The first objective should be to make the best possible use of daylight. External obstructions, heavy net curtains or curtains which, when open, fall within the boundary of the window, can all cause lights to be switched on sooner than necessary. It is often the case that the desire for privacy conflicts with energy-saving imperatives.
Light coloured internal surfaces maximize light reflection. The obsession of the architects of the Modern Movement with light led to the fashion for white, uncluttered interior walls. This reduced dependency on artificial light.
Internal spaces like some bathrooms obviously need artificial light and the same is frequently true of hallways and staircases. One answer is the light pipe. This consists of a tube which is mirrored on its inner surface. A domed rooflight is fitted over the top of the tube and a light diffuser at the bottom. Daylight or sunlight are guided down the tube, reflected by the mirrored surface. Such a device transmits a surprising amount of illumination (Figure 9.1).
Artificial lighting in homes is often inefficient and wasteful. Aesthetic considerations tend to outweigh practicality and economy. For example, we often perform close-up tasks, such as reading or sewing, using the overall light in a room rather than employing appropriate task lighting. This involves eye strain leading to subliminal stress resulting in headaches and ultimately the possibility of eyesight damage. So, this section considers:
• The design of a lighting layout;
• The appropriate types of lamp.
Design for efficiency and comfort
At the outset, it is worth describing the terms used in lighting. There are three basic measures:
Figure 9.1 Light pipe diagram.
• The output of a lamp in terms of its level of illumination is denoted by lumens (lm);
• The level of illumination required in a given space is defined as lux (lx);
• The performance of a particular lamp is described as its ‘lamp efficacy’. The greater the luminous efficacy the better the lamp at converting electricity into light.
Background illumination in a living room should be around 300 lx. A popular means of achieving this is by up-lighters which reflect off a white ceiling, giving a soft, overall light. Point light sources create glare, increasing the contrast between the light source and the background. The eye adjusts to the bright light by reducing the pupil size, making the surroundings even more gloomy. As an aside, it is worth pointing out that a television is a bright light source.
The area surrounding the TV should be at the recommended background level to avoid eye strain which is exacerbated by rapidly moving images. The same applies to computers. Wherever possible background light should be from a reflected source. For circulation spaces, the recommended level is 200 lx.
For task lighting the recommended level is at least 500 lx over an area defined by the task in question.
Types of lamp
Most households still rely on the traditional incandescent tungsten filament lamp. As the name suggests, it generates a great deal of heat. In fact it produces 95 per cent heat at 2000°C within the lamp and 5 per cent light. Consequently it has poor efficacy amounting to 13 lm/W. Life expectancy is around 1000 hours. A slight improvement is achieved by a variation of the type known as tungsten halogen lamps. These are compact units generating a bright point light which makes them popular for task lighting. Their efficacy is raised to 17 lm/W and they have an improved life expectancy of 2000-4000 hours, depending on the model.
For most situations, incandescent lamps should be replaced by compact fluorescent lamps (CFLs). These are a development from the conventional fluorescent tube. These tubes have been shrunk and coiled so they are only marginally larger than a tungsten bulb. They have electronic ballasts which enables them to oscillate at 20 000 cycles per second – way beyond the power of the eye to detect. The old fluorescents oscillated at 50 cycles per second, which made them a major culprit for inducing ‘sick building syndrome’, probably due to the fact that the flicker rate was subliminally perceived. A 15 W CFL is equivalent to a 60 W incandescent bulb. The spectral output is similar whilst it uses only 25 per cent of the energy. Another bonus is that a CFL lasts around 7500 hours.
It is also possible to use dimming controls on CFLs, whereby the electrical consumption is directly proportional to the intensity of light.
CFLs are more expensive than tungsten bulbs, but this is soon offset by the energy savings and longer life. These gains are more marginal for spaces that are used intermittently for short periods, such as store rooms and bathrooms.
Most efficient of all is the latest fluorescent tube called the T5. It is a 15-mm diameter tube with a luminous efficacy of 106 lm/W.
Semi-conductor technology could be about to invade the sphere of lighting in the form of the light emitting diode (LED). The principle is that a small electrical current applied to a diode made from a semi conductor material raises electrons to a higher energy state. The result is the production of photons (particles of light) in the visible spectrum. Up to the present it has not been possible to produce pure white light from an LED. When this breakthrough occurs we can expect a revolution in lighting. LEDs produce 100 lm/W and have a life span of up to 100 000 hours. Furthermore, they are a fraction of the size of existing lamps and will be available in a range of colours.
• Make maximum use of natural light with light coloured wall and ceiling surfaces.
• Light pipes are a useful means of directing daylight to internal spaces, such as bathrooms and toilets.
• With artificial lighting it is important to create the right balance between background and task lighting.
• Glare induces stress.
• For background lighting an indirect source, e.g. reflected off the ceiling, avoids glare.
• In most situations compact fluorescent lamps (CFLs) should replace incandescent bulbs.