Outdoor Lighting Guide

Institution of Lighting Engineers

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781134200238
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

As concern grows over environmental issues and light pollution, this booksatisfies a need for a straightforward and accessible guide to the use, design and installation of outdoor lighting.

Thisall-inclusive guide to exterior lighting from the Institution of Lighting Engineers, recognized as the pre-eminent professional source in the UK for authoritative guidance on exterior lighting, provides a comprehensive source of information and advice on all forms of exterior lighting, from floodlighting, buildings and road lighting to elaborate Christmas decorations. Useful to practitioners and non-experts alike, specialists will value the dependable detail on standards and related design, installation and maintenance problems, whilstgeneral professionalscan find extensive practical guidance on safety issues, the lighting of hazardous areas and avoiding potential difficulties.

Chapter 1

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Visual effects of lighting

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1.1 Introduction

Lighting has three primary functions:

1 To improve visual performance;

2 To improve safety; and

3 To improve the visual environment.

Each of these functions is equally applicable to both indoor and outdoor environment, however; the outdoor environment is a very different place from the indoor environment. It is subject to more extreme environmental conditions and human behaviour. Whilst there is some overlap of task there is a complete overlap of lighting principles and visual effects.

1.2 Light

1.2.1 Lighting levels

The human eye perceives objects by the light that is emitted or reflected by them. With the exception of light sources, the light reaching an observer (the luminance of the object) is the reflected light and is dependent on the light incident on the object (the illuminance), the reflective properties of the object and the position of the observer with relation to the object. These variables give a very large number of possible luminance requirements.

1.2.2 Task performance

The ability to perform a visual task is influenced by the size of the task, the contrast and the vision of the viewer. Whereas a difficult task cannot be made into an easy task, increasing the illuminance generally improves visual performance for a specified task. However, saturation occurs, and beyond a certain value any further increase is superfluous and results in an unnecessary use of energy. The point at which saturation occurs will be higher the more difficult the task e.g. tasks involving very small objects or those carried out at high speed. Saturation is illustrated in Figure 1.1.

Figure 1.1 Relationship of relative visual performance with retinal illuminance in trolands and contrast, for task sizes of 15 µsr (a) and 4.8 µsr (b).

1.2.3 Appearance

In a limited number of outdoor locations the appearance of the lighting will be more important than the task performed. Indeed the actual task may be viewing the appearance of a lit object or the lit environment. Examples are the illumination of buildings and structures and the lighting of town squares. The lighting here is intended to create mood, interpret architecture or give visual stimulation. The art of good lighting becomes as important as the science, and designers have to take account of colour, form, texture and perception.

1.3 Flux, intensity, illuminance, luminance and brightness

Flux is the total quantity of light that a source (e.g. a lamp) emits. It is measured in lumens and is the starting point for general lighting calculations.

The quantity of light emitted in a specified direction is the luminous intensity or simply the intensity of the light in that direction. The existence of luminous intensity diagrams or tables for luminaires allows detailed lighting calculations to be carried out. It is measured in candelas, which are lumens per unit solid angle in the specified direction.

Illuminance is the magnitude of light incident on a surface. It cannot be seen because it has not reached the eye yet. Illuminance is the objective quantity that is most commonly used to specify lighting levels because in most applications it is not possible to specify the position of the observers, the lit objects and the light sources with sufficient accuracy to use luminance, or there are multiple observer positions. Illuminance is measured in lux, which are lumens incident on a point per area of the point.

Figure 1.2 Flux, intensity, illuminance and luminance.

However, lighting practitioners think in terms of luminance, contrast and glare. The amount of light that reaches the eye by reflection or by direct emission from a light source is called luminance. The light reflected from any surface is dependent on the quantity of illuminance, the reflective properties of the surface and the position of the observer with relation to the surface. Luminance is measured in candelas per square metre, which is the luminous intensity per area of the solid angle of the object when viewed by the observer.

Figure 1.2 illustrates all the four terms. Brightness is the subjective response created by the brain's interpretation of what the eye sees.

1.4 Glare

Glare occurs when one part of the visual scene is much brighter than the remainder. The most common causes of glare are inappropriate orientation of luminaires, and the poor selection of luminaire and mounting height combination. In a road environment, dipped vehicle headlamps can cause substantial glare even if the road is well lit. Glare impairs vision, causes discomfort and reduces task performance.

1.5 Positive and negative contrast

Contrast is the assessment of the difference in appearance of two or more parts of a field seen simultaneously or successively. It is the key to vision: if there is no contrast between an object and its background then the object will not be detected. The luminance contrast of an object is

where L₂ is the luminance of the object; and L₁ is the luminance of the background.

Where the object is brighter than the background, there is a positive contrast and the object is seen by direct vision. And where the object is darker than the background, there is a negative contrast and the object is seen by silhouette vision.

1.6 Absorption and reflection

Any light falling on to a surface that is not reflected is either absorbed or transmitted through the object.

If the material does not transmit light, all non-reflected light disappears into the surface and is converted into heat. This is called absorption. The amount of absorption varies according to the angle of incidence, the colour of the light and the physical characteristics (colour, texture, density) of the material. Generally, for higher angles of incidence more light will be reflected.

The colour of a surface is dependent on the light reflected from it, for example a blue surface will reflect incident light in the blue wavelengths of the spectrum and absorb light with other wavelengths.

1.7 Radiation

Light forms part of a complex of physical phenomena included under the heading ‘electromagnetic radiation’. It is therefore closely related to, for example, radio and TV signals, infrared (IR) and ultraviolet (UV) radiations, X-rays and other radiations. These emissions occur at different wavelengths. The major difference between light and these other phenomena is that humans and animals use a collection of the wavelengths to ‘see’ and this is called the visible spectrum. Some animals also use wavelengths in the IR or UV ranges to extend their range of vision.

‘White’ light is a collection of different wavelengths between approximately 380 and 780 nm, which in combination are perceived as white. Most lamps that emit a ‘white’ light do not emit a continuous spectrum of wavelengths but a series of wavelengths of different amplitude. Figure 1.3 shows the relationship between the visible portion of electromagnetic spectrum i.e. visible light and the non visible portions of the spectrum. Figure 1.4 shows the range and proportions of the emission spectrum of a typical fluorescent lamp.

Figure 1.3 Visible portion of electromagnetic spectrum. (see Colour Plate 1)

1.8 Apparent colour

Colour temperature describes how a lamp appears when lit. It is the temperature of a black body radiator that emits radiation of the same chromaticity as the lamp being considered. For complete accuracy, the chromaticity must be on the black body (full radiator) locus, the power radiation curve of a black body.

As very few lamps have chromaticity on the locus, the more useful correlated colour temperature (CCT) is used. It is based on similar chromaticity to a blac...