Light and materials

The interaction of light with materials depends on the wavelength of the radiation and the characteristics of the material. Gases and liquids will allow some light to pass unchanged (although at a slower speed), but will absorb or scatter other wavelengths. For example, the blue of the sky is a result of shorter wavelength blue light from the sun being scattered in the atmosphere and so reaching us from a wider angle. Solid materials will absorb, reflect or transmit light:

• * Absorption – occurs where the frequency of the light photons is similar to the natural frequency of the electrons in the material. Absorption raises the temperature of the material (see chapter 1: Solar gain p. 31).
• * Reflection – occurs where the frequencies of the light photons and electrons are dissimilar. Reflectance is the proportion of incident light which a surface returns: 1 represents complete reflection, 0 complete absorption.
• * Transmission – occurs when the atoms that make up the material contain electrons that do not interact with the photons. Photons from the visible part of the spectrum (wavelengths of 380–780 nm) do not interact with electrons of glass molecules, and therefore are transmitted through the material. However, photons from the ultraviolet part of the spectrum (wavelengths of 10–380 nm) can interact, and are absorbed by standard glass.

Light from a source commonly consists of a range of wavelengths; when it strikes a surface some wavelengths will be absorbed and some reflected (or transmitted). It is only the reflected wavelengths which reach the human eye and are recognised as colour. For example, an object which reflects green light but absorbs all other light will appear green.

The sun

The sun is the main natural source of light on earth: it emits a wide spectrum of EMR, some of which is absorbed by the atmosphere, so that at the earth’s surface the radiation is composed of 53% infrared, 44% visible light, and 3% ultraviolet. Because the earth rotates around the sun and rotates on its own axis (which is 23.4º from vertical) the angle of the earth to the sun changes constantly, resulting in variations in the length of day, the angle of the sun in the sky and the intensity of solar radiation.

The change in day length depends on latitude: at the equator there is no significant variation, but the nearer a site is to the poles, the greater the variation in day length between winter and summer, until at the Arctic and Antarctic circles there is perpetual daylight during the summer and no daylight during the winter. Figure 5-02 shows the annual variation in daylight hours for three locations.

The change in the length of day is linked to the position of the sun in the sky. Figure 5-03 shows the sun position through the year for latitude 55º north and midsummer and midwinter. In the summer, the sun rises and sets over a wider range, and is higher in the sky at noon, so solar radiation is more intense.

Figure 5-02 Variation in Daylight Hours through the Year for Three Locations

Figure 5-03 Seasonal Changes in the Sun’s Path For 55° North

Measuring light

In defining light there are two main characteristics to consider: the intensity (or brightness) and colour.

Intensity

The strength of a light source, its luminous intensity, is measured in candela (cd) (roughly equivalent to the light given out by a candle flame). Luminous intensity is the power of visible light emitted for a solid angle of one steradian (an angle which encompasses roughly 1/12.6 of the surface of a sphere). One candela is 1/683 watt per steradian. In practice the contribution of each wavelength is weighted according to the sensitivity of the eye to different wavelengths.

The total amount of visible light emitted by a source, the luminous flux, is measured in lumens (lm). A light source radiating 1 cd in all directions has a luminous flux of 12.6 lm, while a source radiating with the same intensity but in a hemisphere would have a luminous flux of 6.3 lm. The light received by a surface is expressed as the illuminance, measured in lux (lx), which has the units lumens/m².

The amount of light a surface receives from a source decreases as the distance between them increases (it is inversely proportional to the square of the distance between source and surface). As Figure 5-04 shows, a source producing an illuminance of 160 lux on a surface 1 m away will produce an illuminance of 40 lux on a surface 2 m away, and 10 lux on one 4 m away. That decrease affects the performance of all artificial light sources. (The same principle applies to light from the sun, but the earth’s distance from the sun is so great that any changes produced by variations in the earth’s orbit are negligible for our purposes.)

Figure 5-04 Light Intensity and Distance

Colour

An accurate description of colour has three components: the colour of the light emitted by sources; the colour of the incident surface; and the ‘rendering’ of colour (i.e. how accurately does the colour appear under a specific light source).

