Interior Lighting: Fundamentals, Technology and Application

By Wout van Bommel

This book outlines the underlying principles on which interior lighting should be based, provides detailed information on the lighting hardware available today and gives guidance for the design of interior lighting installations resulting in good visual performance and comfort, alertness and health.

The book is divided into three parts. Part One discusses the fundamentals of the visual and non-visual mechanisms and the practical consequences for visual performance and comfort, for sleep, daytime alertness and performance, and includes chapters on age effects, therapeutic effects and hazardous effects of lighting. Part Two deals with the lighting hardware: lamps (with emphasis on LEDs), gear, drivers and luminaires including chapters about lighting controls and LEDs beyond lighting. Part Three is the application part, providing the link between theory and practice and supplying the reader with the knowledge needed for lighting design. It describes the relevant lighting criteriafor good and efficient interior lighting and discusses the International, European and North American standards and recommendations for interior lighting.

A particular focus is on solid state light sources (LEDs) and the possibility to design innovative, truly-sustainable lighting installations that are adaptable to changing circumstances. The design of such installations is difficult and the book offers details of the typical characteristics of the many different solid state light sources, and of the aspects determining the final quality of interior lighting.

Essential reading for interior lighting designers, lighting engineers and architects, the book will also be a useful reference for researchers and students.

Reviews of Road Lighting by the same author:

"If you are going to design streetlighting, you must read this book....a solid, comprehensive textbook written by an acknowledged expert in the field – if you have a query about any aspect of streetlighting design, you will find the answer here.” – LUX, August 2015

“…a realy comprehensive book dealing with every aspect of the subject well…essential text for reference on this subject” – Lighting Journal, March 2015

• Language: English
• Publisher: Springer
• Release date: Aug 13, 2019
• ISBN: 9783030171957

Book preview

Part IFundamentals

© Springer Nature Switzerland AG 2019

W. van BommelInterior Lightinghttps://doi.org/10.1007/978-3-030-17195-7_1

1. Visual Mechanism

Wout van Bommel¹ 

(1)

Van Bommel Lighting Consultant, Nuenen, Noord-Brabant, The Netherlands

Abstract

A visual sensation is the result of processes in the eye and brain. Light entering the eye is projected on the back of the inner part of the eye, the retina. The retina contains photoreceptor cells: cones and rods. Photopigments in these receptor cells absorb light, resulting in a chemical-electrical signal which travels down a nerve into the visual cortex part of the brain where the visual sensation is invoked. A small area of the retina around the axis of the eye, the fovea, only contains cone cells. The other, peripheral, areas have few cone and many rod cells. The cone cells in the fovea have a one-to-one nerve connection to the brain. Rod photoreceptor cells are located in the periphery of the retina. Many of them converge on a single ganglion cell. Consequently, foveal vision is sharp and peripheral vision is not sharp. The set of rods converging on the same ganglion (the receptive field of that cell) are processed through an opponent mechanism. Colour vision is possible because there are three types of cones, one with sensitivity for reddish, one for greenish and one for bluish light. A colour opponent mechanism processes their signals. Since we have just one type of rod cell, colour vision with rods is impossible. Cones are mainly active at lighting levels larger than some 5 cd/m ² . Vision is then referred to as photopic. The spectral eye sensitivity curve V(λ) defined for photopic vision is the basis for all photometric units.

This chapter explains how the various components of the visual system function to produce visual perception under widely different lighting circumstances. The system uses photomechanical, photochemical and photoelectrical processes. The expression photo refers to the fact that light controls these processes. Photomechanical processes take place in the eye itself. Think of pupil and eye lens changes. Photochemical and photoelectrical processes take place in the photoreceptor cells located in the retina of the eye. These processes are essential for relaying messages from these photoreceptors to the area in the brain where the visual sensation is evoked. The visual lighting effects are a direct consequence of these processes. The lighting professional needs to have a basic knowledge of these processes to understand the relations between lighting and visual performance and comfort.

Chapter 2 will discuss how the spectrum of light influences the perception of colours.

The visual system also evokes non-visual biological effects. This chapter only discusses this subject there where there is a direct relationship with vision. Part II of this book (Chap. 5) will deal with the non-visual biological system.

1.1 Visual Sensation

Already around 1490 Leonardo da Vinci showed in an anatomical drawing that vision is the result of combined processes taking place in the eye and the brain (Fig. 1.1). Da Vinci’s picture represents Greek, Arab and Medieval views on these processes (Gross 1997). Da Vinci added his views based on dissections he carried out on human cadavers (McMurrich 1930). The lens, drawn by da Vinci circularly and centrally located in the eye, is not considered by him as having an optical function. Only in 1602 Johannes Kepler described the paths of light rays in the eye. He defined the eye as an optical device creating an inverted image at the back of the eye where light receptors (photoreceptors) are located.

