 45°,

• so in a polar coordinate system we write A = (81/2 ,45).

• In a polar coordinate system, a point having one or both of its rectangular coordinates negative is specified using values of H > 90.

• Thus the point with rectangular coordinates [–2, –2] becomes (81/2,225) in polar form.


• In the Munsell colour-order system, we write the polar coordinates of any point

• (B in Figure 4.1) as (C,H),

• where C is the distance OB • and H is the angle OB• makes with the positive part of the x-

axis. • C and H are derived from the

rectangular coordinates x and y by Eqn 4.1:


• Colour, however, is three-dimensional.

• In a rectangular coordinate system the third dimension is introduced by

• adding a third axis, perpendicular to those of x and y and passing through O.

• If exactly the same thing is done in a polar coordinate system, it becomes

• a cylindrical coordinate system in which,• by convention, the principal axis (OP) of

the cylinder is oriented vertically (Figure 4.2).


• The general point is now• D = (V,C,H),• where V is the distance OL• (L being the point where the principal

axis intersects the horizontal plane containing the point D),

• C is the distance LD • and H is measured anticlockwise from

the reference plane OPQR, bounded by the principal axis.

• (Cylindrical coordinates, though perhaps unfamiliar, have the ad making the structure of colour space much easier to work with; it is worth persevering

• with the concept.)

Munsell colour-order system: concept(V=0,10)

• In the Munsell system the vertical axis of the cylinder is the V-axis.

• Its lower end (V= 0) represents the perfect black, • a term often used to indicate a uniform reflectance of

0%, • and its upper end (V = 10) an approximation to the

white of the perfect reflecting diffuser.

• The CIE defines the latter as the ideal isotropic diffuser

• (that is, radiation reflected from it is equal in intensity in all directions in the hemisphere in which it occurs)

• with a uniform reflectance of 100% .


• The intermediate points on the V-axis• represent the infinite number of • achromatic colours (that is, colours that

resemble only black and white )

• corresponding, inter alia, to uniform percentage reflectances of R

• (0 < R < 100), which are perceived as blacks (if R ≈ 0), whites (if R ≈ 100) and greys.


• All colours with a given V, whether achromatic or chromatic (chromatic being the opposite of achromatic, that is, colours possessing hue, even if only slightly ), fall on

• the horizontal plane that contains the given V. • The V-coordinate of a coloured surface is determined by• its lightness, which is a function of the total reflectance of the

surface, weighted according to the response of the human visual system to stimuli of different wavelengths.

• The lightness of a colour is a measure of how it would appear, for example, in a black and white photographic print, provided all the processes leading to the print exactly emulated the human visual process.

• If two colours, say an orange and a grey, appear identical in such a print they have the same lightness, and hence the same V.

Munsell colour-order system: concept(ELEMENTARY)

• elementary colour• There are six such colours:• white, black, red, yellow, green and blue.

• We may thus identify two elementary achromatic colours • (white and black)

• and four elementary chromatic colours• (red, yellow, green and blue).

Munsell colour-order system: concept(HUE)

• Hue is then defined as the attribute of a chromatic colour

according to which it appears to be similar to one of the elementary chromatic colours

or to a combination of two of them


• The Munsell system has a qualitatively similar hue circuit.

• It surrounds the achromatic axis

• but it is a circle in which equal steps do correspond to visually equal differences in hue.

Munsell colour-order system: concept (HUE)

• In it, the five so-called Munsell principal hues –

1. red (R),

2. yellow (Y),

3. green (G),

4. blue (B)

5. and purple (P) • are equally spaced around the

circle • and arranged clockwise in the

order given when viewed from ‘above’ (Figure 4.3).


• Lying halfway between each pair of adjacent principal hues

• is one of the five intermediate hues • (YR, GY, BG, PB and RP).

• Together, the principal and intermediate hues constitute the ten major hues of the system.

• Each is subdivided into ten equal parts,

• so that the whole circle is divided into a total of 100 equal angular intervals.