The majority of light sources emit light in a band of wavelengths, which is determined by the temperature of the source. Colder sources, such as candles and fires, produce predominantly red and orange light, while hotter source, such as the sun, produce bluer light.

The wavelengths of light produced by a particular source can be matched with those from an ideal light source (known as a black body radiator) at a given temperature. That gives the colour temperature of the light source. Figure 5-05 shows the colour temperatures for common light sources. The colour temperature can also be used to describe light which is not emitted by hot sources, such as light-emitting diodes (LEDs).

Figure 5-05 Typical Colour Temperatures of Light Sources

There have been numerous attempts to create systems consistently defining colours (something which even a quick glance at a paint chart shows can be difficult). Most systems define a ‘colour space’ (a mathematical attempt to describe the relationships between colours), then add a measure of the intensity of the colour (saturation) and sometimes its lightness. The main systems in use today are:

• * Munsell – considers hue, based on the dominant part of the spectrum (red, yellow, green, blue, purple), chroma (the strength of the colour from neutral grey (0) to saturation), and value, between perfect black (0) and perfect white (10).
• * NCS (Natural Colour System) – uses a colour space with white, black, red, yellow, green and blue, together with an expression of colour intensity (chromaticness) and a black–white axis.
• * DIN – uses a colour space defined by hue, saturation and darkness.
• * RAL – a system of 1688 colours defined on hue, lightness and chroma.
• * BS 5252 – a system originally designed for coordinating colours within buildings. It comprises 237 surface colours, defined with a hue (numbered 00 to 24), greyness (letters A to E) and weight (an additional number). So 00 A 01 is a colour described as ash grey/oyster. BS 4800 contains a table which aligns BS, NCS and Munsell.
• * CIE L*a*b* – uses three parameters: L* refers to lightness, while a* and b* denote colour. The system is designed to enable differences between colours to be expressed, as well as describing colours.

The final aspect of colour is colour rendering: how a colour appears under a light source and whether colours ‘look right’. The commonest measure is the colour rendering index (CRI), which compares the appearance of standard colour samples illuminated by the light source with their appearance illuminated by a standard light source. The maximum CRI is 100, which indicates a light source identical to standardised daylight (see: Colour rendering p. 127).

Physical health

Ultraviolet (UV) light has higher energy levels than visible light (see above: The sun and Figure 5-01 p. 120) and affects the skin and eyes. Light in the UV part of the spectrum encompasses a range of wavelengths that have different physical effects, and is therefore categorised by type; the best known being UVA (315–380 nm) and UVB (280–315 nm). In animals, moderate exposure to UVA increases the production of melanin skin pigment, which protects against UVA and UVB, but high exposure to UVA can result in melanoma (skin cancer).

UVB exposure is vital for humans to maintain healthy levels of vitamin D; however, overexposure to UVB causes sunburn and direct damage to DNA, leading to skin cancer, and can also damage the eyes.

Less well known is UVC, which is also harmful, but the UVC wavelengths of sunlight are filtered out by the earth’s atmosphere, so there is no natural exposure.

Vision

The primary human perception of light is through the eye, with stimuli relayed to the brain for processing. The retina at the back of the eye is covered with photoreceptors – light sensitive cells known as rods and cones. Rods are sensitive to the presence of light and primarily detect motion, while cones are sensitive to colour. When light levels are low, vision is provided mainly by rods, but at the higher levels required in buildings the cones predominate. Roughly 5% of people are colour blind in the red/green region, which should be taken into account when designing colour schemes.

Physiological effects of light

A third type of photoreceptor, ganglion cells, send signals of light and dark directly to the hypothalamus, the part of the brain that regulates many physiological functions. Those signals help to regulate the circadian systems which control sleep patterns, changes in body core temperature and some hormone secretion. Prolonged absence of daylight, particularly in the morning, disrupts the circadian systems, leading to poor sleep quality and depression. Low light levels in winter can result in seasonal affective disorder (SAD), which is characterised by depression, lack of energy and increased appetite.

Those physiological effects must be taken into consideration – especially for buildings which people are likely to occupy for long periods, such as hospitals and carehomes.