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Fig. 1.1

Anatomical study by Leonardo da Vinci, approx. 1490 (Windsor Castle, Royal Library)

In da Vinci’s drawing, the middle part of the brain processes the light collected by the eye. He considered that part of the brain as the site that receives input from all five sense organs: sensus communis (common sense). Today we know that different locations in the brain process the different senses. In these locations, the effect of the senses is produced: the sensation. In the case of the sense of sight, this is the visual sensation representing the scene in front of the eye. The area in the brain where the final processing for vision takes place lies not in the middle of the brain, as da Vinci thought, but in the lower rear end of it. It is called the visual cortex. Recent research indicates that there exists some communication between the different sense areas of the brain: multisensory integration (Macaluso and Driver 2005; Witten and Knudsen 2005; Marrelec et al. 2008).

1.2 Optics of the Eye

Figure 1.2 shows a cross section of the eye. The outer surface of the eye, the sclera also called the white of the eye, consists of hard white tissue giving rigidity to the eye ball. It bulges out at the front where it is translucent. This part is named the cornea. Light enters the eye through the cornea and travels through a circular diaphragm formed by the iris. The colour of the iris tissue determines the colour of the eye. The owner of the eye depicted in Fig. 1.2 has brown eyes. The opening of the iris is called the pupil. A circularly shaped muscle running through the iris can change the size of this opening. The change is controlled by the amount of light entering the eye and is one of the mechanisms of adaptation to different light levels. Just as with a diaphragm of a camera the actual size of the pupil also influences the size of the area in front of the eye that is seen sharp: a smaller pupil size results in a larger field of depth. The eye lens projects an inverted image from the scene in front of the eye on the back of the inner part of the eye, the retina. An object on the line of sight, and thus on the axis of the eye, is projected at the position of the retina where the fovea is located. The shape of the lens is changed, dependant on where the eye focuses on, from flat to more spherical by a system of muscles. In this way, the eye can project a sharp image on the retina from a distant part of a scene (flat lens shape) or a more nearby part (more spherical shaped lens). This process of adjusting the lens in dependence of the distance viewed is called accommodation. The inner part of the lens contains transparent fibres with proteins of the crystalline type. This is the reason that the eye lens often is referred to as the crystalline lens. The large area of the eye behind the lens, called the vitreous body, is filled with a vitreous, transparent, colourless gel existing for some 99% of water. It presses the retina against the back of the eye so that it stays in place.

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Fig. 1.2

Cross section of the eye

The retina at the back of the eye can be compared with the light-sensitive film in conventional cameras or with CCD cells in electronic cameras which transform light into a chemical action (paper or plastic film) or an electric action (CCD cell). In the retina of the eye, the projected image is transformed into neural activity. The actual transformation takes place in some 100–130 million photoreceptor cells located in the retina. The dark-brown layer behind the retina, called the choroid, absorbs the light which is not transformed into a neural action. In this way, disturbing internal reflection of light within the eye, stray light, is prevented. The choroid layer has the same function as the black interior of a camera. Another function of the choroid is nourishing the internal parts of the eye, in particular, essential for the retina. For this purpose, it contains many blood vessels.

1.3 Retina and Photoreceptors

The retina is a very thin tissue with a thickness between 0.1 and 0.3 mm (about the thickness of two sheets of paper). It consists of many different layers. Figure 1.3 shows the most important ones. Surprisingly the layer of the photoreceptor cells, the cones and rods, is not located at the front but at the back of the retina. It means that light must pass through different layers of neuron cells (ganglion and collector cells) before reaching the photoreceptor cells. These layers also contain many blood vessels, in Fig. 1.3 indicated as pink shaded.

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Fig. 1.3

Part of the retina with photoreceptor cells and their connections. Collector cells are bipolar, horizontal and amacrine cell types. The pink-shaded area represents blood vessels

The human photoreceptor cells for vision can be distinguished, according to function and geometry, into two categories of cells: cones and rods.

The eye has some 5–6 million cones and 100–120 million rods. The cone and rod-shaped outer part of the photoreceptor cells have hundreds of thin membrane plates that contain photopigment molecules (Fig. 1.4). They are called opsins. The cones and rods have a different type of opsin called, respectively, photopsin and rhodopsin. The tips of the photoreceptors are in contact with the pigment epithelium layer at the back of the retina (Fig. 1.3). This layer provides vitamin A to the photoreceptors which chemically binds with the opsin molecules to make them photosensitive. Vitamin A is converted from beta-carotene, available from food as, for example, carrots. The opsin molecules can now absorb a photon of light and trigger a cascade of chemical reactions, ultimately changing the electrical state of the photoreceptor cell. This process is called phototransduction. The resulting electric-chemical signal is transmitted through a series of different types of collector cells towards the ganglion cells. Here the signals undergo a first processing, and the result is forwarded through the optic nerve into the brain towards an area called visual cortex (Fig. 1.3). Here the signal is processed into a light and colour sensation (Weston 1949; Tovée 1996; Kolb 2003).

