Light can be produced by an electric discharge in a gas-filled transparent tube. The discharge is started by applying a high voltage across the electrodes at each end. This ionizes the gas filling, enabling an increasing current to flow, and resulting in further ionization. The radiation produced depends on the materials in the tube and on the gas pressure. Its spectrum is dis-continuous, and comprises bands of radiation at specific wavelengths. Phosphor coatings on the inside wall of the tube may be used to absorb some of the radiation and re-emit it at different wavelengths—especially to convert ultraviolet radiation to energy in the visible range.
With all discharge lamps additional equipment is required in the electrical circuit. This produces an initial high voltage to start the discharge, then limits the current during operation and controls the power factor. The power factor depends on the relationship between voltage and current in an a.c. circuit, and affects the efficiency of the equipment.
The combined efficacy of a lamp and its control circuit determine the energy efficiency.
The colour appearance of a discharge lamp is specified by the correlated colour temperature (CCT): the temperature in kelvins of the black-body radiation that appears closest to the colour appearance of light from the lamp. A colour temperature below 3300 K is often describes as warm, between 3300 and 5300 K as intermediate, and over 5300 K as cold.
The CIE general colour rendering index (R
a ) provides a measure of a lamp’s colour rendering quality around the hue circle, quantified on a scale from 0 to 100. Because it has a continuous spectrum the incandescent lamp is used as a reference, and is assigned R
a=100. Lamps with an R a greater than 90 are considered to be very good, and are used where accurate colour matching or discrimination is required. Those with an R
a in the range 80–89 are appropriate where accurate colour judgement is necessary or where good colour judgement is required for reasons of appearance. Lamps with an R a below 80 should be used only where colour quality is of little importance.
The most commonly used discharge lamp is the low-pressure fluorescent tube. This uses primarily a mercury discharge, which emits a large part of its energy as ultraviolet radiation. The inside wall of the lamp tube is lined with a phosphor powder, which absorbs the ultraviolet and re-radiates the energy in the visible spectrum.
Since its introduction in the early 1940s the lamp has gone through many developments. Much work has been done on phosphor compositions, and now it is possible to have lamps with colour appearances ranging from warm to cold. The lamp is efficient, with a high efficacy, although the actual value depends on the phosphor composition. Typically, a modern fluorescent lamp using multi-phosphor technology with good colour rendering (R a=80) has an efficacy of 80–100 lm/W. The life is usually 8000–10 000 hours, but the light output reduces gradually with age.
Originally the electrical circuit for fluorescent lamps included a wire-wound inductive ballast, a starter switch and a power factor capacitor. Recently, electronic circuits have been introduced. These operate lamps at very high frequencies, 20–30 kHz rather than the 50 Hz of the normal mains supply. The very high frequency improves energy efficiency, but comfort is also improved because lamp flicker is undetectable. A further advantage of electronic circuits is the ability to regulate or to dim the light output. The light from the lamp can be controlled to vary with daylight or dimmed for special conditions.
The fluorescent tube has near instant switch-on and restrike, especially with electronic control. The light output increases from switch-on, usually reaching its maximum after a few minutes, but the change in output after switch-on will be hardly noticed.
The output of a fluorescent lamp depends on the temperature of the coolest spot on the bulb wall and therefore on the ambient temperature. If a fluorescent lamp is to be used at low ambient temperatures (in a cold store, for example) or at high ambient temperatures (such as in a bakery) then the lamp light output will be different in the two extremes from normal operating temperatures.
28 The design of lighting
In the compact fluorescent lamp (or CFL) the lamp tube is folded and combined with an integral control circuit to form a discharge lamp with a volume similar to that occupied by an equivalent incandescent lamp. The early versions, developed during the 1980s, were heavy because of the integral control gear, but they formed an important breakthrough in lamp development. There has since been considerable improvement, and now it is possible to have CFLs with either integral or separate control gear. It is likely that this type will become one of the most commonly used lamps, and a direct replacement of incandescent lamps for many purposes.
Compact fluorescent lamps have good colour-rendering properties and come in a range of different colour appearances. Lamp life is high, typically 8000–10 000 hours, but the output decreases with age. Efficacy is typically 50–70 lm/W: this is high by comparison with incandescent lamps but less than that of the tubular fluorescent.