Psychological effects of light

Light can subtly affect mood and perception. We think of reds and oranges as warm and blue as cold: which is at odds with the physical reality (see above: Colour p. 123). We feel the difference in quality of light between a summer’s morning and a winter afternoon. This is probably the subtlest interface between physics and the human experience of buildings, and as such, the most difficult to codify.

Some of these psychological effects can be addressed by effective use of daylighting. The design of artificial lighting should also address these.

Principles of good lighting

Lighting conditions should enable the occupants of a building to carry out their activities safely and comfortably, taking into account their physiological and psychological needs. The lighting requirement will vary according to the function of a space and the tasks carried out within it.² Four key criteria for achieving good lighting conditions are: illuminance, modelling (the balance of direct and diffuse light), colour rendering and visual contrast of surfaces.

Illuminance

There must be sufficient light for the activities that are to be undertaken in the space. Table 5.1 gives broad guidelines on illuminance for common activities. Illuminance should be measured at the work plane which, for some applications, may well be vertical (e.g. warehouses or libraries). Areas where specific tasks are carried out may require more illumination than is needed for the whole space (e.g. a reception desk in a foyer), in which case task-specific lighting will be needed. The area immediately surrounding a task area (0.5 m beyond) should be illuminated as the final column of table 5.1. The background area (up to a further 3 m from the task area) should be illuminated to at least 1/3 of the immediate surrounding area.

Table 5.1 Recommended Illuminance Levels

Light should also be evenly distributed within a space. The distribution of light is affected by the amount of light reflected back from surfaces: the following values of reflectance are recommended:³

• * Ceilings: 0.7–0.9
• * Walls: 0.5–0.8
• * Floors: 0.2–0.4.

Differences in illuminance can be used to draw attention to features: for example, in display lighting a ratio of 5:1 between background and feature illuminance provides a definite distinction, while 30:1 provides a dramatic effect.

Too high a level of illuminance within a space can result in glare on surfaces, which is visually disturbing. Poorly positioned window and inadequately shaded luminaires may produce distracting reflections on computer.

Daylight

Daylight has a value beyond the illumination of spaces and of tasks. In a day-lit room the brightness varies with time, the colours are rendered well and the direction of lighting gives good three-dimensional modelling.⁵

Daylight has two components:

• * Direct sunlight, which falls on a surface
• * Skylight, the diffuse light produced by scattering of sunlight in the atmosphere

Direct sunlight is beneficial, but to avoid visual or thermal discomfort, it should not fall on people working, or on visual tasks. The orientation of the building and its openings should take account of the direction of sunlight at all seasons (see above: The sun p.121). Where occupants have a reasonable expectation of sunlight a room should receive sunlight for at least a quarter of all daylight hours (5% in winter). In north-facing rooms and dense urban settings the absence of sunlight is acceptable.

Skylight can provide good general illumination and the design of the building should ensure that there is not too great a contrast between illumination inside and outside. As a rule of thumb, daylight will penetrate a room for a distance equal to 2.5 times the window head height, which typically gives a penetration of about 6 m. A dual-aspect building can therefore have a 12 m deep plan and still rely on daylighting.

The extent of daylight penetration can be measured by the ‘no-sky line’, which is the line at which the sky cannot be seen from the height of the working plane (taken as 0.70 m in offices, 0.85 m in houses); this is illustrated in Figure 5-06.

Figure 5-06 Daylight Measurements

The amount of daylight a space will receive can be assessed using the average daylight factor (ADF), which expresses the illuminance on an internal surface as a percentage of a standard outside illuminance (see box: Calculating the average daylight factor p. 129). The ADF can be used as a rough guide to the need for artificial light:

• * ADF 5% or more – the room will be well lit; some artificial lighting may be required at the start and end of the day
• * ADF 2–5% – the room will have a predominantly day lit appearance, but is likely to require some artificial light during the day
• * ADF less than 2% – the room will be gloomy and will require artificial light for most of the day

In single-storey buildings, and on the top floors of multistorey buildings, rooflights can be used to provide daylight beyond the range possible for windows. It may also be possible to transmit daylight into the back of a room using a sun pipe – a reflective tube which has a rooflight (usually on a pitched roof) at the upper end and a ceiling-mounted diffuser at the lower end.