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Fig. 1.4

Cones and rod photoreceptor cells. From left to right: synapse through which electric-chemical signals pass, the nucleus and the lamellae membrane containing photopigment molecules, making the cone cells colour sensitive (Note that Fig. 1.3 shows the cells completely coloured to make them easier to distinguish)

While a photopigment absorbs a photon, the colour of the photoreceptor bleaches and the photoreceptor is temporarily depleted (exhausted). As a consequence, when, because of high lighting levels, a large part of the photopigment is bleached, the probability of photon absorption decreases and the cell is less active. In the case of rods, the probability tends to be zero at lighting levels more than some 5 cd/m²: the rods are saturated and no longer active as light receptors (Walraven et al. 1990). This is important because rods have a much larger sensitivity to light than cones. If rods remained active at high lighting levels, tremendous glare would be the unwanted result. When the light level decreases, a reverse chemical reaction reactivates them. This reactivating process is very slow. It is the reason that adaptation from high to low lighting levels may take minutes and in the case of adaptation to very dim light even half an hour. In this way, rods provide for vision only at low to extremely low lighting levels.

Vision with rods is of low acuity and monochrome. Cones provide for high-acuity colour vision at higher lighting levels. Colour vision with cones is possible because there are three types of cones each with a different type of photopigment, one with maximum sensitivity for blueish, one for greenish and one for reddish light.

The small thin and pit-shaped area of the retina around the axis of the eye with a diameter of around 1.5 mm is called the fovea (see again Fig. 1.3). It receives light from within a cone of approximately 2° centred on the axis of the eye (CIE 2010). The fovea only contains cone cells. The other, peripheral, areas of the retina have few cone cells and many rod cells. Cone cells in the fovea are much thinner and closer packed than cone cells outside the fovea. Figure 1.3 illustrates that every cone in the fovea has a one-to-one direct connection with the optic nerve and the brain. Outside the fovea, the area of peripheral vision, there is no such one-to-one direct connection with the brain. Here, many cells converge on a single collector cell, and different collector cells converge in turn on a single ganglion cell before the combined signal travels to the brain. Some 120 million photoreceptors converge on approximately 1 million ganglion cells. The convergence increases with distance from the fovea (Watson 2014). On the area outside the fovea, an average of some 100 rods and cones converge on the same ganglion cell. This combination of signals increases the sensitivity of rod vision considerably, making vision possible under very dim lighting conditions. On the other hand, the message sent to the brain loses information about the exact location from where the light originates. Consequently, peripheral vision with rods is highly sensitive at low light levels but results in blurred images and thus low-acuity vision. Foveal vision, on the other hand, results in sharp, high-acuity vision but needs higher lighting levels. This is because of the thinner, densely packed, cones and the one-to-one connection with the brain. High-acuity vision is here also helped by the fact that the fovea area is free from blood vessels and the ganglion cells are displaced to leave a clear area for the light to pass through (Fig. 1.3). This is the reason that the fovea has the shape of a pit with a rim and is the thinnest area of the retina.

Table 1.1 summarises the most significant differences between the foveal and peripheral parts of the retina

Table 1.1

Comparison of the foveal and peripheral retina (adapted from Mann 2016)

1.4 Spectral Sensitivity

1.4.1 Cones and Rods

As described in the previous section, the photopigments in the cones and rods (photopsin and rhodopsin) absorb incident light and transmit as a result of this an electric signal to the brain where then the visual sensation is evoked. The rods and three types of cones contain a different kind of photopigment. They each have a different absorption spectrum and thus different wavelength sensitivity. The photopigments of blue cones absorb light with relatively short wavelengths in the blue range of the spectrum. These cones are referred to as S-cones, where S stands for short wavelength. The S-cone type of opsin is called photopsin of the type cyanolabe, usually, more simply, referred to as the S-cone opsin. The spectral sensitivity of S-cone opsins has a pronounced peak in the short part of the spectrum (blue light) as shown in Fig. 1.5. Spectral sensitivity curves are sometimes shown on a linear scale (Fig. 1.5, top) and sometimes on a logarithmic scale with better information about the lower sensitivities (Fig. 1.5, bottom). The photopigments of the green and red cones provide a broader sensitivity spectrum (Fig. 1.5). The green one peaks in the middle-wavelength part of the spectrum and the red one in the long-wavelength part. Consequently, these opsins are called M-cone and L-cone opsins, formally called photopsins of the type chlorolabe and erythrolabe, respectively. The spectral sensitivities, shown in Fig. 1.5, are measured relative to light entering the cornea. They thus also take filtering by eye media in front of the retina, such as the eye lens, into account. The cone sensitivities are formally referred to as cone fundamentals.

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Fig. 1.5

Relative spectral sensitivities for light at the outer surface of the eye for the three types of cones and for the rods. Basis: 32-year-old standard observer; rod curve: CIE (2015); cone curves: CIE (2006). Bottom figure in logarithmic units

The sensitivity spectrum of rods coincides with the green-bluish part of the spectrum (Fig. 1.5). Their opsins are called rod-opsins (or rhodopsins). The sensitivity curves shown in Fig. 1.5 are valid for a 32-year-old observer.

The spectral sensitivities of the three different cone types form the basis for colour vision. Since we only have one type of rod cell, colour vision with only rods is impossible: achromatic vision.

