Fundamentals of Human-Computer Interaction

Fundamentals of Human-Computer Interaction aims to sensitize the systems designer to the problems faced by the user of an interactive system. The book grew out of a course entitled ""The User Interface: Human Factors for Computer-based Systems"" which has been run annually at the University of York since 1981. This course has been attended primarily by systems managers from the computer industry. The book is organized into three parts. Part One focuses on the user as processor of information with studies on visual perception; extracting information from printed and electronically presented text; and human memory. Part Two on the use of behavioral data includes studies on how and when to collect behavioral data; and statistical evaluation of behavioral data. Part Three deals with user interfaces. The chapters in this section cover topics such as work station design, user interface design, and speech communication. It is hoped that this book will be read by systems engineers and managers concerned with the design of interactive systems as well as graduate and undergraduate computer science students. The book is also suitable as a tutorial text for certain courses for students of Psychology and Ergonomics.

• Language: English
• Publisher: Academic Press
• Release date: Jun 28, 2014
• ISBN: 9781483276755

Book preview

Fundamentals of Human–Computer Interaction

ANDREW MONK

Department of Psychology, University of York, York, UK

London Orlando San Diego New York Toronto Montreal Sydney Tokyo

Table of Contents

Cover image

Title page

Computers and People Series

Copyright

CONTRIBUTORS

PREFACE

ACKNOWLEDGEMENTS

PART ONE: THE USER AS A PROCESSOR OF INFORMATION

INTRODUCTION TO THE USER AS A PROCESSOR OF INFORMATION

Chapter 1: Visual Perception: an Intelligent System with Limited Bandwidth

Publisher Summary

1.1 INTRODUCTION

1.2 LUMINANCE, CONTRAST AND BRIGHTNESS

1.3 COLOUR SENSITIVITY

1.4 THE VISUAL SYSTEM AS A SPATIOTEMPORAL FILTER

1.5 PERCEPTION AS AN ACTIVE PROCESS

1.6 SUMMARY

1.7 FURTHER READING

Chapter 2: Reading: Extracting Information from Printed and Electronically Presented Text

Publisher Summary

2.1 INTRODUCTION

2.2 THE COGNITIVE PSYCHOLOGY OF READING

2.3 LEGIBILITY

2.4 SPECIAL PROBLEMS ASSOCIATED WITH READING FROM CRT DISPLAYS

2.5 SUMMART

2.6 FURTHER READING

Chapter 3: Human Memory: Different Stores with Different Characteristics

Publisher Summary

3.1 INTRODUCTION

3.2 SHORT TERM MEMORY STORES

3.3 LONG TERM MEMORY

3.4 SUMMARY AND CONCLUSIONS

3.5 FURTHER READING

Chapter 4: Thinking and Reasoning: Why is Logic So Difficult?

Publisher Summary

4.1 INTRODUCTION

4.2 DEDUCTIVE REASONING

4.3 INDUCTIVE REASONING

4.4 SUMMARY

4.5 FURTHER READING

PART TWO: THE USE OF BEHAYIOURAL DATA

INTRODUCTION TO THE USE OF BEHAVIOURAL DATA

Chapter 5: How and When to Collect Behavioural Data

Publisher Summary

5.1 THE VALUE OF BEHAVIOURAL DATA

5.2 WHEN TO COLLECT BEHAVIOURAL DATA

5.3 BEHAVIOURAL MEASURES

5.4 SELECTING SUBJECTS

5.5 DESIGNING EXPERIMENTS

5.6 SUMMARY

5.7 FURTHER READING

Chapter 6: Statistical Evaluation of Behavioural Data

Publisher Summary

6.1 INTRODUCTION

6.2 TESTING FOR DIFFERENCES BETWEEN MEANS

6.3 CORRELATION

6.4 SUMMARY

6.5 FURTHER READING

Chapter 7: Example of an Experiment: Evaluating Some Speech Synthesisers for Public Announcements