Munsil COLOR (Hue)

• The attribute of Munsell hue (H) may thus be specified by means of an angular scale of 0 < H ≤ 100 with 100

• (equivalent to 0) representing a hue midway• between RP and R, R itself being at H =5, YR at H = 15, and so on. • More usually, however, a system is used in which one of the major hues is

given preceded by • a number n (0 < n ≤ 10).• If n = 5, the major hue itself is indicated (5R, for example, denotes the major

hue red). • A designation of n > 5 implies• a hue clockwise from the given major hue• (and n < 5, anticlockwise); thus, for example, 7R denotes a red shade that is

yellower than the major red hue 5R

Munsil COLOR (Chroma)

• To define the significance of the dimension C of the Munsell system, we return to the chromaticity diagram.

• The colours of the spectrum locus and the nonspectral purples • resemble one of the elementary chromatic colours• (or more usually only two adjacent ones).

• Most colours of a given lightness, however, also resemble that• achromatic colour which has the same lightness.

Munsil COLOR (Chroma)

• An orange colour, say, may possess• only the attributes of redness and

yellowness • in a given ratio,

• whereas a brown which possesses these two attributes in the same ratio also has the attribute of greyness.

• This leads us to two further definitions:

(a) a full chromatic colour is a colour

that resembles only the elementary chromatic colours, and (so) does not at all resemble grey

(b) chroma is the attribute of a colour that expresses the degree

by which it differsfrom an achromatic colour(white/black) of the same lightness .

The Munsell chroma (C) of a colour dictates

• The Munsell chroma (C) of a colour dictates• the distance from the achromatic axis • At which it is placed in the system.

• It is a measure of the extent by which the colour differs

• from the achromatic colour of the same V.

• The orange colour mentioned above has a• higher C than the brown and hence lies further from

the achromatic axis,

• all truly achromatic colours having C = 0.

Munsell colour-order system: realisation

collection of coloured specimens

or colour atlas,

defined as the arrangement of

coloured specimens according to a colour-order system


• On each chart chips are displayed at intervals of

• V = 1 and C = 2,• arranged with those in each row of

constant V • and those in each column of constant

C (Figure 4.4).

• The inequality of the intervals of V and C arises because although

• Munsell designed the three fundamental attributes of his system with the aim of perceptual uniformity,

• he deliberately chose scales such that V = 2C = 3H at C = 5.


• The scaling of H relative to that of • V and C needs to be qualified• (at C = 5) because

• Munsell colour space is specified by a cylindrical coordinate system


• The chips of highest C on each Munsell constant-H chart form a curved boundary,

• such as that in Figure 4.4, • which is different for each chart. The boundaries of

each pair of adjacent charts are similar,

• however, so that all 40 boundaries form a smooth, but

• irregular, three-dimensional locus • called a colour solid: that is, a three dimensional• representation of that part of colour space which

can be achieved by means of coloured objects .


• The chips of highest C on each Munsell constant-H

• In each hue chart, the chip of highest C• for each V • has a lower C than • that of the optimal colour stimulus for this H and V.

• The C of real surface colours is necessarily lower because

• they are produced using real colorants, • which neither absorb nor reflect perfectly at all visible

wavelengths.


• Additionally, in any colour atlas the maximum C illustrated is

• restricted by the need for the colour of the chips

• to be maintained throughout a production run,

• to be reproduced between runs,

• and to be stable to the various agencies to which they are exposed during use.

COLOR SPACES

1. CIE xyY colour space

2. Judd triangular and MacAdam rectangular UCS diagrams

3. Hunter Lαβ and Scofield Lab colour spaces

4. Adams chromatic value colour space

5. Hunter Lab colour space

6. Adams–Nickerson (ANLAB) colour space

7. Early cube-root colour spaces

8. CIE 1960 UCS diagram and CIE 1964 (U*V*W*) colour space

9. CIE 1976 UCS diagram, CIELUV and CIELAB colour spaces

10. Residual non-uniformity of CIELUV and CIELAB colour spaces

Color order system

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