The combination of directional sunlight and skylight means that daylight provides good modelling of faces and objects. Daylight gives good colour rendering, although the balance of light in the spectrum will change on both a daily and seasonal basis.

Calculating the Average Daylight Factor

The ADF on the working plane of a room can be calculated using the equation:

Where:

T is the diffuse light transmittance of the glazing, including the effects of dirt

Aw is the glazed area of the window in m², net of the frame and glazing bars

θ is the angle of the visible sky (degrees), measured from the centre of the window, as shown in Figure 5-07

A is the total area of the ceiling, floor and walls, including windows, in m²

R is the area-weighted average reflectance of the interior surfaces. A figure of 0.5 can be used as a rough value for a room with a white ceiling and walls of average reflectance

Artificial light

Artificial lighting should provide a visual environment which is:

• * Safe – people can move around the space easily
• * Efficient – people can carry out their tasks properly and unhindered
• * Comfortable – lighting does not strain the eyes through glare, and does not form veiling reflections or flicker

Good lighting design can provide an inspiring and creative visual environment. The three principles outlined above (see: Principles of good lighting p.126) form a good starting point.

A lamp’s efficiency is expressed by its luminous efficacy, which is the amount of light emitted for each watt of electricity, measured in lumens per watt (lm/W) (see below: Light sources, p. 131 for typical values). The efficiency may also be quoted as a luminaire luminous efficacy, which takes account of the power absorbed by the luminaire’s circuits and the physical effect of the luminaire on light output from the lamp.

Illuminance

The amount of light provided by an individual luminaire will depend on its type and configuration, and the design of shades and reflectors. Photometric data sheets for luminaires contain intensity tables which give the intensity of light produced at a range of angles. The intensity values are used to calculate surface illuminance. While lighting calculations are now usually carried out using specialist software, the number of luminaires required for large installations can be estimated using the lumen method.

The Lumen Method

A rough calculation of the number of lamps required for a space, N, can be carried out using the following equation:

Where:

E is the illuminance level required

A is the area at the working plane, in m²

F is the average luminous flux from each lamp, in lumens

MF is the maintenance factor, allowing for the reduction in output caused by deterioration and dirt

UF is the utilisation factor, which is the proportion of light from the lamp which will illuminate the working plane. (The utilisation factor depends on the room configuration and the luminaire design.)

For example, an office measuring 20 m × 10 m is to be illuminated to a level of 500 lux, using luminaires containing lamps with an output of 15,000 lumens, a maintenance factor of 0.75 and utilisation factor of 0.5. The number of lamps required is:

or, 18 lamps.

Light sources

There are four main types of artificial light sources used in buildings:

• * Incandescent lamps – light is produced by passing an electrical current through a tungsten filament, which makes it glow. The majority of the radiation emitted is infrared. These lamps have a colour temperature of about 2700–3000 K, giving a warm, yellow light. Colour rendering is good. Luminous efficacy: 14–16 lm/W.
• * Halogen lamps – these are incandescent lamps with halogen gas (iodine or bromine) in the enclosure to extend the filament life. They are hotter than incandescent lamps, so produce light at a high colour temperature. Luminous efficacy: 20–35 lm/W.
• * Fluorescent lamps (including compact fluorescent lamps, CFLs) – an electric current is passed through mercury vapour to ionise the gas, producing photons which make the fluorescent (phosphor) coating on the glass glow. The colour temperature is in the range 2700–6500 K depending on the composition of the phosphor coating. Colour rendering also varies with coating: the CRI can be as low as 50, but daylight fluorescents can approach 100. Luminous efficacy: 50–100 lm/W.
• * LEDs – these are semi-conductor chips which emit light when a current is passed through them. They do not have a conventional colour temperature. Colour rendering is improving as the technology develops. They have a long life span (approximately 25,000 hours compared with 1000 for incandescent). Luminous efficacy: 50–90 lm/W.

Light in the bigger picture

Interactions with Light