Publisher Summary

7.1 INTRODUCTION

7.2 EXPERIMENT ONE – METHOD

7.3 RESULTS

7.4 CONCLUSIONS FROM EXPERIMENT ONE

7.5 EXPERIMENT TWO

7.6 SUMMARY AND GENERAL DISCUSSION

7.7 FURTHER READING

PART THREE: THE USER INTERFACE

INTRODUCTION TO THE USER INFERFACE

Chapter 8: Work Station Design, Activities and Display Techniques

Publisher Summary

8.1 INTRODUCTION

8.2 INPUT DEVICES

8.3 OUTPUT DEVICES

8.4 FACILITY OR FEATURE SELECTION TECHNIQUES

8.5 DISPLAY TECHNIQUES

8.6 SUMMARY

8.7 FURTHER READING

Chapter 9: Dialogue Design: Characteristics of User Knowledge

Publisher Summary

9.1 INTRODUCTION

9.2 FIELD STUDIES OF SYSTEM USE

9.3 EXPERIMENTAL STUDIES OF SYSTEM USE

9.4 APPLICATION OF FINDINGS

9.5 SUMMARY

9.6 FURTHER READING

Chapter 10: User Interface Design: Generative User Engineering Principles

Publisher Summary

10.1 INTRODUCTION

10.2 PROBLEMS IN INTERACTIVE SYSTEM DESIGN: MOTIVATION FOR A BETTER WAY

10.3 INTRODUCING GENERATIVE USER-ENGINEERING PRINCIPLES

10.4 EXAMPLES OF GUEPS

10.5 A WARNING AGAINST PSEUDO-GENERATIVE PRINCIPLES

10.6 SUMMARY

10.7 FURTHER READING

Chapter 11: Future Uses of Future Offices

Publisher Summary

11.1 INTRODUCTION

11.2 SETTING THE SCENE

11.3 SCENE ONE – THE FIRST CONSULTATION

11.4 SCENE TWO – ON LOCATION IN CAMDEN TOWN FRIDAY MORNING THE NEXT WEEK

11.5 SUMMARY

11.6 FURTHER READING

Chapter 12: Speech Communication: The Problem and Some Solutions

Publisher Summary

12.1 SPEECH AS A MEDIUM FOR COMMUNICATION

12.2 SPEECH ARTICULATION AND RECOGNITION: HOW DO PEOPLE DO IT?

12.3 SPEECH PRODUCTION AND RECOGNITION: HOW CAN MACHINES DO IT?

12.4 SUMMARY

12.5 FURTHER READING

Chapter 13: Speech Communication: How to Use It

Publisher Summary

13.1 INTRODUCTION

13.2 MACHINE-GENERATED SPEECH

13.3 VOICE RECOGNITION

13.4 INTERACTIVE SYSTEMS

13.5 SUMMARY

13.6 FURTHER READING

Chapter 14: Human Factors Problems in the Design and Use of Expert Systems

Publisher Summary

14.1 INTRODUCTION TO EXPERT SYSTEMS

14.2 HOW EXPERT SYSTEMS WORK

14.3 ACQUIRING KNOWLEDGE FROM THE HUMAN EXPERT

14.4 REPRESENTATION AND USE OF KNOWLEDGE BT THE SYSTEM

14.5 USER INTERFACE DESIGN

14.6 SUMMARY

14.7 FURTHER READING

GLOSSARY

REFERENCES

Author Index

Subject Index

Computers and People Series

Edited by

B. R. GAINES

The series is concerned with all aspects of man–computer relationships, including interaction, interfacing modelling and artificial intelligence. Books are interdisciplinary, communicating results derived in one area of study to workers in another. Applied, experimental, theoretical and tutorial studies are included.

