Principles of Colour and Appearance Measurement

Visual Measurement of Colour, Colour Comparison and Management

Asim Kumar Roy Choudhury

• Publisher: Woodhead Publishing
• Year: 2014
• Print ISBN: 9781782423676
• eBook ISBN: 9781782423881
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Colour and appearance perceptions are very complex psychological phenomena. Written by one of the foremost authorities in the field, this major two-volume work addresses the key topics required to understand the issues and manage colour effectively. Principles of colour appearance and measurement Volume 2 addresses the visual measurement of colour, methods of comparing colours, and the management of colour in industry.Volume 2 begins with an overview of the visual measurement of colour. Chapter 1 discusses means of colour communication and various visual attributes of colour. Chapter 2 then focuses on several popular colour order systems, and chapter 3 discusses various colour difference formulae and their use in colour comparison and control. Subsequent chapters review instrumental colorant formulation, metamerism, chromatic adaptation and colour constancy, methods of shade sorting and digital colour reproduction.- Addresses the means of colour communication and the various attributes of colour- Examines colour order systems and the methods of colour comparison- Reviews the management of colour in industry

1

Visual measures of colour

A.K. Roy Choudhury Govt. College of Engineering and Textile Technology, Serampore, India

Abstract

Instrumental colour parameters are very useful for quality control and colour matching purposes. However, they have poor correlation with visual parameters of colour. Visual colour order systems, or colour notations, are very useful for effective communication, comparison, recording and formulation of colours. The chapter discusses how colours are assessed visually. Visual perception is a psychological phenomenon which is difficult to measure directly. Hence, various sets of visual colour parameters are proposed by colourists that are not mutually convertible.

Key words

colour naming

colour order systems

visual colour attributes

colour atlas

Pantone

Colour Harmony Manual

1.1 Introduction

‘Artists can colour the sky red because they know it’s blue. Those of us who aren’t artists must colour things the way they really are or people might think we’re stupid.’ – Jules Feiffer.

Colour is the visual perceptual property corresponding in humans to the categories red, green, blue and others. Colour derives from the spectrum of light (distribution of light power versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Colour categories and physical specifications of colour are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection and emission spectra. By defining a colour space, colours can be identified numerically by their coordinates. Colour is the element that is produced when light, striking an object, is reflected back to the eye.

Berlin and Kay (1969) described a pattern in naming ‘basic’ colours (such as ‘red’, but not ‘red-orange’ or ‘dark red’ or ‘blood red’ which are ‘shades’ of red). The authors theorized that as languages evolve, they acquire new basic colour terms in a strict chronological sequence; if a basic colour term is found in a language, then the colours of all earlier stages should also be present. All languages that have two ‘basic’ colour names distinguish dark/cool colours from bright/warm colours. The next colours to be distinguished are usually red and then yellow or green. All languages with six ‘basic’ colours include black, white, red, green, blue and yellow. The pattern holds up to a set of 12: black, grey, white, pink, red, orange, yellow, green, blue, purple, brown and azure (the colour of the sky on a bright, clear day – the hue halfway between blue and cyan). The work achieved widespread influence. However, the constraints in colour term ordering have been substantially loosened, both by Berlin and Kay in later publications, and by various critics. Barbara Saunders (2000) questioned the methodologies of data collection and the cultural assumptions underpinning the research.

The colour names always seem to appear in a specific order of importance across cultures – black, white, red, green, yellow and blue. ‘If a population has a name for red, it also has a name for black and for white; or, if it has a name for green, it also has a name for red,’ said researcher Francesca Tria, a physicist at the ISI Foundation in Turin, Italy. But if a population has a name for black and white, that does not necessarily mean they have a name for red. To solve the puzzle of this colour name hierarchy, Tria and her colleagues devised a computer simulation with pairs of virtual people, or ‘agents’, who lacked the knowledge of names for colours. One agent, the speaker, is shown two or more objects, invents a name for a colour to describe one of the objects, and refers to the item by that colour. The other agent, the hearer, then has to guess which item, and thus colour, the speaker referred to. Scientists repeated this until all the agents came to a consensus on colour names. A key feature of this simulation was its adherence to the limits of human vision. Our eyes are more sensitive to some wavelengths of light, or colours, than others. The agents in the simulation were not required to distinguish between hues that a human eye could not tell apart. ‘Roughly speaking, human eyes can tell apart two colours only if their wavelengths differ at least by a certain amount – the ‘just noticeable difference’, Tria said.