1.4.2 Photopic and Scotopic Vision

It has already been mentioned that successful transformation of light into signals to the brain is very much dependent upon the light level to which the eyes are adapted. At adaptation levels larger than some 5 cd/m² the photopigments of the rods are completely inactive. What remains is pure cone vision. Vision under these conditions is called photopic vision. Lighting levels of most indoor lighting installations are in the photopic vision range. At lower lighting levels the activity of rods gradually increases until, at some 0.005 cd/m², they have reached their maximum sensitivity. Therefore, at adaptation levels between 5 and 0.005 cd/m² vision is a combination of rod and cone vision. Vision under these conditions is called mesopic vision. Most road lighting applications result in mesopic vision. At lighting levels lower than some 0.005 cd/m² the sensitivity of the cones is far too small to play a role in vision so that only the rods determine vision. Vision under these conditions is monochrome and of low visual acuity. This type of vision is called scotopic vision. Only at night in areas without any artificially light and with no full moon vision is scotopic.

The overall spectral sensitivity under photopic vision, important for interior lighting applications, is not a simple summation of the sensitivities of the three cone types as given in Fig. 1.5. Cones are not evenly distributed over the retina (Fig. 1.6). Blue S-cones are located more at the outside border of the fovea. The number of the three cone types is also different: there are substantially less blue S-cones (only 5–10% of all cones). The absolute sensitivity of blue S-cones is considerably lower than that of the other cone types.

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Fig. 1.6

Distribution of the S-cones (blue), M-cones (green) and L-cones (red) in the fovea

All this means that the overall spectral sensitivity is dependent upon the type of sensation or action the light is expected to deliver. Examples are creating brightness, enabling detection and enabling small detail vision. The size of the visual field plays a role: for a small field (often relevant for seeing small details) the fovea (with cones only) plays a bigger role than in the case of a larger visual field (often relevant in creating a bright environment). In 1924 CIE, as international standardisation body for light and lighting, defined a spectral sensitivity curve on the basis of the brightness of a 2° (foveal) visual field. The curve was published in 1926 as the CIE spectral luminous efficiency function V(λ) for photopic vision (CIE 1926). Figure 1.7 shows the V(λ) curve. The peak is at 555 nm in the yellow area of the wavelength range. Appendix A gives the values of this curve per 5 nm. For purposes where a larger visual field is relevant, CIE defined a spectral sensitivity curve for a 10° visual field (CIE 2005). In 1951, CIE published a spectral sensitivity curve for scotopic vision, again based on brightness, but here for a 20° visual field (CIE 1951). It is called the spectral luminous efficiency function V′(λ) for scotopic vision. It peaks at a shorter wavelength of 507 nm (Fig. 1.7).

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Fig. 1.7

Standardized relative spectral sensitivity curves V(λ) and V′(λ) for photopic and scotopic vision, respectively (CIE 1926, 1951). Bottom figure in logarithmic units

The photopic spectral sensitivity curve V(λ) is used as a weighting function to convert radiometric units (such as radiant power expressed in watt) into photometric units (such as luminous flux expressed in lumen). The lumen is thus the radiant energy of visible radiation as sensed (weighted) by the eye under photopic vision conditions. The other light units, candela and lux, are also based on the same V(λ) curve.

1.5 Receptive Fields

Ganglion cells do not simply transmit the signals from the photoreceptors towards the brain. In the ganglion cells, the input signals from the photoreceptors undergo the first processing. The output of a ganglion cell is a series of voltage spikes (neural action potentials) that travel down the optic nerve towards the brain where further processing finally leads to the visual sensation. The output signal of the ganglion cells is determined by the rate of voltage spikes and not by the voltage value. Firing voltage spikes at different rates is the means with which communication, in general, takes place in the brain. Especially outside the fovea, many photoreceptor cells converge on a single ganglion cell. This is because the horizontal cell type of collector cells connects with several photoreceptors (Fig. 1.8). The total area of photoreceptor cells converging on a particular ganglion cell is called the receptive field of that cell. Kuffler (1953) was the first to map receptive fields by radiating tiny spots of light on the retina and measuring with miniature electrodes electrical activity in ganglion cells. Figure 1.8 shows a sketch of receptive fields of two different ganglion cells.

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Fig. 1.8

Receptive fields of two ganglion cells

Further down in the periphery of the retina, the receptive fields become gradually larger, because there more photoreceptors converge on a single ganglion cell. The sketch of Fig. 1.8 may suggest that the dimensions of a receptive field are large, but they are very small. Remember that the retina contains more than 100 million photoreceptors and 1 million ganglion cells. A so-called bipolar cell connects the centre part of the receptive field directly with the ganglion cell (see again Fig. 1.8). The other photoreceptors of the same receptive field have a more indirect connection via a horizontal cell which interconnects the photoreceptors of the receptive field. This area of the receptive field is referred to as the surround area.

Signals from the receptive field are not just passed along but are processed by the ganglion cell to facilitate interpretation of a scene. For this interpretation, light-dark patterns or contrasts, particularly at the edges of objects and surfaces, are important.