On Becoming a Personal Scientist: Interactive computer elicitation of personal models of the world, Mildred L. G. Shaw 1980

Communicating with Microcomputers: An introduction to the technology of man–computer communication, Ian H. Witten 1980

Computing Skills and the User Interface, M. J. Coombs and J. L. Alty (eds) 1981

The Computer in Experimental Psychology, R. Bird 1981

Fuzzy Reasoning and Its Applications, E. H. Mamdani and B. R. Gaines (eds) 1981

Recent Advances in Personal Construct Technology, Mildred L. G. Shaw 1981

Intelligent Tutoring Systems, D. Sleeman and J. S. Brown (eds) 1982

Principles of Computer Speech, I. H. Witten 1982

Designing for Human–Computer Communication, M. E. Sime and M. J. Coombs (eds) 1983

The Psychology of Computer Use, T. R. G. Green, S. J. Payne and G. C. van der Veer (eds) 1983

Fundamentals of Human–Computer Interaction, Andrew Monk (ed) 1984, 1985

Copyright

Academic Press Rapid Manuscript Reproduction

Copyright © 1985, by Academic Press Inc. (London) Ltd.

all rights reserved.

no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

ACADEMIC PRESS INC. (LONDON) LTD.

24–28 Oval Road

LONDON NW1 7DX

United States Edition published by

ACADEMIC PRESS, INC.

Orlando, Florida 32887

British Library Cataloguing in Publication Data

Fundamentals of human-computer interaction.

1. Interactive computer systems

I. Monk, Andrew

001.64 QA76.9.158

ISBN 0-12-504580-8

ISBN 0-12-504582-4 (pbk.)

LCCCN 84-45602

printed in the united states of america

85 86 87 88 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Peter Bailey,     Department of Psychology, University of York, Heslington, York, Y01 5DD

Phil Barnard,     MRC Applied Psychology Unit, 15 Chaucer Road, Cambridge, CB2 2EF

Nick Hammond,     Department of Psychology, University of York, Heslington, York, Y01 5DD

Charles Hulme,     Department of Psychology, University of York, Heslington, York, Y01 5DD

Alison Kidd,     R19.2.1, British Telecom Research Labs., Martlesham Heath. Ipswich, IP5 7RE

G. Reinhard Kofer,     K KE ST, Siemens AG, Office Software Prototyping Group, Hofmannstr. 51, 8 Munchen 70, West Germany

Antony Lo,     British Telecom Research Labs., Martlesham Heath, Ipswich, IP5 7RE

Andrew Monk,     Department of Psychology, University of York, Heslington, York, Y01 5DD

Peter Reid,     Software Sciences Ltd., London and Manchester House, Park Street, Macclesfield, Cheshire, SK11 6SR

Harold Thimbleby,     Computer Science Department, University of York, Heslington, York, Y01 5DD

Peter Thompson,     Department of Psychology, University of York, Heslington, York, Y01 5DD

Neil Thomson,     Thomson Computer Services, 71 The Mount, York

John Waterworth,     British Telecom Research Labs., Martlesham Heath, Ipswich, IP5 7RE

PREFACE

The task of interacting with a ‘machine’, typically a computer-based system, is no longer restricted to a small group of specialists. With the advent of computer-controlled telecommunications and access to large databases using teletext, most members of the community will, in the relatively near future, have routine daily interactions with computer systems. There is an increasing awareness of the case made by Human-Computer Interaction (HCI) specialists that the design of the user-machine interface in any interactive system is crucial for its efficiency and acceptability, and therefore for its commercial potential.

The assumption that motivates this book is that the time has come for these experts to share some of their specialist knowledge with those at the sharp end of the design process; for example, the system designers and programmers. It is not enough simply to consult an HCI specialist when problems arise. If effective interactive products are to be built and sold, all the personnel involved in the design process must be aware of the basic issues and principles involved.

The aim of this book is to sensitise the systems designer to the problems faced by the user of an interactive system. We hope that it will be read by systems engineers and managers concerned with the design of interactive systems as well as graduate and undergraduate computer science students. The book is also suitable as a tutorial text for certain courses for students of Psychology and Ergonomics.

