Xin Total Colour Management in Textiles

1. Auflage 2006
ISBN: 978-1-84569-108-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 246 Seiten

Reihe: Woodhead Publishing Series in Textiles

Managing colour from the design stage to the finished product can be a difficult activity as colour perception is subjective and can therefore be inconsistent. Total colour management in textiles covers all aspects of managing colour from the design stage to the final product, ensuring that the designer's vision is fulfilled in the finished colour. There have been many new developments in the area of colour measurement and colour perception which are discussed. These include discussion of the sensory effect of colour for design and use in product development, and digital colour simulation. - Concentrates on managing colour from the design stage to the finished product - Provides a comprehensive discussion on the many aspects of textile colour management - Written with an industrial applications viewpoint

Autoren/Hrsg.

Xin, John H

Leseproben

1 Colour perception
S. Westland; V. Cheung    University of Leeds, UK 1.1 Introduction
Colour exists only in the mind; it is a perceptual response to light that enters the eye either directly from self-luminous light sources or, indirectly, from light reflected by illuminated objects. The nature of light and the spectral reflectance properties of objects are therefore described in the first part of this chapter. The second part of the chapter is concerned with the physiology and functional properties of the retina in the human eye. Light that enters the eye is sampled by three classes of light-sensitive cells in the retina known as cones. In order to understand colour, it is necessary to appreciate that the effective spectral sensitivities of these cones are not static; rather, they change with the illumination conditions and the responses of spatially neighbouring cells, to name but two factors. Furthermore, the three classes of signals from the cones are processed by the neural pathways that lead from the retina to various areas of the cortex in the brain. Although our understanding of colour processing in the human visual system is sufficient to allow us to predict when two spectrally dissimilar objects will be a visual match, it does not allow us to make reliable predictions of colour appearance. In this chapter, three current problems for the science of colour vision are described: colour contrast, colour constancy and colour appearance. 1.2 The nature of colour
Light is a form of energy. Specifically, it is that part of the spectrum of electromagnetic radiation that our eyes are sensitive to. Radio waves and X-rays, as well as ultraviolet and infrared radiation, are all part of the spectrum of electromagnetic radiation but the human visual system is only capable of sensing a very narrow band of wavelengths in the approximate range 360–780 nm (a nanometer is 10- 9 metres). The light from any source can be usefully described in terms of the relative power emitted at each wavelength in the visible spectrum. Figure 1.1 shows the wavelengths of the visible spectrum and the colours with which we normally associate the wavelengths. However, Newton was famously aware that ‘the rays are not coloured’. By this phrase Newton meant that light is not intrinsically coloured; short-wave light, for example, has no intrinsic property by which it is blue but, rather, it may induce in us the sensation of blueness. Under some circumstances, however, short-wave light may appear black or some colour other than blue. It is therefore clear that colour cannot be understood without a study of the properties of the human visual system, since colour exists only in the brain. 1.1 The electromagnetic spectrum. Only radiation in the range 360–780 nm is visible to the human eye. The spectral power distribution of daylight varies with geographical position, atmospheric conditions, and with the time of day and year but the set of daylight power distributions is very similar to that emitted by a blackbody1 heated at different temperatures (Judd et al., 1964). For many light sources it is useful to refer to the temperature (usually expressed in Kelvin) of the blackbody whose radiation most closely resembles that of the light source. This temperature is called the correlated colour temperature (Sinclair, 1997). The radiation of north sky daylight on a cloudy day has a correlated colour temperature of about 6500 K, whereas the light from a tungsten filament bulb has relatively more power at the long wavelengths, which gives it a much lower correlated colour temperature. 1.3 The physical basis of colour
