There are several light sources to consider when lighting your home. They each have different characteristics, advantages and disadvantages. The technical term for a removable light source is a 'lamp', though in practice 'light bulb' is used for light sources which have the shape of a conventional bulb.
As technology has progressed, artificial light sources have become cleaner, easier to control, cheaper and more efficient. The naked flame, candles, gas and pressure mantle lights have been replaced by electric lighting. While the the earlier types of lighting still have some applications, the focus here is on the main practical light sources for the home.
Daylight - Light from the Sun
Light from the sun is created by nuclear fusion. It is perceived as extremely bright light - direct sunshine - or diffused through clouds. The intensity and colour temperature (see Unit 1 - The Nature of Light for explanations of colour temperature and render) of sunlight changes throughout the day. At sunrise and sunset when the sunlight has further to travel through the earth’s atmosphere, the light has a warmer colour temperature because the blue light is scattered by atmosphere. Light from the sun is free, consumes no natural resources, has a colour temperature ‘in tune’ with the human activity cycle, and has excellent colour render. The intensity of sunlight can be controlled by blinds or curtains and there is some scope for transporting sunlight using light tubes or sun pipes. But it is, of course, impossible to control the rotation of the earth or the weather, both of which affect the brightness and availability of sunlight. Significantly increasing the amount of sunlight in a room or space is likely to require structural work - adding or enlarging windows. Without building work, sunlight can be maximised by limiting obstructions and shadows and using pale surface finishes. There are certain circumstances - notably watching television - where the intensity of sunlight will need to be effectively limited.
Three light sources (left to right): incandescent, compact fluorescent and LED
Filament or Incandescent Light Bulbs
Filament light bulbs emit light when an electric current heats the tungsten filament - incandescence is the emission of light as a result of something being heated. Filament bulbs were the principal form of domestic lighting in the UK from the early 20th century. Filament bulbs have a warm colour temperature which becomes warmer when they are dimmed and have excellent colour render. They are available in a range of bulb shapes and filament styles and emit light evenly around the bulb. Although incandescents have been eclipsed by newer technologies, the quality of the light they emit may mean they still have a place in a lighting scheme. They are suitable for mood or decorative lighting - perhaps above a dining table. But: they are inefficient at turning electrical power into light and have a relatively short lifespan. There are now better alternatives for practical illumination. The positive characteristics of filament bulbs - high colour render (CRI >98), fast start, low flicker, and dimming - are a useful starting point against which to measure energy efficient alternatives. Low energy alternatives will last longer and, of course, use less energy, but they are likely to disappoint if they do not get close to the positive aspects of incandescents.
Halogen Light Bulbs
Tungsten halogen lamps are similar to incandescents but the filament is surrounded by a halogen gas which deposits evaporated tungsten back on the filament, thereby increasing lamp life (generally, 2,000 to 10,000 hours) and preventing the blackening of the inside of the glass. Halogen bulbs generally provide cooler and whiter light than conventional filament bulbs. As with filament bulbs, they have a high colour render, fast start time and low flicker. They can take the same shape as conventional filament bulbs or more compact forms suitable for recessed ceiling lights and spotlights. The smaller forms (such as MR16) are generally low voltage and require a transformer either separate or integrated into the light fitting. Halogen reflector or spotlight lamps tend to have a narrow beam angle: unlike filament bulbs, the light is emitted in a more or less narrow cone from the light source. This may mean that multiple light sources - as with ceiling downlights - are required to ensure good coverage.
Spectra of three light sources (left to right): incandescent, compact fluorescent and LED
The brightness of halogen lamps means they are suitable for practical illumination but lack the aesthetic appeal of conventional filament bulbs. The development of more efficient LED light sources in the same forms/shapes used by halogens mean that halogen use will be limited to applications where LEDs cannot be used, for example lighting ovens.
Fluorescent Tubes or Compact Fluorescent Light Bulbs
Fluorescent strip lights remain the dominant form of lighting in many commercial and institutional buildings. Electricity is used to excite mercury vapour in a glass tube which then emits ultraviolet light - fluorescence. A phosphor coating on the inside of the tube converts the ultraviolet light to white light. Fluorescent strip lights require a ballast to start them which slows the start time. Different phosphor coatings can be used to change the colour temperature of the light from the tubes and options commonly include 'warm white' (c 3,000K), 'cool white' (c 4,000K) and 'daylight' (c 6,500K). Fluorescent strip lights can achieve a CRI of around 80 which is acceptable for most applications. The key advantages of fluorescent lamps are that they are extremely economical in terms of installation, operation and maintenance. Although the aesthetics of fluorescent light fittings and the quality of light produced means they have limited application in the home, their practical and economic advantages means there may still be a place for them in utility rooms, garages etc. Compact fluorescent light bulbs (or CFLs) use the same technology as fluorescent strip lights but in a form which allows them to be used as direct replacements for incandescent light bulbs - the tube is folded or twisted to fit the dimensions of conventional bulbs. CFLs are widely available with standard screw (E27) or standard bayonet (B22d) caps and the electronic ballast needed to start the lamp is integrated into the base of the bulb. As with incandescents, they emit light evenly around the bulb
CRI values of three light sources (left to right): incandescent, compact fluorescent and LED
While compact fluorescents were heralded as the eco-friendly replacement for filament bulbs, the performance of most CFLs - especially the cheap ones - is hugely disappointing. When compared to incandescent, they are very slow to reach full brightness, have poor colour render (colours look dull or washed out) and dimming performance is mixed. While CFL quality has improved, it is clear that LED technology now offers better performance for practical lighting.
LED Light Bulbs
The development of the blue light emitting diode (LED) in the early 1990s made it possible to produce white light from LEDs. The subsequent rapid improvement of LED technology makes it clear that LED will be the dominant form of lighting - either as LED light bulbs to replace CFL and incandescent light bulbs or integrated into light fittings. As with CFLs, the performance of LED bulbs can be disappointing. There are several factors which go into producing LED light sources which have a warm colour temperature, high CRI and which are efficient and long-lasting. The LED itself - the chip and orange phosphor coating which shifts the colour of the light from the chip - needs to be designed and manufactured to ensure a consistent light output at the right colour temperature with a high colour render. Because LED light sources are 'flat' and produce a narrow beam of light, the number and positioning of the LED chips is crucial to ensure an even light distribution. Light distribution can improved in a number of ways, most commonly by the use of a translucent diffuser either as part of the LED bulb or integrated into the light fitting. LED light sources run on low voltage direct current (DC) and a driver is needed to convert mains alternating current (AC) to DC. Driver quality is crucial to ensure low flicker, smooth dimming and the long-life of the chips. And, as with all integrated circuits, LED chips fail if they get too hot. Unless the lamp has an effective means of heat dissipation - by a heat sink or in the design of the lamp - the light quality will degrade rapidly and the chips will fail. Properly designed and manufactured LED light bulbs are available in range of colour temperatures with high CRI, low flicker and good dimming performance.
Except for limited decorative applications, it is increasingly likely that people will choose LED lighting for their homes. The next unit of Lighting 101 (Unit 3 - Light Fittings and Luminaires) looks at how light sources are integrated into fittings which are then fixed to walls and ceilings in the home, connected to a power supply and which then distribute the light.