Principles of Colour and Appearance Measurement

Object Appearance, Colour Perception and Instrumental Measurement

Asim Kumar Roy Choudhury

• Publisher: Woodhead Publishing
• Year: 2014
• ISBN:
  9780857099242
• Topic: Design
• Subtopic: Industrial Design

Colour and appearance perceptions are very complex psychological phenomena. Written by one of the foremost authorities in the field, Principles of Colour and Appearance Measurement is a major two-volume work addressing the key topics required to understand the issues and manage colour effectively. The book addresses how objects appear to viewers, how viewers perceive colour, and the major types of instrumentation used to measure colour.

Chapters detail the characteristics of light sources and object colour and appearance attributes. They encompass the complexities of human visual perception, including the various causes and types of colour blindness, and other unusual visual phenomena. The book also covers colour measurement instruments and methods, as well as fluorescence and whiteness.

Principles of Colour Appearance and Measurement is a comprehensive resource for designers, colour technologists, colour quality inspectors, product developers, and anyone who uses colour in their work.

• Addresses the key topics required to understand the issues of colour measure and management
• Examines how viewers perceive colour and how objects appear to them
• Reviews the major types of instrumentation used to measure colour

1

Characteristics of light sources

Abstract:

This introductory chapter discusses the importance of light sources in visual perception. Various radiometric and photometric measures of illumination have been explained. The characteristics of various incandescence and discharge lamps are discussed and compared. Colour temperature, luminous efficacy, colour rendering and colour preference indices of light sources are also discussed. Theoretical light sources or illuminants are explained.

Key words

light sources; illuminants; radiometry; photometry; black body; colour rendering index; colour preference index

1.1 Introduction

According to the International Lighting Vocabulary, the definition of light is: ‘Any radiation capable of causing a visual sensation directly’. Light, or visible light, is electromagnetic radiation visible to the human eye and responsible for the sense of sight (CIE, 1987). Visible light has wavelength in the range of about 380 nanometres (nm) to about 740 nm, with a frequency range of about 405–790 THz. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not (Kumar, 2008).

Light is everywhere. Light is at once obvious and mysterious. We are showered with yellow sunlight during daytime, and saved from the darkness by incandescent and fluorescent lamps. We need light to see the objects surrounding us. It carries information from the world to our eyes and brains. Colours and shapes are indispensable parts of nature, yet light is a perplexing phenomenon when we study it more closely. The brain and eyes act together to make our visual perception extraordinary powerful. We see movies as sequences of still pictures and the pictures as arrays of dots. We catch glimpses of its nature when a sunbeam angles through a dust-filled room and when a rainbow appears after a storm. Light acts as particles that stream from a source. This explains how shadows work. Light also acts like waves – ripples in space. This explains how rainbows work. In fact, both are light. This ‘wave–particle duality’ is one of the most confusing and wonderful principles of physics (Greiner, 2001). Many scientists have spent lifetimes developing physical, biological, chemical, and mathematical explanations for these principles.

1.2 Process of visual perception

Visual perception is the ability to interpret the surrounding environment by processing information that is contained in visible light. The resulting perception is also known as eyesight, sight or vision. The various physiological components of vision are referred to collectively as the visual system. All creatures have the ability to sense the surrounding world, but in various ways and degrees. We may envy the bloodhound’s exceptional nose, but humans possess visual prowess that is unsurpassed in its ability to detect and make sense of patterns (although it does not match the eagle’s sight in distance). Our eyes and brains work as a team to discover meaningful patterns that help us make sense of the world.

There are three ingredients or elements in the process of visual observation of an object as shown in Fig. 1.1. They are:

• a light source

• an object

• a human observer.

1.1 Object colour and appearance recognition process.

All three factors influence the colour and appearance visualised by the observer. A source illuminates the object and is characterised by the emitted energy at different wavelengths, which is denoted by the term spectral power distribution (SPD). When light falls on an object, the light beam is modified by absorption, scattering and other physical processes, depending on the physical and chemical construction of the object. Ultimately, the light reaches the eye of the observer in the form of reflected or refracted light. Photosensitive pigments in the eye absorb the light energy. This gives rise to nerve impulses which are transmitted to brain. The human eye–brain mechanism makes rapid and continuous evaluation of object appearance and colour. The light, which enters our eyes, contains the characteristic imprints of both the light source and the object.