Ganglion cells compare signals arriving from an inner circular area of the receptive field with signals arriving from the outer circular area (the surrounds) of the same receptive field. One type of ganglion cell increases its output (excites is the term normally used) when the centre circle of its receptive field is illuminated but decreases (inhibits) its output when the surrounds are illuminated. This type of cell is called an ON-centre ganglion cell. Figure 1.9b shows the situation of such a cell when the light completely overlaps with both the centre and the surrounding part of the receptive field. The ganglion cell shows only spontaneous activity, just as in the case with no light at all (Fig. 1.9a). Figure 1.9c shows the situation of an ON-centre ganglion cell when only the centre part of the receptive field is illuminated. The ganglion cell fires a series of voltage spikes of high rate that travel to the brain (an ON response). Contrary to this, when the light illuminates only the surround of the centre of the receptive field (Fig. 1.9d), the firing of spikes stops completely (an OFF response). When both the centre and the surrounds are partly illuminated (Fig. 1.9e), spikes fire at a rate higher than the spontaneous rate, but at a lower rate than in the situation with all light in the centre. When the centre and part of the surround are illuminated (Fig. 1.9f), again the cell fires at a rate higher than the spontaneous rate. There is another type of cell, called OFF-centre ganglion cell, that processes signals in the opposite way: light in the centre of the receptive field of an OFF-centre cell decreases or stops the firing of spikes while light in the surround increases the firing rate (Fig. 1.9g–i). The number of ON-centre and OFF-centre ganglion cells is more or less the same.

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Fig. 1.9

Voltage firing rate of ON-centre and OFF-centre types of ganglion cells in dependence of a light spot (yellow circle) at different areas of the receptive field of the cell. The voltage-spike scale indicates with a yellow bar when the light spot is on. (a) Rest situation (no light) with spontaneous activity; (b–f) ON-centre type of ganglion cells; (g–i) OFF-centre type of ganglion cells

The centre-surround ON-OFF processing by the retinal ganglion cells as described here enables detection of light-dark transitions and thus edge detection of bright objects or light sources. Figure 1.10 illustrates this by showing how a bright circular object (or light source) of uniform luminance interacts with a group of receptive fields of neighbouring ganglion cells.

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Fig. 1.10

Edge detection of an object or light source of uniform luminance (yellow circle) through the processing of receptive fields by retinal ganglion cells

The bright object (light source) overlaps entirely with the inner ganglion cells. The output activity of these inner cells is consequently suppressed to spontaneous activity only, as is shown in Fig. 1.9b. Many ganglion cells that overlap with the edges of the object overlap partly with both the centres and surrounds. Their output activity is, therefore, higher, as shown in Fig. 1.9e, f, h, i. By processing the information from the receptive fields in this way, the edge of the object is detected. Only that information is forwarded to the visual cortex. Since the retina contains more than a million ganglion cells, the edge of the bright object overlaps with a much larger number of ganglion cells than sketched in Fig. 1.10. The ganglion cells are also much more densely packed. Therefore, the edges of the objects or light sources are in reality very sharply marked by the ganglion cells.

From the uniform parts of the object or light source, less information is forwarded by the ganglion cells to the brain so that the brain is less engaged. A multitude of smaller bright objects of light sources (as matrix LED luminaires) excite more ganglion cells by the larger number of edges and consequently engage the brain more. Image processing computer software for image compression and automatic object recognition use a similar process.

Glare has much to do with light-dark transitions. Chapter 4 (Sect. 4.​4) will discuss recent studies that take the centre-surround processing of receptive fields by ganglion cells, as a fundamental-physiological basis for the development of glare models.

The opponent mechanism of ON-OFF signal processing in ganglion cells is also typical for colour processing of visual information further down the visual pathway in the brain itself. It is discussed in the next section.

1.6 Colour Vision

The fact that the fovea of the retina contains three different types of cones, each with a different sensitivity in the short (S), medium (M) and long (L) wavelength range, makes that we can see colours. The spectral sensitivities have been shown in Fig. 1.5. The three-dimensional character of colour vision is called the trichromatic theory. It is also referred to as the Young-Helmholtz theory after the nineteenth-century developers of the theory. Young and Helmholtz demonstrated that each colour can be produced by mixing different amounts of light of the three primary colours: red, green and blue (RGB). With two colours this is impossible, while more than three colours are not needed. Mixing different colours of light is called additive colour mixing, this in contrast with mixing paint that is called subtractive colour mixing. With additive mixing, the result is brighter than the individual components and white can be obtained (Fig. 1.11, left). With subtractive mixing, the result is darker than the individual components (the paints absorb light) and eventually black is obtained (Fig. 1.11, right).