The book has grown out of a course entitled ‘The User Interface: Human Factors for Computer-based Systems’ which has been run annually at the University of York since 1981. This course has been attended primarily by systems managers from the computer industry. The enthusiasm and constructive criticism of these people has done much to shape this book. Thanks are also due to all my colleagues at York, in particular to Karin Carter for her work on the references, to Peter and Jenny Bailey for their help ‘editing the editor’, to my wife, Ruth Monk, and also to Charles Hulme for their considerable help and encouragement and to Marilyn Glicker, Fay McDonald and Annelies Campbell for their help with the production of the book.

ACKNOWLEDGEMENTS

Acknowledgement is made to the Director of Research, British Telecom Research Laboratories, for permission to publish Chapters 7, 13 and 14. Acknowledgement is also due to the following for illustrations for figures:

1.4 – from Pearson, D.E. (1975). Transmission and Display of Pictorial Information, Pentech Press, Figure 6.4 The CIE Chromaticity Diagram.

1.6 – from Schiff, W. (1980). Perception: An Applied Approach. Boston: Houghton and Mifflin Co. Table 1.4 Techniques for measuring visual acuity, and optimum resulting values (p. 28).

1.10 – from Endeavour (1972) 31, 88–94, Digital computers and image processing, Figure 7. (Photographer Harry Andrews.)

1.11 – from Anstis, S.M. (1974). A chart demonstrating variations in acuity with retinal position. Vision Research, 14, 589–592, Figure 3 (p. 591).

* 1.17 – from Lindsay, P.H. and Norman, D.A. Human Information Processing (1st edition). New York: Academic Press. Figure 1–9. (Photographer R.C. James.)

2.1 – from Tinker, M.A. Bases for Effective Reading. Minnesota: University of Minnesota Press.

Table 9.1 – from The Presentation Graphics Feature and Interactive Chart Utility (SC 33-011-0), IBM, Winchester, England.

---

*Originally published in J. Thurston and R.G. Carraher Optical Illusions and the Visual Arts, c. 1966. Litton Educational Publishing, Inc. and reprinted by permission of Van Nostrand Reinhold Company.

PART ONE

THE USER AS A PROCESSOR OF INFORMATION

Outline

INTRODUCTION TO THE USER AS A PROCESSOR OF INFORMATION

Chapter 1: Visual Perception: an Intelligent System with Limited Bandwidth

Chapter 2: Reading: Extracting Information from Printed and Electronically Presented Text

Chapter 3: Human Memory: Different Stores with Different Characteristics

Chapter 4: Thinking and Reasoning: Why is Logic So Difficult?

INTRODUCTION TO THE USER AS A PROCESSOR OF INFORMATION

The aim of this book is to introduce concepts and examples which allow the designer to think more clearly about the problems faced by the users of interactive systems and their solution. This first section presents over-views of four areas from Cognitive Psychology.

Cognitive Psychology, or ‘Human Information Processing’ as it is sometimes known, is central to the development of the kind of sensitivity to human factors problems that the book aims to create. It provides the conceptual framework needed to think about the abilities and limitations of the user. If the ‘human’ part of the human-computer interaction is to be viewed as a part of a complete information processing system then it is necessary to describe him in terms of his information processing capacities. This perspective is not commonly taken outside the behavioural sciences and is developed in detail to ensure a full appreciation of the rest of the material in the book. It is illustrated by describing the strengths and weaknesses of the human information processing system as they relate to the user interface.

The first two chapters are concerned with how we take in information. This kind of human information processing has been studied extensively under the heading of ‘Perception’. Chapter 1 discusses such topics as visual acuity, colour vision and our sensitivity to changes in the visual array. The visual system can be usefully characterised as having a limited bandwidth in the temporal and spatial domains and this view is developed to explain the usual recommendations about display parameters such as letter size and refresh rates. Chapter 2 summarises some of the findings from experimental studies of people reading printed and electronically presented text. This chapter also includes a review of work on special problems arising from the use of VDTs.