1.3 Optics

Optics is the branch of physics dealing with the behaviour and properties of light, including its interactions with matter, and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light (McGraw-Hill, 1993). Practical optics is usually done using simplified models. It is broadly divided into three fields based on their method of theoretical treatments:

• Geometric optics deals with the geometric aspects of propagation and interaction of light, i.e. the rectilinear aspect of light. It treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.

• Physical optics is a more comprehensive model of light, which includes wave effects. Phenomena such as diffraction and interference cannot be explained by geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the nineteenth century led to the discovery that light waves were in fact electromagnetic radiation.

• Quantum optics applies quantum mechanics to optical systems for explaining both wave-like and particle-like properties. When considering light’s particle-like properties, the light is modelled as a collection of particles called ‘photons’.

1.3.1 Quantum or corpuscular theory

The absorption and emission properties of light can be best explained by the concept that light exists as a series of energy packets known as photons, and its inherent energy may be expressed as follows:

[1.1]

where h is Planck’s constant (6.6254 × 10⁻²⁷ erg sec).

1.3.2 Wave theory

In physical optics, light is considered to propagate as a wave. Sir Isaac Newton held the theory that light was made up of tiny particles. In 1678, Dutch physicist, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction the light travels, and therefore formulated a way of visualising wave propagation. This became known as ‘Huygens’ Principle’. Huygens’ theory was successful as a theory of light wave motion in three dimensions. Huygens suggested that light wave peaks form surfaces, like the layers of an onion. In a vacuum, or other uniform media, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than go in a straight line. Newton’s theory came first, but the theory of Huygens better described early experiments. Huygens’ principle lets us predict where a given wave front will be in the future, if the present location of the given wave front is known.

A large number of optical phenomena can be explained by assuming that light consists of waves. Maxwell (J. Clerk Maxwell, 1831–79) first proposed a definite relation between light and electricity. Wave theory describes how electromagnetic radiation propagates in the form of waves. The phenomenon of interference, diffraction and polarisation can be explained by this theory. The electromagnetic theory of light was established on the basis of the relation between electricity and magnetism. Light was proposed to be a type of electromagnetic radiation.

1.3.3 Electromagnetic radiation

Electricity and magnetism are two components of electromagnetism: a changing magnetic field produces an electric field, and a changing electric field produces a magnetic field. This connection was first explained by Faraday and Maxwell – Einstein saw electricity and magnetism as frame-dependent aspects of a unified electromagnetic force. An accelerating charge produces electromagnetic waves (radiation). Both electric and magnetic fields can transport energy. Electric field energy is used in electrical circuits, e.g., released in lightning. Magnetic field carries energy through a transformer.

Electromagnetic radiation (EMR) is a form of energy emitted and absorbed by charged particles, which exhibit wave-like behaviour as they travel through space. EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation. Some key features of EMR are:

1. Electromagnetic (EM) waves are special since they do not need a medium to propagate through.

2. All electromagnetic waves travel at the same speed through space (the speed of light).

3. When something creates energy it also emits radiation. Depending on the amount of energy, the object will emit different types of electromagnetic radiation.

4. Gamma rays, X-rays, ultraviolet waves, light, infrared rays, microwaves, and radio waves are all electromagnetic waves with different wavelengths.

5. Some substances absorb EM waves, some reflect them, and others transmit them.

In a vacuum, EMR propagates at a characteristic speed, the speed of light. Light has a very high but finite speed (c). In vacuum it travels at a speed of 186 000 miles.s⁻¹. The speed reduces as light travels through a medium and is inversely proportional to the refractive index of the medium. The velocity of any wave can be expressed by the following equation:

[1.2]

where f = frequency i.e. the number of complete waves that occur in a second, and λ = wavelength i.e. the distance after wh...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Author contact details
• Woodhead Publishing Series in Textiles
• 1. Characteristics of light sources
• 2. Object appearance and colour
• 3. Colour and appearance attributes
• 4. Principles of colour perception
• 5. Unusual visual phenomena and colour blindness
• 6. Colour measurement instruments
• 7. Using instruments to quantify colour
• 8. Issues in measuring whiteness and fluorescence
• 9. Instrumental measures of whiteness
• Index