Best Practice > Lighting
Building regulations require a certain percentage of dedicated low-energy lamps, and external lighting must have a minimum luminous efficacy of 40 lumens per watt.
These include the conventional “light bulb”, sometimes referred to as GLS (General Lighting Service). Other types include R80 and R100 reflector lamps and also halogen lamps. Light is produced by passing an electric current through a very thin filament of wire such that it glows. The filament is protected from the air (and thus combustion) by a vacuum or neutral gas in the glass envelope. However, over time, the filament will degrade and burn out, hence the frequent need to replace the lamps. The relative inefficiency of tungsten filament lamps is due to the fact that less than 10% of their energy consumption results in light output – the rest is heat! The 100 W tungsten filament bulb has just been phased out in England and Wales under a recently adopted European directive.
Tungsten halogen lamps, which have been used in vehicle headlights for some years, have become very common in domestic as well as retail and commercial settings, due to the intensely bright light they produce from a very compact lamp. This means they can be used in fittings which could not possibly accommodate a GLS or CFL and can thus more easily contribute to the interior design themselves – not only by the light they produce, but by their very form. Two types are typically found, the linear type (as used in outside security lighting) and the conical type. Low voltage versions (of the conical type) can be used safely in areas such as bathrooms and kitchens where moisture is more likely. However, a downside in safety terms is that the lamps run at a high operating temperature, required for the “halogen cycle”. The halogen cycle is the process whereby tungsten from the filament is burnt off and combines with the halogen gas during operation, being deposited back on the filament on cooling. The high operating temperature raises safety issues, especially where small children are concerned, and also contributes to potential overheating of spaces.
However, even the low voltage conical types are not low energy, as the current is raised to compensate for the lower voltage, giving the same power consumption (Wattage) as the mains voltage type – typically around 50W. Due to the high operating temperature, much energy is wasted as heat and the lamp life is relatively short, especially when the heat is not able to escape from around the lamp. As they are quite expensive to replace, overall operational costs of halogen lamps are high. If replacing low-voltage type conical halogen lamps in an existing fitting, use the most efficient types available. Firstly, the type with an axial filament allows more light to be emitted from the lamp. Secondly, a dichroic reflector allows heat to escape from the sides of the lamp while light is directed out through the lens. A newer type with an infra-red coating actually reflects heat back into the lamp, thus maintaining the required high operating temperature at lower energy consumption – typically 35W for an equivalent 50W basic conical halogen type.
These are, in effect, thin fluorescent tubes coiled up. Mounted on a bayonet or screw fixing, they can fit into a standard domestic fitting. Alternatively they can come with 2- or 4-pin connectors (PL types) which are compatible with a dedicated fitting only. The electric current “excites” gas molecules causing them to fluoresce. While early CFLs all had a high colour temperature (see below) and thus produced a very white light, there are CFLs available now which have lower colour temperatures. This, combined with instant warm-up and reduced flicker on start (further disadvantages of early CFLs) mean that they are now much more acceptable in a domestic setting. In addition, while early CFLs were bulky and often not compatible with light fittings, there is a much greater range of CFLs now available in different sizes and formats, from reflector lamps, to candle-type and even a glass-enveloped type, resembling (and hardly any bigger than) a conventional “light-bulb”. The advantages of CFLs are two-fold. Firstly, they last much longer – generally at least 8 times longer than conventional “light bulbs”. Secondly, they use much less energy – typically a fifth of the energy used by a tungsten filament lamp giving the same light output. While they also cost much more (at present) the over-cost will be saved many times over during the lifetime of the lamp, due to the lower energy usage and longer lamp life. To get the most out of CFLs, it pays to buy the better quality lamps, which are usually a bit more expensive than the lowest priced ones, as the latter do not have the same lamp-life and therefore will not give you such good savings. The Energy Saving Trust (see Useful Contacts section) maintains a list of approved lamps which have been tested for quality and life-span.
CFLs come in two basic forms. One has the control gear integrated into the lamp (self-ballasted) while in the other (non-self-ballasted) it is incorporated into the fitting. The self-ballasted type (depending on the end-cap) can be installed into either a conventional bayonet or screw fitting. The non-self-ballasted PL type, with a 2- or 4-pin end cap, requires a dedicated fitting which cannot accept a conventional end-cap. The self-ballasted type is obviously the one to choose if you wish to use existing conventional fittings. However, if starting from scratch, a dedicated fitting accepting only a non-self-ballasted lamp should be considered.
Light-emitting diode (LED) light sources are poised to make a major impact on lighting in buildings. You will have noticed LEDs in use in traffic lights and rear lights of buses and certain cars, applications requiring only luminance (being seen). LEDs have also begun to make an appearance in buildings, though again they have been used mainly for their luminance – for example to indicate circulation routes and on stair treads. However, due to recent breakthroughs in brightness and efficiency of white LEDs, these are now beginning to compete with incandescent and fluorescent lighting sources in terms of illuminance (lighting of spaces).
The basic LED is a solid state device containing a chemical compound which emits coloured or white light when an electric current passes through it. Though LEDs have been around for at least 30 years, their use has been limited to applications such as electronic indicators, displays and toys. These recent advances however have led to predictions of a global market share of 60%, worth £52 billion by 2020.