The third and fourth chapters are concerned with how we store and manipulate information. Human memory is conventionally viewed as a collection of stores of various kinds which have different characteristics. Knowing how these stores function makes it possible to minimise the users’ memory problems. Similarly, knowledge of human reasoning processes makes it possible to understand how we manipulate information. Human memory is discussed in Chapter 3 and reasoning in Chapter 4.

CHAPTER 1

Visual Perception: an Intelligent System with Limited Bandwidth

Peter Thompson

Publisher Summary

This chapter discusses the visual perception of an intelligent system with limited bandwidth. Visual stimuli, be they objects in the real world or patterns on a cathode ray tube, can be described in terms of their size, shape, orientation, color, and movement. In many of these dimensions, the humans’ visual systems act like a filter, responding to some parts of the dimension while remaining oblivious to others. An appreciation of these limitations is important because it guards humans against requiring visual system to perform beyond its physical constraints and because it enables to achieve an economy of representation when constructing visual displays—one do not need to generate those parts of a pattern that will be invisible to the visual system. Many of the constraints on vision are established in the early stages of visual processing and can be traced to aspects of the anatomy and physiology of the peripheral visual system. Other constraints appear at a cognitive level. At this higher level, visual processing appears to be an active process that uses past experience and expectations to aid in its construction of a plausible visual world from seemingly ambiguous sense-data. The rigidity of the physical limitations of the peripheral visual system makes recommendations on the physical dimensions of visual stimuli possible. Whether the use of color, highlighted words, or flashing symbols will help or hinder the user is often impossible to determine outside the particular task being considered. Fortunately, these decisions can be made after reasonably simple experiments have been carried out.

1.1 INTRODUCTION

Vision is our primary sense. Man learns about his environment largely through his eyes, but the human visual system has its limitations and its quirks; and an appreciation of these is an important prerequisite for making the most efficient use of our visual sense to communicate information from the world to our higher cognitive centres.

The message of this chapter is two-fold. Firstly, the visual system acts as a low-pass spatio-temporal filter; this means that there are many things that the eye does not see and that any visual display need not transmit. Secondly, vision is an active process, constructing our visual world from often inadequate information. It is little wonder therefore that the eye is sometimes deceived.

An appreciation of both the physical and the cognitive constraints upon the visual system is important when designing visual displays.

Light

Visible light is that part of the Electro-magnetic spectrum to which our eyes are sensitive, the visible range lying between wavelengths of 400–700 nanometres (nm). At the short-wavelength end of the visible spectrum is ‘blue’ light and at the long-wavelength end is ‘red’ light. The perceived brightness and colour of light depend largely on the physical intensity and wavelength of the light. However, most of the light we see is reflected light, whose perceived brightness and colour depend upon the properties of the surface from which it is reflected, as well as the properties of the illuminant: generally dark surfaces absorb most light, light surfaces absorb little light; ‘red’ surfaces absorb most short wavelengths and ‘blue’ surfaces absorb most long wavelengths, but this is not always the case.

Our Visual Apparatus

For the purposes of this chapter the human visual system comprises the eyes and those areas of the brain responsible for the early stages of visual processing. The human eye focuses an image of the world upsidedown on the retina, a mosaic of light-sensitive receptors covering the back of the eye (see Figure 1.1a). The photoreceptors of the retina are of two different sorts: rods, which are very sensitive to light but saturate at high levels of illumination, and cones, which are less sensitive and hence can operate at high luminance levels. Humans have three different types of cone, each optimally sensitive to a different wavelength, which allow us to have colour vision. Most of each retina’s 7 million cones are concentrated in the fovea, the small area of the retina (about 0.3 millimetres in diameter) upon which fixated objects are imaged. The rods, 120 million in each retina, predominate in the periphery. Figure 1.1b shows the distribution of rods and cones on the retina. The point from which the optic nerve leaves the retina is devoid of all photoreceptors, this is called the ‘blind-spot’, a surprisingly large area of retina totally insensitive to light. The blind spot is located in the temporal retina, that is, in the half of the retina closer to the temple. The other half of the retina is known as the nasal retina, being closer to the nose. The whole of the retina is covered by a network of blood vessels which lie between the visual receptors and the world. However, these are not usually seen because the visual system rapidly ceases to respond to stimuli which remain unchanging on the retina.

