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The design of public space in urban centres in the UK and Europe has dramatically improved over the last decade, largely due to a growing appreciation of the crucial role that good design can play in the development of successful and sustainable communities, particularly as part of regeneration projects.

Lighting adds a further qualitative layer to new and creative approaches to urban design, not simply as a cosmetic or graphic addition, but as a vital part of the scheme.

This growing discipline is used at many scales - from large strategic plans to small interventions - and is beginning to change the face of our cities and towns after dark, providing lighting solutions as a clear contextual response to programme and site. A small revolution has been occurring in the design of the 'nightscape'.

Lighting is increasingly playing a key role in the success of many urban spaces, making them as vital and active after dark as they are by day, particularly where the 24-hour economy is well developed.

The pace of technological change and an increased interest in the field of urban lighting have led to an improved understanding of the opportunities and challenges that face the designer when considering the illumination of the public realm. While natural light is important to the character of any public space - with changing patterns of sunlight not only creating contrast and texture, but also revealing form and surface - the requirement to introduce artificial lighting brings new possibilities, offering the chance to 'reinterpret space'.

The design of artificial lighting can create dynamic layers of contrast, texture and colour on a temporary or permanent basis; it can make space more legible and reinforce the formal and informal relationships of a scheme, which might be less evident by day. Recent responses range from the highly subtle to the overtly theatrical.

Good lighting is not simply a visual question of providing the exactly right quality or quantity of light; it is a complex design activity requiring the correct balance between a wide range of often conicting criteria, which might include access for people with special needs; safety and security (including integration with CCTV); the control of energy use; and the reduction of light pollution.

The final lighting solution is determined by satisfying the various criteria while retaining a creative approach; its ultimate success lies in the final selection of the light sources, fittings and control systems.

THE LIGHT SOURCE The specification of light in terms of quality (colour appearance and colour rendering) and quantity (luminous ux) is determined by the selection of the light source itself: the lamp. The specifier faces a range of choices of light sources that may be employed in the external environment, all of which are available in a wide range of wattages and forms.

Ceramic metal-halide lamps are now often used for roadway, parking and pedestrian lighting, as well as the lighting of landscape features and building facades.

Their whiter appearance and accurate colour-rendering properties make them more appropriate for use in publicrealm projects than the more traditionally employed highpressure or low-pressure sodium products, even though they are less efficient.

Compact and linear fluorescent lamps can also be considered for various applications. Their range of colour temperatures and good colour-rendering properties, combined with their energy efficiency, makes them ideal sources, as long as their intensity can be carefully controlled.

Light emitting diodes (LEDs) are increasingly popular for outdoor use because of their relatively low light output with high visual intensity. While their advantages of long life and low energy use are well known, they are vulnerable to extreme changes in temperature and are only as good as the base quality to which they are supplied. Like many solid-state technologies, the quality of LEDs varies widely depending on their origin. White LEDs also often have problems with colour stability.

THE LIGHT FITTING The light fitting (also known as the 'optical instrument' or luminaire) holds the light source and controls, directs and distributes the light in the required manner. While light fittings come in a wide range of shapes and sizes, it is their optical design that dictates how the light is delivered.

Internal reflectors control the direction of the light and the 'shape' of its distribution; they also control glare, which is one of the main problems in designing outdoor schemes.

During the day, the appearance of the light fittings is critical to the character of the space and needs to be attractive and well integrated into the language of the design.

However, light fittings should not be selected for their aesthetic appearance alone; the way in which they deliver the lighting package must be considered. Light fittings that look good and perform well are becoming easier to find, but usually come at a high price.

CONTROL SYSTEMS How the lighting is to be controlled must be considered.

Traditionally, street lighting has been switched on and off by local photocells, often mounted on the fittings, which ensure that lighting is available from dusk until dawn. Automated switching via simple programmable time-clocks may be provided to ensure that more decorative or specialist applications, such as the lighting of landscape features or building facades, switch off at the point at which schemes become less populated. This has the additional advantage of reducing energy consumption, conserving lamp life and, in some cases, improving residential amenity.

DESIGN ISSUES There is also a range of design issues to be considered:

Scale. Like most aspects of design, lighting has scale; the difference with lighting is that its scale is both tangible (the light source and fitting) and intangible (the light it produces). A considered hierarchy of these two elements can contribute to the perceived scale and character of any space and also give importance to individual elements within the streetscape.

Glare. The uncomfortable brightness of a light source when viewed against a dark background can cause a reduction in visual performance, including eye irritation. Glare can be kept to a minimum by specifying light fittings with an accurate cut-off angle that can be focused on to a specific area or object.

Protection. The degree of protection of any light fitting can be found by using a simple and widely recognised rating system that defines the ingress characteristics of a fitting in respect of both dust and water.

Quality. The build quality of lighting equipment is essential to the durability and robustness of the scheme. Lighting is no different from any other aspect of urban design in that the specification of high-quality equipment considered fit for purpose may be expensive.

Vandalism. While no light fitting is truly vandal proof, a range of measures (such as the use of protective cowls, louvres, grilles and the careful detailing of fixings) can at least help overcome the problem of basic day-to-day abuse.

Installation. The ease of installation not only saves time and money on site, but also often means that the final lighting detail will be integrated with the final scheme.

Cleaning and maintenance.

Any good lighting scheme must be cost effective and easy to run. Future maintenance must be considered at all stages of the design and include a clear and consistent strategy for the replacement and cleaning of lamps, ballasts, accessories and spare parts.

Energy use. The conservation of energy is becoming an increasingly critical issue in the design of exterior lighting, and the designer is therefore now required to balance the need for efficiency with improved quality.

Light pollution. This is an increasing problem within our cities and towns. Not only does it contribute to sky glow, which conceals our natural view of the sky, but it also creates a nuisance through light trespass and damage to local ecologies.

Integration. The proper detailing of the lighting scheme, including its associated electrical infrastructure, is essential to a successful outcome. The consideration of lighting should not be left too late in the design process, but included from the outset.

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