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Fig. 1.11

Left: Additive mixing of primary colours (RGB) gives all the complementary colours and white. Right: Subtractive mixing of the complementary colours gives the primary colours (but darker) and black

As has been discussed, each cone in the fovea connects to its own ganglion cell in the retina. Thus, contrary to rods, cones do not converge in the retina. They do, however, converge on other type of ganglion cells further down the visual pathway at a kind of substation located in the central part of the brain (the thalamus). This substation is called lateral geniculate nucleus (LGN). Like the ganglion cells in the retina, these so-called LGN-ganglion cells have a receptive field in the retina. This field consists of cone cells only. De Valois et al. (1966) were the first to map, with microelectrodes, the connection between LGN-ganglion cells in the brain and cones in the fovea. An LGN-ganglion cell processes the signals from the cones with a similar opponent ON-OFF mechanism as the retinal ganglion cells discussed in the previous section. Retinal ganglion cells process opposing brightness signals arriving from the rods in the receptive field (light on and light off, respectively). LGN-ganglion cells process opposing colour signals arriving from the cones in the receptive field. Already in the late nineteenth century, Hering (1878) suggested that colour vision is based on two pairs of opposing colours: blue and yellow on the one side and red and green on the other side. This suggestion came from the observation that the colours bluish-yellow and reddish-green do not exist, while bluish-green (purple) and reddish-yellow (orange) do exist. The fact that a person who is colour-blind for blue is also colour-blind for yellow and that the same holds for colour-blindness to red and green also supported Hering’s suggestion. The LGN-ganglion cells indeed process the signals from the cones by comparing the opposing colours blue and yellow and red and green, respectively (Hubel 1995). Figure 1.12 illustrates the mechanism. Yellow is available for processing by adding the signals from red-sensitive cones (L-type) and green-sensitive cones (M-type). A third class exists, namely that of opposing black and white in which the three signals of the red, green and blue sensitive cones are added. As in the case of the ON-centre and OFF-centre retinal ganglion cells, each opposing colour class also has two types of LGN-ganglion cells.

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Fig. 1.12

The opponent colour processing mechanism of LGN-ganglion cells receiving signals from cones. Light of the right colour in the centre (+) of the receptive field increases the voltage firing rate of the ganglion cell, while light of the right colour in the surrounds (−) decreases the voltage firing rate

Light of the right colour in the centre of the receptive field (+) increases the output of the cell (increases the rate of voltage spikes). Light of the right colour in the surrounds of the receptive field (−) decreases (inhibits) the output signal (reduces the rate of spikes).

Quite some physiological aspects of the colour opponent mechanism are not yet known (Conway 2014; Wuerger and Xiao 2016). Important discoveries are being made on further colour processing mechanisms in the visual cortex itself (Johnson et al. 2008; Harada et al. 2009; Wandell and Chichilnisky 2012; Mély and Serre 2017).

1.7 Pupillary Reflex

The section about photoreceptors mentioned that the photochemical bleaching and regeneration process taking place in the pigments of cones and rods is responsible for the possibility of the eye to adapt to a wide range of dark and light. Because of adaptation, vision is possible over a gigantic range, from a moonlit scene (with less than 0.1 lux) to a sunlit one (with even more than 100,000 lux). The relatively slow photochemical change in photoreceptors is not the only process that is responsible for adaptation. Fast neural changes account for the first few seconds of adaptation. Pupil size change in dependence on the amount of light (the pupillary reflex) also plays a role in the adaptation process, although only for a relatively small amount. The changing size of the pupil has, however, a significant influence on the quality of the retinal image, especially regarding the depth of field. Until recently, it was thought that it is the rods that control the pupil size. One of the reasons for this speculation was that the pupil does not change in a restricted field of view situation when rods, being located in the periphery of the retina, cannot play a role.

As recent as 2002, a novel photoreceptor type has been discovered in the retina of the eye (Berson et al. 2002). These cell types appear to have a more dominant role in changing the pupil size than rods. Surprisingly, these cells are ganglion cells. Ganglion cells are, just like rods, located in the periphery of the retina, outside the fovea. Of the circa 1 million ganglion cells of the eye, some 1–5% appears to be photosensitive (Lucas et al. 2013). These photosensitive ganglion cells have, contrary to cones and rods, no direct contact with the pigment epithelium layer of the retina. They contain their own opsin photopigment type, melanopsin, which makes them intrinsically photosensitive (Provencio et al. 2000; Lucas et al. 2003). They are called photosensitive retinal ganglion cells, pRGCs. These novel cells have no direct function for visual image forming. They have pathways to areas of the brain, different from the visual cortex. They are essential for synchronising circadian rhythms of the body and therefore of great significance for lighting effects on health. Part II of this book will discuss in detail about the subject light and health and the role of pRGCs.

It is now known that a particular subtype of pRGC cells have a pathway towards an area in the brain (called OPN) known to play a role in driving the pupillary reflex (Lucas et al. 2001; Berson 2003; Chen et al. 2011; Güler et al. 2008; Takahashi et al. 2011). Rods, cones and pRGCs all contribute to controlling the pupil size (McDougal and Gamlin 2010; CIE 2015). Their relative contributions are not constant but change with, among other things, light level, light duration and light spectrum. Research on the interplay of cones, rods and pRGCs in driving the pupillary reflex is ongoing. Of course, the change of pupil size affects the amount of light reaching the retina and the field of depth of observers. Therefore, pRGCs play, apart from their essential role in the relationship between light and health, also a role in the relationship between light and vision.