The researchers found that the time agents needed to reach consensus on a colour name fell into a distinct hierarchy – red, magenta-red, violet, green-yellow, blue, orange and cyan, in that order. This hierarchy approximately matches the colour name order seen in real cultures. This hierarchy of colours also matches the limits of human vision, with the human eye being more sensitive to red wavelengths than those for blue, and so on.

‘Our approach suggests a possible route to the emergence of hierarchical colour categories’ Tria told Live Science. ‘Humans tend to react most saliently to certain parts of the spectrum, often selecting exemplars for them, and finally come the process of linguistic colour naming, which adheres to universal patterns resulting in a neat hierarchy.’

Tria and her colleagues detailed their findings online in the Proceedings of the National Academy of Sciences (Choi, 2012).

Colour is subjective, since it is generated within the visual cortex. Unlike the sensations of taste, smell or feeling, colour is not a characteristic of objects, but of the light that enters our eyes from the objects. Objects are visible or seen coloured only when light reaches our eyes after interaction with them. The same object may be seen in different colours when observed under varying lights. In the absence of light, all colours disappear. The common attribution of colours as properties of objects is largely a matter of memory and in most cases those refer to some form of sunlight. Daylight is a mixture of direct sunlight and the scattered component or skylight. We say that snow is white, soot black, blood red, because under ordinary conditions of life, the objects appear to be of these hues. While specifying colour, it is, therefore, essential, to mention the specific nature of illumination and viewing.

1.2 Means of colour communication

It is not very clear how colour names developed historically. One of the two prevailing opinions is that people of all societies became aware of different colours or colour categories and then named them in the same sequence: white and black, red, green, yellow, blue, brown, purple, pink, orange, grey (Berlin and Kay, 1969). Others think that all colour names are group cultural achievements and there is little common thread.

Many colour words are related to materials, such as orange, ultramarine, olive, malachite green, bottle-green, peanut-green, sea-green, etc. These common names refer to the colours of various common objects, which can be quickly recognized and memorized by most people. Some names reflect poetic invention, such as Cuban Sand, Ashes of Rose, Blue Fox and so on. But such colour names are very approximate, unreliable and temporary. Their meaning also changes with observer, time, place, style, technology, language, culture, etc.

It is common practice to describe colour in terms of hues, such as red, yellow, etc., along with tone or secondary hue, such as greenish, bluish, etc., and the amount of light reflected such as dark or pale. However, when we describe a colour as ‘dark greenish blue’, the description is very inadequate, as there may be many thousands of such colours. The problem was realized long ago.

The accurate description of colour is essential for communication and for accurate reproduction of colours across a wide range of products. The colour of any object is commonly registered or recorded in two ways, namely:

1. Preserving coloured physical samples

2. Recording in terms of common colour names

Physical samples of paint panels, patches of printing inks, coloured papers, fabrics, yarns or fibre, etc. are frequently used in the trade. Collections of such colour samples are very useful as examples of colour product if the number of colours required is fairly limited. A good example of such use is the dye-manufacturer’s ‘shade cards’. Shade cards carry numerous coloured objects on specific substrates (e.g. piece of paper or various textile materials) along with procedures and names of the colourants to be used. However, the exemplifications are very limited. They are restricted to the specific type of colourant or substrate, and cannot be used for general reference.

It is common practice to describe colour in terms of hues such as red, yellow, etc., along with tone or secondary hue such as greenish, bluish, etc., and the amount of light reflected, such as dark or pale. However, when we describe a colour as ‘dark greenish blue’, the description is very inadequate, as there may be many thousands of such colours. The problem was realized long ago (Roy Choudhury, 2000).

Colour dictionaries are created for several purposes:

• Standardized colour names facilitate specification, purchase and use of coloured goods, markers, etc.

• Companies e.g. Pantone register colours (and names), providing formulas for inks, plastics, toners and paints to guarantee uniformity and accuracy of colour for their clients’ products. These colour designations are often just numbers and letters, requiring search through process guides to find a particular shade.

• The combinatorial colour dictionaries underlying the Munsell, OSA-UCS and GIA colour scales allow field-workers to encode and communicate colour from visual observation. These systems endeavour to partition their colour spaces into equally distinguishable regions with a named colour at the centre of each.

• A set of c...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Woodhead Publishing Series in Textiles
• 1: Visual measures of colour
• 2: Popular colour order systems
• 3: Colour-difference assessment
• 4: Instrumental colourant formulation
• 5: Metamerism and shade sorting
• 6: Chromatic adaptation and colour constancy
• 7: Digital colour reproduction
• Index