FIG. 1.1a Cross-section through the human eye. The cornea and lens of the eye focus light on the sensitive retina. Objects which we look at directly are imaged on the fovea.

FIG. 1.1b The distribution of rods and cones in the human retina, left eye.

Visual Angle

The most widely used measure of image size is the degree of visual angle, see Figure 1.2. An object 1 unit in length placed at a distance of 57 units from the eye produces an image of approximately 1 degree of visual angle upon the retina. Both the sun and the moon subtend angles of about 0.5 degrees (30 minutes). The fovea covers an area of about 1–2 degrees, roughly the size of your thumb-nail at arm’s length, and the blindspot an area of about 5 degrees.

FIG. 1.2 Retinal image size expressed in terms of its visual angle. Objects with same visual angle have the same size on the retina.

1.2 LUMINANCE, CONTRAST AND BRIGHTNESS

Luminance generally refers to the light reflected from a surface, and is an objective measure of radiance. It depends both upon the illuminance, that is the light falling on the surface, and upon the reflectance, that is the light-reflecting properties of the surface. Luminance is easily measured by a photometer and is defined by the Commission Internationale de l’Eclairage (C.I.E.), in units of Candelas per square metre, although the Foot-Lambert is a commonly used alternative unit (1 cd/m² = 0.2919 fL).

Contrast has been defined by the C.I.E. as follows:

Contrast=(Lo−Lb)Lb

where

LO = object luminance

Lb = background luminance

This definition allows contrast to take a negative value, e.g., the printing on this page.

Contrast is often defined differently by visual scientists and physicists:

Contrast=(Lmax−Lmin)(Lmax+Lmin)

where

Lmax = maximum luminance

Lmin = minimum luminance

This contrast, often called the Michelson Contrast can only take on a value between 0 and 1.

Luminance and contrast are physical measures, whereas brightness is a psychological, subjective response to light; its apparent lightness or dimness. Generally the higher the luminance the brighter a light will appear. However this is not always the case. Perceived brightness on one part of a display depends on the brightness of adjoining areas, see Figure 1.3a. Furthermore the juxtaposition of high and low brightnesses can produce disturbing visual effects, Figure 1.3b.

FIG. 1.3a Simultaneous brightness contrast. The two small grey squares have the same luminance but different brightness.

FIG. 1.3b The Hermann Grid. Ghostly dots appear at the intersections of the lines. Note that no such dot appears at an intersection if that point is fixated.

The classic method for measuring brightness involves measuring the difference in luminance which produces a ‘just noticeable difference’ (j.n.d.), in brightness. The threshold luminance, dL, which can be discriminated from a background luminance, L, obeys Weber’s Law:

dLL= k

where k, the Weber Fraction, has a value around 0.01–0.02 for the range of luminances encountered in typical displays.

This simple relationship is complicated by the adaptation level of the visual system. Our vision in dim illumination is mediated by rods because of the low sensitivity of cones. Because we have very few rods in the fovea we are relatively insensitive to objects at the point of fixation when using rod vision – our maximum sensitivity is shifted about 20 degrees into the periphery where rods have their greatest density, see Figure 1.1b. Astronomers have long been familiar with the fact that very faint stars in the night sky cannot be seen when looked at directly – they only become visible when imaged away from the insensitive fovea. In normal day-time vision, photopic vision, the rod visual pigment, called rhodopsin, is bleached by the high light levels, and only recovers slowly when we move into low levels of illumination. This process, known as dark-adaptation, can take up to 30 minutes to reach maximum sensitivity. Anyone who has entered a dark cinema will have experienced the temporary blindness caused by the combination of the low light level and bleached rods. Moving from the dark into bright light, known as light adaptation, is achieved very rapidly as cones begin to operate and the rods are bleached.