1.8 High-Level Vision

How we experience a scene is not solely dependent on the physical retinal image. Memory and experience also play a role. The photograph of a canyon, shown in Fig. 1.13, illustrates this. Because we have learned, unconsciously, that outdoors shadows are always cast by light coming from above, we see a river in the bottom of a canyon. After turning the photograph upside down, the brain explains the bright and dark patterns so that it seems that shadows are cast again by light from above. The upside-down photograph, wrongly, is seen as that of a mountain ridge. Objects or complete scenes have cues that, through experience, help us not only to detect but also to recognise them properly. Vision which includes cognitive processes that incorporate experience with objects, materials and scenes is called high-level vision. This is in contrast with vision that only relates to the retinal image (Kosslyn 1987; Adelson 2000; Cox 2014).

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Fig. 1.13

Canyon changes into mountain ridge by turning photograph upside down

1.8.1 Perceptual Constancy

High-level vision is responsible for the phenomenon of perceptual constancy. This term is used for the ability of the visual system to perceive stable lightness and colour of objects and scenes under widely different lighting conditions. The most commonly used example is that of black coal, which continues to be experienced as black when the light level changes from very low to extremely high. We experience a piece of white paper that receives so little light that its luminance is lower than that of brightly lit black coal adjacent to it, still as white. A painted wall with uneven lighting is experienced as a wall of uniform colour, not as a wall that is irregularly painted with different tints of paint. The lightness property of the wall, like the blackness property of the coal, remains constant. High-level vision neglects here, for an important part, the illumination, but appraises the reflective property of the wall. The same holds for the piece of crumpled paper shown in Fig. 1.14 experienced as the same white, notwithstanding the dark shadows on the paper.

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Fig. 1.14

Crumpled paper experienced as having the same lightness, in spite of the shadows with lower illuminance and luminance values

Lightness is one of the fundamental attributes of objects, which possess perceptual constancy. Colour is another object attribute that knows perceptual constancy. Here chromatic adaptation plays a major role in how we perceive colours. Chromatic adaptation keeps colours of a scene partially constant under changing colour and amount of the illumination. Experience may help, additionally, with colour constancy. A banana lit with a bad colour-rendering lamp may not look appetising, but we easily recognise it as a yellow banana.

1.8.2 Maintaining Constancy

Lighting can help secure visual constancy or can break it (Lynes 1971; Coaton and Marsden 1997; Cuttle 2008; Boyce 2014). The example of the canyon picture of Fig. 1.13 has already shown that unusual sharp shadows can break perceptual constancy. Many optical illusions are based on breaking perceptual constancy. Stage lighting is sometimes designed on purpose to break perceptual constancy to create drama. The bad man effect obtained with strong lighting from below the actor’s face is an example of this. Similarly, in display lighting, breaking perceptual constancy may be used to attract attention. Of course, in daily life, it is important to maintain perceptual constancy. Cuttle (2008) describes this importance aptly: Our lives would become chaotic if objects changed from black to grey to white when carried from shade to full light.

Lynes (1971, 1994) gives recommendations for lighting a scene to help maintain perceptual constancy:

Adequate illuminance, also on the surroundings of the object or scene being viewed

Good colour rendering

Limit glare

Obvious, but not necessarily visible, light sources

Avoid sharp shadows

Reveal surface textures

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© Springer Nature Switzerland AG 2019

W. van BommelInterior Lightinghttps://doi.org/10.1007/978-3-030-17195-7_2

2. Colour

Wout van Bommel¹ 

(1)

Van Bommel Lighting Consultant, Nuenen, Noord-Brabant, The Netherlands

Abstract

Solid-state light sources offer far more possibilities to engineer lamp spectra to suit different colour quality requirements than gas discharge lamps did. Accurate lamp colour specification based on perceived colour has therefore received renewed attention. This concerns in the first place the specification of different types of white light sources. Coloured LEDs are more and more used in interior spaces so that also an accurate specification of coloured light sources is needed. For the specification of chromaticity coordinates of light sources, the CIE x–y chromaticity diagram (CIE colour triangle) is the basis. It is based on the standard CIE 1931 colorimetric observer, defined with colour-matching functions. Correlated colour temperatures of light sources, as a characterisation of the tint of whiteness, are easily obtained from the x–y chromaticity coordinates. MacAdam ellipses, in a more uniform u′–v′ chromaticity diagram, are the basis for the binning process in the LED manufacturing process.

A wealth of new research on colour science is available as a basis to replace some colour concepts that have been developed between the 1930s and 1960s. New uniform three-dimensional colour spaces have been introduced. The CIECAM02-UCS colour space is proposed as a basis for a novel two-metric colour-rendering system with a fidelity index R f and a gamut index R g . Here, R g is a measure of colour saturation.

Vector graphics visualise the colour properties of light sources. They represent an indispensable new tool for the lighting designer in the LED era.

2.1 Perceived Colour

Perceived colour is a subjective result of a physical colour stimulus in the form of direct light or light reflected from a surface. The stimulus and the observer condition together determine how colour is perceived. The stimulus is characterised by the spectrum and quantity of light and the size, shape and surroundings of the stimulus. The observer condition concerns especially the adaptation state of the observer. The colour stimulus of surfaces of objects is determined by the light reflected from its surface which, in turn, depends on the spectral composition of the surface and the spectral properties of the light incident on the surface. Perceived colour thus changes with the light incident on the surface. As a consequence, an object surface does not have one real colour.

