Colour

The following sections are included:

• Contents
• Introduction

When the word ‘eye’ is used it is usually taken to mean the human eye, or perhaps similar eyes in higher life forms such as other mammals, birds and fish. However, nature provides many types of eye, some of which are just light-detecting organs while others produce images, but with widely varying resolutions. Here we restrict our attention to two types of eye that produce images of moderate-to-high resolution. In the following chapter the evolution of the mammalian eye will be described; in the evolutionary process leading to eyes such as ours, other eye forms occurred, some of which are present in extant organisms, and the creatures with these eye forms will be identified.

The following sections are included:

• The Nature of Darwinian Evolution
• The Reaction of Plants to Light
• The Evolution of an Image-Forming Eye

The following sections are included:

• The Nature of Light
• The Scientific Nature of Colour

The following sections are included:

• The Chromaticity Diagram
• Colour Addition
• Why are Objects Coloured?
• Subtractive Colour
• Colour Printing

Most individuals can distinguish the complete range of colour present in the CIE diagram although, sometimes, different individuals may describe the same distribution of wavelengths presented to them in different ways. For example, cyan may be described by one individual as a greenish-blue while another would say it was a bluish-green. This may reflect a slight difference in what they see, but both individuals could be described as having normal colour vision. Each of them would see a complete spectrum running from red to violet, but the spectra would be slightly distorted versions of each other. Here we first describe the characteristics of normal colour vision, with the caveat just given, then the different types of colour vision present in creatures other than hominids, and finally the various kinds of colour blindness that occur.

It might be thought that the colour, saturation and intensity perceived due to the light falling on any small region of the retina, of a size that could just be resolved, would depend only on the responses of the three types of cone in that region and on nothing else. However, that is not the case and the perception of colour and saturation in any small region depends on the stimulation of other parts of the retina, even those quite far from the region of interest. Another factor affecting what is seen at any instant is what the eye has been exposed to in the past. We now describe some of these effects.

In this chapter and the next, we are going to describe the development of pigments, mostly used in an artistic context and dyes, mostly used for colouring textiles or other everyday objects. Pigments are usually inorganic materials, very often derived from naturally-occurring minerals and are in the form of a suspension of fine particles in a fluid in which they are insoluble. By contrast a dye is either liquid in form or is disolved in some solvent in which it is used and it is usually an organic compound. It is sometimes possible to convert a dye to a pigment by precipitating a soluble dye by adding a metallic salt; the resultant is then called a lake pigment — for example, crimson lake.

The following sections are included:

• Dyeing and Mordants
• Natural Dyes
• Synthetic Dyes

The following sections are included:

• The Origin of Pottery
• Coloured Decoration of Pottery
• The Origin of Glass
• Coloured Glass
  • Stained-glass windows
  • Decorative glassware

The following sections are included:

• The First Projected Coloured Image
• Land Projected Images
• General Comments Concerning Perceived Colours

The process of photography consists of creating an image of a scene in some way and then storing the image so that it can be viewed whenever required. The first stage, that of producing an image, goes back a surprizing long way but photography, as we know it today in its monochrome version, is just over 200 years old and colour photography about 100 years old. Here we describe the progress from the first ways of producing a monochrome image to the first early successful ways of producing and storing coloured images.

We have dealt with three ways of producing coloured images. The first, due to Maxwell, uses colour addition with three primary colours to produce a projected coloured image. The second is that of Ducos du Hauron that uses a subtractive colour process involving overlapping pigments behaving like three coloured filters. The third process, the one designed by Lippmann, uses the interference of light to produce a coloured image. To have colour photography, in the sense that John Herschel envisioned it, we need to have a process that provides a negative from which any number of coloured positives can be produced, giving a true rendering of the colour of the original scene.

The following sections are included:

• Persistence of Vision
• The Birth of Cinematography
  • The work of Louis Le Prince
  • Other pioneers of cinematography
• The Introduction of Colour
  • Dufaycolour
  • Technicolour
• Digital Film Technology
• Colour, Black-and-White or Both

The concept of television is much older than its implementation in a practical form. In 1873, the phenomenon of photoconductivity was discovered in the element selenium. When light falls upon a photoconductive material its electrical conductivity increases with the intensity of the light. This raised the possibility that an image, formed by variations of light intensity, could be converted into a variable electrical signal, based on the variation of the conductivity of photoconductive material, and transmitted over wires and then reformed into an image.

Here, we shall briefly describe the development of television from its crude beginnings to a more complicated and effective black-and-white form and then, finally, illustrate how colour has been incorporated.

Light displays are used for many purposes but mainly either for illuminated signs over shops, hotels and public buildings or for entertainment. There are a number of ways of producing the lighting display and here we discuss the more important techniques for doing so.

The following sections are included:

• Colour as a Safety Tool
  • Traffic lights
  • Railway signals
  • Identifying gas in cylinders
  • Colours of hospital pipelines
  • Underground utility colour codes
• General Use of Colour
  • Resistors
  • The London Underground map
• Commercial Uses of Colour
  • Colour labelling of food
  • Colour and advertising

The following sections is included:

• Index