Advantages of High Luminance

Most normal viewing conditions span photopic (cone) vision, when luminance is above 10 cd/m², and mesopic vision (when both cones and rods are operating), between 0.001–10 cd/m². When viewing a VDT the level of light adaptation will be determined by some weighted average of the room luminance and the display luminance. The brightest parts of the screen are unlikely to exceed 1000 cd/m² and the total range of luminance encountered with a VDT will be less than 2 log units.

There seems to be good evidence that having a high level of display luminance is advantageous because:

(i) acuity increases with luminance.

(ii) pupil diameter decreases with increased luminance; this decreases optical distortion and improves depth of field, c.f. reducing the aperture on a camera when taking a photograph.

(iii) discomfort from reflected glare may be reduced. The penalty paid by increased luminance is that flicker sensitivity increases (see section 1.4). Discomfort from direct glare may increase with increased display luminance.

1.3 COLOUR SENSITIVITY

There are three types of cone in the human retina, one is most sensitive to short wavelengths (the ‘blue’ cone), one to the middle range (the ‘green’ cone), and the other to long wavelengths (the ‘red’ cone). Each cone type is broadly tuned in its sensitivity and responds to a wide range of wavelengths so that most of the colours we see will have excited all three mechanisms to some extent. When coloured lights are mixed together colour addition takes place, which is quite different from the colour subtraction which results from mixing paints. Red and green lights mix together to give yellow; red and green paints mix to give a rather nasty grey-brown. Yellow surfaces will excite the red and green cones a great deal but will have little effect on the blue cones. Because of the distribution of cones in the retina, colour vision is best in the fovea and less good in the periphery. Indeed, in the far periphery we have no colour sensitivity at all.

Colour Naming

No dimension of the visual world illustrates the limitations of the visual system better than colour. We are able to see only a small portion of the Electromagnetic spectrum, between about 400 – 700 nm, even though shorter wavelengths, in the ultraviolet, can be ‘seen’ by some insects and longer wavelengths, in the infrared, by certain reptiles. Furthermore our ability to discriminate different wavelengths as different hues is limited. This ability varies enormously over the visible spectrum. With just three cone types we are able to discriminate around 128 just noticeable differences (j.n.d.) of wavelength. We are most sensitive in the yellow and blue/green portion of the spectrum where a change in wavelength of only 1 nm can be discriminated. At the extremities of the spectrum the j.n.d. rises to as much as 20 nm.

The discriminations described above are made at equal luminance and saturation; that is, the discrimination is made on the basis of wavelength alone. Colour comprises three aspects of a stimulus, its hue, its brightness and its saturation, saturation being the amount of white light added to the saturated hue. Thus pink is an unsaturated red. Although we can discriminate around 128 different hues, we can discriminate many thousands of colours. Indeed, 8000 colour names are listed in the ‘Methuen Handbook of Colour’, but it is unlikely that anyone would correctly identify ‘Wafted Feather’ or ‘Angel Wing’ without some training.

An important constraint in the use of colour is a cognitive one – we cannot reliably assign generally accepted names to many of the colours we can discriminate. Grether and Baker (1972) have listed ten spectral colours which can be reliably identified without extended training; these are listed below:

Of course we could add black, white, and a few non-spectral colours like brown to this list.

Measuring Colour

Because we have 3 cone mechanisms human colour vision is trichromatic; that is, any colour can be specified as a mixture of 3 ‘Primaries’. The primaries do not have to match the spectral properties of the cone pigments; indeed, as long as none of the primaries is simply a mixture of the other two, any three colours can be used. In C.R.T. colour displays the primaries (i.e., the phosphors) are commonly red, green and blue.

The C.I.E. has laid down standard