When light strikes an object, some light is always reflected from the surface, at the boundary between the object and air, because of the change in refractive index as the light passes from air to a more dense medium. This surface reflectance has the same relative spectral power distribution as the illumi nating source and may be diffuse or specular in nature. Diffuse reflectance, where the light is dispersed in many different directions, occurs when the surface is rough, whereas smooth glass-like surfaces give rise to specular surface reflectance where the angle of reflection is equal to (but with opposite sign) the angle of incidence of the illumination. The light that is not reflected at the surface enters the body of the object, where further interactions take place. If the material is transparent, some light will pass through the material and emerge at the other side. The most common processes that reduce transparency are absorption and scattering. Absorption is a process whereby light is removed by an interaction with the molecules of the object at an electronic level. Most objects are coloured because this absorption process is more efficient at certain wavelengths than at others, in a way that depends upon the properties of the molecules (Zollinger, 1999). Scattering is a kind of reflection that occurs when particles (or air bubbles) are present in the material. The amount and directional nature of the scattering depends upon the size of the particles and their refractive indices (relative to the medium in which they are contained). Many opaque (non-transparent) white materials are manufactured by adding particles of a white pigment such as titanium dioxide, which has a particularly high refractive index. Translucency is a visual phenomenon that can give materials a milky or cloudy appearance and occurs when the material is partially transparent but exhibits scattering. Further details about the physics of light and its interaction with materials is provided by Nassau (1983) and Tilley (2000) or, for an explanation at the level of quantum electrodynamics, Feynman (1990). The proportion of light reflected by a sample can be measured using a reflectance spectrophotometer and represents the (physical) colour fingerprint of the sample. A spectrophotometer typically measures the proportion (sometimes expressed as a percentage) of light reflected by the object at each of several equally spaced wavelength intervals. Commercially available instruments typically measure at 31 wavelength intervals centred at 400 nm, 410 nm, 420 nm,…, 690 nm, and 700 nm.2 Most reflectance spectra are smooth functions of wavelength so that it is reasonable to measure the reflectance at wavelength intervals of 5 nm or even 10 nm with little loss of information (Maloney, 1986). For non-fluorescent materials, the spectral reflectance factors are independent of the intensity or spectral distribution of the light source that is used by the spectrophotometer. That is to say, if a given object reflects 50% of the light at a given wavelength, this is independent of whether the incident illumination contains 100 or 1000 units of power at that wavelength. The spectral reflectance factor is obtained by comparing the intensity of the reflected light for an object at a given wavelength with the intensity of the light reflected by a perfect Lambertian diffuser.3 1.4 The human colour vision system
The light that is reflected by objects or emitted by light sources enters the eye, where it may be absorbed by visual pigments in the photoreceptors, or cones, contained within the retina. The spectral sensitivities of the pigments in the three cone classes play a significant role in the nature of our colour perception. However, colour perception can only be fully understood if the processes that take place in the nervous system that transmits the retinal signal to the occipital lobe of the brain’s cortex are studied. A brief review of physiological processes that are important for colour vision is given in this section. 1.4.1 The human eye
The eye is an approximately spherically shaped organ that contains an aperture and a light-sensitive inner lining called the retina. The aperture is at the front of the eyeball and allows light to enter where it can be focused by the lens onto the retina. The front of the eye is covered by a clear layer of tissue known as the cornea through which light must pass before it can enter the eyeball. The main function of the cornea is to protect the eye from injury; however, it also acts to refract the light so that it is focused appropriately at the retina.4 About two-thirds of the focusing of light by the eye is carried out by the cornea (Meek, 2002). the lens – a crystalline structure that is suspended by the ciliary muscles (see Fig. 1.2) – being responsible for the remainder. 1.2 Schematic diagram of the human eye. The shape of the lens can be changed as it is squeezed by the ciliary muscles as a way of focusing a sharp image of the scene on the retinal layer that coats the inner surface of the eye. The iris can change size, so that the area changes from about 50 mm2 in dark conditions to about 10 mm2 in bright sunlight. Although the pupil area can change by as much as a factor of 10 in response to the light intensity, in fact the range of illuminances in which the visual system operates covers many orders of magnitude (e.g. 10 lux in a darkened room to 100 000 lux in brilliant outdoor sunlight). Therefore, the change in pupil size can only play a minor role in the adaptation of the visual system to changes in light intensity. Light is focused onto the retina which includes specialised cells, known as rods and cones, that contain photopigments that undergo a...