Colour

How We See It and How We Use It

Michael Mark Woolfson

• Publisher: WSPC
• Year: 2016
• eBook ISBN: 9781786340870
• Topic: Physical Sciences
• Subtopic: Optics & Light

Colour makes our lives more interesting — how dull it would be in a black-and-white world! It pleases us aesthetically, entertains us and is useful to us. This unique book aims to describe the scientific nature of colour and light, and how we see it, in an accessible and easily understandable style. The evolution of the eye, science of colour and technical visual systems are all broken down into readable chapters, with clear images and illustrations provided for reference. The book then goes on to discuss the innate tendency of humankind to produce artistic works as conceived, realised and augmented through the use of colour. Focussing on broad forms of artistic entertainment — painting with pigments and dyes, colour and light in photography and cinematography, light displays and colour in television — this book then delivers a comprehensive review of what colour means and has meant in the creative arts.

Contents:

• Eyes
• The Evolution of the Eye
• The Science of Colour
• The Range of Colour
• The Visual System and Colour
• Perceived Colour and Environment
• Painting and Painting Pigments
• The Development of Dyes
• Colouring Pottery and Glass
• Projected Coloured Images
• Early Colour Photography
• Colour Photography
• Colour Cinematography
• Colour Television
• Coloured Light Displays
• Practical Uses of Colour

Readership: General readership, physicists interested in the colour science, opthalmologists, art historians.

Chapter 1
Eyes

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.

1.1 The Compound Eye

The most abundant life form on earth is arthropods (insects and crustaceans) and the eyes of most of them — compound eyes — are based on a completely different principle from the human eye. An electron micrograph of a typical compound eye, that of the fruit fly Drosophilidae, is shown in Figure 1.1. Each of the tightly-packed units forming the surface of the eye is a separate lens, constituting the light receiving end of an ommatidium, a basic light-detecting structure (Figure 1.2).

Below the ommatidium lens is a crystalline cone that further concentrates the light in a downward direction. The arrangement of lens and crystalline cone ensures that the ommatidium is detecting light coming in from only a very narrow angular range in one direction. This light then passes through a cluster of visual cells that convert it into an electrical signal of strength proportional to the intensity. At the end of each visual cell there are nerve fibres forming the optic nerve. Around each cluster of visual cells there are other cells containing an opaque pigment which prevents light passing from one ommatidium to its neighbours that, if it happened, would cause blurring of the image. Since each ommatidium receives light from only a small region of the field of view, the resultant from all the ommatidia is a halftone picture, formed as a grid of small dots of different intensities. An example of a low resolution image formed in this way is given in Figure 1.3. Some insects have eyes with very large numbers of ommatidia and hence produce images of reasonable resolution but at best they attain only one or two percent of the resolution of the human eye and at worst they see the world as rather indistinct patches of light.

Figure 1.1 The surface of the eye of the fruit fly

Figure 1.2 A schematic representation of an ommatidium of a compound eye

Figure 1.3 A fly’s-eye image of Charles Darwin

The compound eye performs particularly well in the detection of motion — hence the difficulty of swatting a fly. As an object moves across the field of view, different ommatidia switch on and off, something that insects are programmed readily to detect. However, a slow approach with a transparent container, such as a drinking glass, is not detected and an insect can easily be trapped in this way.

1.2 The Human Eye

Although we are describing here a human eye it is, in fact, an eye form shared by other mammals, birds and other types of creature, such as the octopus. There are many variants of this form of eye and some creatures have eyes that, to meet their survival requirements, have evolved to be much more efficient than those of humans. However, all these variants are similar in their basic structure and there are three distinct components of the human visual system:

(i) An optical system that produces an image,

(ii) A neural network that converts the image into a stream of electrical impulses that are sent to the brain,

(iii) An interpretation system, located in the visual cortex of the brain that converts the electrical signals into a form through which the field of view is visualized.

Components (ii) and (iii) are involved in colour vision, the details of which will be dealt with in Chapter 4.

1.2.1 The optical system

The optical system is very simple and produces an image in the same way as by a convex lens, as illustrated in Figure 1.4 where the image of an arrow is projected onto a screen.

For an object at some specified distance from a particular lens a sharp image is produced at a particular distance from the lens and if the screen is closer to, or further from, the lens then the image would be blurred. For objects at different distances sharp images could only be formed by having screens at different distances. Alternatively, if the screen had to be at a fixed distance from the lens, sharp images could be formed only if there were a different lens, of different focal length, for each object distance.

A simplified diagram of the human eye is shown in Figure 1.5; the combination of cornea and lens produces a sharp image on the retina, acting as a screen. Since the distance of the retina from the lens is fixed and the objects being viewed can vary from being very close to very far (effectively at an infinite distance), there is a mechanism to vary the focal length of the lens. This is done by the ciliary muscles that, by squeezing the lens by varying amounts, change its shape and hence its focal length. The contribution of the cornea in the focusing process does not change but in a normal eye the total variation in the focal length of the lens is sufficient to allow clear images of both near and far objects. This variation is called accommodation. Some individuals are unable to cover the full range of distances and need spectacles to deal with the problem; those only able to bring close objects into focus suffer from short-sightedness, or myopia, while those only able to focus on far objects suffer from far-sightedness, or hyperopia. Another very common problem is astigmatism where the curvature of the cornea is different in different directions. This means that light rays coming from a point on the object do not focus at a point on the retina. However, this problem is normally so slight that it causes no difficulties to an individual. Where it is more severe then, once again, suitable spectacles can compensate for it.