Three attributes can describe perceived colour: hue, saturation (or chroma) and lightness (or brightness in the case of light source colours). Figure 2.1 visualises these attributes for the colours green and red arranged according to the Munsell system for specifying surface colours (Munsell 1929). Munsell arranged the different hues as the pages of a book (e.g. 5G green, 5R red and 5YR yellow-reddish). Hue refers to the monochromatic spectral colours, red, yellow, green, blue and combination of adjacent pairs of these colours (including blue-red, i.e. purple). Saturation or chroma refers to the colourfulness or vividness of the actual hue. Lightness or brightness relates to the total amount of light of the colour stimulus. All colours meet at the axis of the Munsell book where they are perceived as tints ranging from black to grey to white. By gradually reducing the brightness of a colour stimulus, finally all colours appear black: a red colour via reddish brown, an orange colour via brown, yellow via yellowish brown, green via olive green and white via grey.

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Fig. 2.1

Visualisation of the three attributes of perceived colour: hue, saturation and lightness. Adapted from the Munsell Book of Colour (Munsell 1929). Munsell used the term chroma for saturation and value for lightness

The influence of brightness on perceived colour is substantial when the brightness of the colour stimulus changes while the remaining parts of the visual scene do not change. When the brightness of the whole scene together with the colour stimulus changes, the effect on the perceived colour is smaller. This is because of perceptual-colour constancy as discussed in Sect. 1.​8.​1 of Chap. 1.

2.2 Colour Specification

2.2.1 CIE x–y Chromaticity Diagram

2.2.1.1 Chromaticity Coordinates

In 1931 CIE defined a numerical system for the definition of colours (CIE 1932; Schanda 2007; ISO/CIE 2019b). The system allows for the calculation of colour points, called chromaticity coordinates x and y, from the spectrum of a colour stimulus (a light source or light reflected from a coloured surface). The x–y chromaticity coordinates define the position of the colour stimulus in a rectangular colour diagram or colour triangle (Fig. 2.2). The x–y coordinates correspond to the values of the horizontal and vertical axis of the triangle, respectively.

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Fig. 2.2

CIE x–y chromaticity diagram with the spectral wavelengths on the spectral locus. Correlated colour temperature lines are indicated on the blackbody locus. E: equal-energy white point; D50 and D65: daylight chromaticity points; R, G and B: CIE colour-matching stimuli. Chromaticity coordinates of a point on a line (example: P) can be obtained by a mixture of two different spectra with chromaticity points lying on the line on either side of that point (e.g. points A and C or A and F). Point D indicates the dominant wavelength of point P

The monochromatic spectral colours (indicated with the corresponding wavelengths) are located on the outer border of the colour triangle. It is called the spectrum locus. Here the colours are most saturated. Purples are no spectral colours but mixtures of red and blue or violet. Chromaticity coordinates falling in this area of the diagram can therefore not be characterised by a spectral wavelength. By moving inwards to the centre of the diagram, the colours become less saturated until in the centre of the triangle white is obtained. It is the point where all spectral colours, each with the same energy, are mixed. It is called the equal-energy white point (point E in the diagram). The chromaticity points of two standardised types of daylight sky, D50 and D65, are also shown. The CIE x–y colour triangle has been developed so that the different hues occupy large enough areas and the white point is located rather centrally in the diagram.

The x–y chromaticity point of a mixture of light of two different spectra lies on the line connecting the chromaticity points of those two spectra. In the example shown in Fig. 2.2, the spectrum corresponding to colour point P can be obtained by a mixture of spectra corresponding to chromaticity points A and C. The relative quantities (luminances) of the two components A and C determine the exact location of point P on the line. The same colour point P can also be obtained by mixing, in the appropriate quantities, the spectra corresponding to other points on the line, for example, points A and F. Different lines can be drawn through point P giving more mixtures resulting in the same colour point. Thus, many different mixtures of spectra can give the same chromatic coordinates.

The curve through the centre of the colour triangle is the blackbody locus (also referred to as Planckian locus). Section 2.3 discusses the blackbody locus and its practical meaning. Section 2.4 deals with the dominant wavelength, represented by point D.

2.2.1.2 Standard Colorimetric Observer

The chromaticity diagram is based on the trichromatic theory, the basic law of colour vision already dealt with in Chap. 1, Sect. 1.​6. This law states that each colour can be produced by mixing different amounts of light of three primary colours: red (R), green (G) and blue (B). The CIE system is based on colour-matching tests done by observers in the late 20s of the last century (Wright 1928–29; Guild 1931). From these tests, CIE defined a so-called standard colorimetric observer. In the tests, observers were asked to match the colour of two test stimuli. Figure 2.3 shows the principle for the example of a light-blue test stimulus.

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Fig. 2.3

Principle of colour-matching tests with red (R), green (G) and blue (B) primaries.