Cold-cathode lamps have an unheated filament but require high-voltage control gear.
They have long, thin tubes, which can be bent into signs or shaped to fit architectural features. Their life is long, 30 000 hours, but their efficacy is typically about 50 lm/W.
When filled with gases other than mercury vapour, and the phosphor coating omitted, they can produce light in various colours.
The high-pressure discharge lamp has a small discharge tube contained within a tubular or elliptical outer bulb; there is not necessarily a fluorescent coating. It is much smaller than the tubular fluorescent lamp but operates similarly; it also requires ancillary electrical control equipment to initiate the discharge, control the current, and correct the power factor. A high-pressure discharge lamp takes a few minutes to achieve full light output; if switched off and back on again, there is a delay before the lamp cools sufficiently for the arc to form again, unless hot restrike control gear is provided.
The high-pressure mercury lamp was the first to be introduced, in the 1930s. It was used almost exclusively for street lighting, because although it was efficient compared with incandescent sources it had a very poor colour appearance and rendering. The lamp comprises a quartz arc tube contained within either an elliptical or a reflector-shaped outer bulb. It has long life, typically 10 000 hours, and a reasonable efficacy, 40–60 lm/
W. In the lamp’s basic form the colour performance is poor, but this is significantly improved in versions of the lamp with phosphor coating inside the bulb.
In the 1960s a development of the mercury high-pressure discharge lamp occurred that raised its colour quality to the point where it could be used where colour rendering and appearance were important. In the metal halide lamp, halogens are added to the mercury vapour. To do this successfully requires advanced manufacturing technology. Initially the colour appearance of some lamps tended to change through life: this was particularly noticeable when several were used in the same installation and could be easily compared, but the technolo-gy has been developed intensively and the major lamp makers have minimized this effect.
The lamp comes in a range of shapes and sizes, including versions with integral reflectors.
Some lamps have very small arc tubes, which makes them ideal for precise optical control lumi-naires, such as spotlights for sports lighting. The lamp life is typically 8000–10 000 hours with an efficacy of 70–100 lm/W. In addition to the ‘white’ metal halide lamp, coloured lamps, particularly blue and green, are produced by some manufacturers for decorative purposes.
The other main group of lamps is based on a discharge through sodium vapour. The low-pressure sodium lamp has a bright orange monochromatic light. It is used only where colour is unimportant and high efficacy is required; motorway lighting is the major
application. The low-pressure sodium lamp is available in a range of sizes, all with tubular bulb shapes. Typically it has a life of 8000–10 000 hours and an efficacy of 100–200 lm/W.
Lamp scientists had known for some time that if a sodium discharge at high pressure could be created, a much improved colour performance could be achieved. As the pressure of a sodium discharge is raised, the spectrum of monochromatic radiation at low pressure expands to produce a broadband distribution. The problem was to find a light-transmitting material that could contain the highly corrosive sodium at high pressure. In the 1960s a translucent ceramic material, sintered alumina, was developed. Research has continued on this, making possible further increases in arc pressure and hence even better colour performance. Currently the high-pressure sodium lamp is available in a range of sizes and bulb shapes. The best colour versions are described as ‘White SON’. The lamp has a long life, typically 8000–10 000 hours, with an efficacy of 70–120 lm/W.
The development of lamps is a continuing process, and two new types of lamp have been recently introduced. The mercury induction lamp depends on energizing a mercury discharge using a magnetic field. Because this eliminates the need for electrodes, which deteriorate with time, the lamp life can be extremely long: typically 60 000 hours is quoted by the manufacturers. The lamp is basically a fluorescent tube and has a similar colour quality, with an efficacy of approximately 60 lm/W.
The sulphur microwave lamp is an electrodeless source, which uses microwaves to create light from a sulphur and argon bulb filling. The prototypes produce 450 000 lm for an energy consumption of 5.9 kW: an efficacy of approximately 76 lm/W. The life is estimated to be 10 000 hours, based on the life of the magnetron that generates the microwaves. The spectral distribution is continuous across the whole spectrum range, with reasonable colour performance. With a source of this power a system is necessary to distribute the light within buildings; the sulphur lamp cannot be used as direct replacement for existing sources until small sizes can be manufactured. A 1 kW version is commercially available.