Using a tenth of the energy of an incandescent lamp of comparable brightness, LEDs could reduce global electricity consumption for lighting by 50% over the next 20 years. Additional benefits include longevity, durability and compactness as well as minimal heat production.
Replacement LED lamps are available for standard mains, GU 10 and low voltage halogen fittings. However, these may not achieve the anticipated long life due to excess build up of heat in the LED junction. New fittings designed specifically for LED lamps include heat sinks, which overcome this problem.
Lighting manufacturers and designers refer to the “colour temperature” of a lamp, which refers to the temperature to which a “black body” object must be heated until the colour of the light is matched. Yellower and pinker light thus has a lower colour temperature (below 3000K) while that of bluer and whiter light is higher (over 4000K). Tungsten filament lamps therefore have a lower colour temperature than fluorescent tubes. This is contrary to the way we think of the quality of light from incandescent sources, which appears warmer and that from fluorescent sources, which appears cooler. (Daylight has a colour temperature in the mid-range). Fluorescent tubes are often seen as looking too commercial for a domestic setting, due partly to their bulk and partly to the quality of light produced. While early CFLs all had a high colour temperature and thus produced a “cool” light, there are CFLs available now which have lower colour temperatures, from 2600-3000K.
A terms sometimes confused with colour temperature is “Colour Rendering”, which describes the accuracy with which a light source shows the true tone of a colour (based on daylight rendering). Some lamps give better colour rendering than others, though not always across the whole spectrum. Fluorescent lamps tend not to give good colour. However, full spectrum lamps are available, at a cost premium, which give excellent colour rendering.
There is generally no need for elaborate lighting controls in a domestic setting, as it should be a simple matter to switch lights on and off manually as you need them. However, it may be worthwhile to consider installing an extra switch where you have, say, two ceiling fittings in the same room, or ceiling and wall fittings, for example. The cost of the extra wiring at construction stage will be negligible, while the potential savings over the life-time of the building may be considerable. This may be especially worthwhile if, say, one fitting is in an area better served with daylight. You then have the choice of having either lamp on at any given time, instead of having to have the light on in the well-daylit part of the room when it is not needed, just because the other part of the room requires light. These days, infra-red (IR) switches and fittings are available which do away with wires altogether. This may be especially worth thinking about in a retrofit situation, though the units will require batteries.
In addition most dimmer switches work not by reducing the energy consumption of the lamp but by diverting it, via an electrical resistance. Thus the lamp will use just as much energy when dimmed as it does at full brightness. Instead of relying on dimmer switches, think in terms of providing light from different locations – wall fittings, standard uplighters, central ceiling rose fittings, etc. If you then have these fittings separately switched, you may adjust the overall lighting levels and distribution in the room – and thus the “mood” – without the need for dimmer switches. Provide wall and ceiling fittings first where possible, separately switched from beside the door, so that they may be more easily turned off when leaving the room.
Bulk-head fittings can always be fitted with CFLs, to which there are unlikely to be any aesthetic objections. Photocell/daylight control is an option, to prevent operation during daylight hours. Security lighting, which is usually controlled on a photocell and PIR (passive infra-red) or other type of motion/presence detector, is almost always of the linear halogen type – this should be limited to 150W, which is quite sufficient in most domestic situations.
Daylight factor is a means of quantifying the amount of daylight available in a space. The daylight factor, expressed in percent, is the fraction of daylight entering the room compared to the level of daylight outside from an unobstructed, uniformly overcast sky (think of the middle of a playing field on a bright but overcast day). The overcast sky is used as the basis since the distribution is even from all directions. The Chartered Institution of Building Services Engineers (CIBSE) and British Standard
BS8206: Part 2 give recommended daylight factors for different rooms.
The above-tabled values reflect the different requirements for daylight and general lighting levels in different rooms of the house. However, a daylight factor of 2% is a generally- recommended minimum aesthetic level for daytime-occupied spaces, while a value of 4% can displace electric lighting at most times of day.
Be wary though, that if the daylight factor is very high on one side of the room and much lower on the other, this can also lead to visual discomfort, due to the contrast between the two parts of the room. For visual comfort, a uniformity ratio of not less than 0.4 is often recommended – that is, the lowest value should not be less than 0.4 of the highest (though the BS8206 and CIBSE recommended values represent a uniformity ratio of only 0.3-0.33). This may suggest balancing a large area of glazing on one façade with a smaller area on another.
Daylight Factor Calculation
A sample calculation for daylight factor, applied to a whole room, is as follows;
Daylight Factor = T * Aw * θ / [A * (1 – R²)]
T is glazing transmittance (0.72 for clean double glazing, 0.65 for low E triple glazing)
Aw is net glazing area (i.e. not including frame) m²
θ is the visible sky angle (measured in degrees between vertical and a line from the centre of the window to the top of the furthest obstacle – see figure 61)
A is the total area of room surfaces (including the window wall and window itself) m²
R is the average reflectance of surfaces in the room (0.5 for a light room)
So, say you have a room 4m wide by 5m long by 3m high, with a double-glazed window of clear glazing area 2m² with no obstructions, then;
Daylight Factor is 0.72 * 2 * 90 / [94 * 0.75] = 1.84%
This tells you that the average daylight factor is just on the border of good day lighting levels.
If you were to add another window of the same area, the result would be;
Daylight Factor is 0.72 * 4 * 90 / [94 * 0.75] = 3.68