Figure 1.4 Producing an image on a screen with a simple convex lens

Figure 1.5 A schematic representation of a human eye

The interior of the eye is occupied by a transparent colourless gel called the vitreous humour, all contained within the combination of the sclera (Figure 1.5) and cornea, forming the outer shell of the eye. The cornea is clear, because it has to admit light, but the sclera is a milky-white colour, giving what is known as the white of the eye. The eye operates best at moderate light levels. It is comparatively inefficient at very low light levels where there is too little light energy to give a strong visual signal, and at very high light levels, which saturate the visual system and give rise to glare and an indistinct, if bright, image. The iris, the coloured disk that gives eye-colour, controls a variable circular aperture that, by opening and closing, adjusts the amount of light falling on the retina. This aperture, known as the pupil, is seen as a black circle in the centre of the iris. In dim light the pupil opens wide to let through as much light as possible while in very bright conditions it closes up to reduce the amount of light entering the eye.

1.2.2 The photoreceptors

In order to interpret the image formed on the retina, the information contained within it must be transformed into electrical impulses that are then transmitted to the brain. To perform the initial stage of this task the retina consists of a tightly packed array of photoreceptors, each of which converts the light energy falling on it into electrical pulses. There are about 130 million photoreceptors, most closely packed in the fovea region of the retina (Figure 1.5) with the density falling off with distance from the fovea. This gives the greatest resolution of the visual image in the centre of the field of view; the field of view covers almost a complete hemisphere but peripheral vision gives only a vague impression of what is present.

There are two kinds of photoreceptors. The first is rods, on account of their long rod-like shape, and they are extremely sensitive and are capable of recording very low light levels. They give what is known as scotopic vision that would operate, for example, if a scene were being viewed by moonlight. The second kind, which are stubbier in form, are known as cones and they operate at high levels of illumination to give what is known as photopic vision. Apart from their sensitivity, rods and cones differ in another important respect — the way in which they discriminate colour. The rods do not differentiate coloured objects but represent them in shades of grey, although objects seen by moonlight seem to have a slightly bluish tinge. By contrast the cones give fully chromatic vision. This is obtained through the agency of three types of cone, containing different visual pigments, with different colour responses and the way that they give the perception of colour will be explained in Chapter 4.

1.2.3 The functioning of nerve cells

The way that nerve cells function is quite complex and involves physical mechanisms triggered by chemical agents. Here we just give a simple description of a typical neuron (nerve cell) which will indicate the general form of some processes that occur in the eye.

Figure 1.6 A schematic nerve cell

A schematic neuron is shown in Figure 1.6. At one end are the dendrites that receive information from other neurons. At the other end is the axon, a channel along which electric impulses from the neuron travel and the terminus of which connects with other neurons through their dendrites. The interior of the cell contains potassium ions, K⁺ — potassium atoms with one electron removed — the positive electrical charges of which are only partly balanced by negatively charged molecules. Outside the cell there is an excess of sodium ions, Na⁺, and when the neuron is in a resting state the electrical potential outside the cell is greater than that within. The ions can pass through the membrane, which acts as a sheath for the cell, but chemicals within the membrane act like ion pumps, a separate one for each ion, so maintaining the relative concentrations of the two kinds of ion. A cell in this situation is said to be polarized.

The axon-dendrites connection is called a synapse and for all cells, except those in the brain, the signal is transmitted across a synapse by chemicals known as neurotransmitters; in the brain signals are passed from one neuron to the next by an electric current, which gives a much faster response. The neurotransmitters cross the synaptic gap and pass along the dendrites into the main body of the neuron where they attach themselves to chemical receptors. Depending on the process that occurs, when the neurotransmitter arrives the activity of the cell can be either stimulated or inhibited. The action of a stimulated cell may be followed by reference to Figure 1.7.

Stimulation occurs when the neurotransmitter opens up sodium channels so that sodium ions enter the cell. Now the inside becomes more positive and the outside less positive so the potential difference between the outside and inside is reversed. The cell is then depolarized and when the depolarization potential difference reaches a certain level, known as the threshold potential, an electrical impulse passes along the neuron. As sodium channels open up in one part of the cell the potential difference reaches the critical ...

Table of contents

• Cover page
• Title
• Copyright
• Contents
• Introduction
• Chapter 1 Eyes
• Chapter 2 The Evolution of the Eye
• Chapter 3 The Science of Colour
• Chapter 4 The Range of Colour
• Chapter 5 The Visual System and Colour
• Chapter 6 Perceived Colour and Environment
• Chapter 7 Painting and Painting Pigments
• Chapter 8 The Development of Dyes
• Chapter 9 Colouring Pottery and Glass
• Chapter 10 Projecting Coloured Images
• Chapter 11 Early Colour Photography
• Chapter 12 Colour Photography
• Chapter 13 Colour Cinematography
• Chapter 14 Colour Television
• Chapter 15 Coloured Light Displays
• Chapter 16 Practical Uses of Colour
• Index