The CFL versus GLS debate

Published:  08 April, 2008

CFL Compact fluorescent
Fig. 1: A good example of lamp information on packaging, showing how compact fluorescent lamps are regarded as being one of the most energy-efficient lighting sources.

Richard Forster delves into the issues comparing compact fluorescent and tungsten-filament lamps — and in doing so challenges conventional perceptions concerning efficiency and mercury.

The domestic lighting is remarkably conservative and has been dominated by unchanged tungsten-filament lamps (GLS) for about 80 years. These same lamps have long been abandoned by industry and commerce for very sound reasons. Lighting of the home is different only in that the burning hours per lamp is much less and typically only a few hundred hours per annum compared with 2500 hours for offices, 4000 hours for street lighting and 8000 hours for 24/7 supermarkets.

Why has the GLS lamp retained its popularity when other light sources use less electricity, last longer and provide cheaper lighting? The most obvious substitution is the compact fluorescent lamp (CFL). Because of its widespread acceptance for commercial lighting the lamp industry saw no strong reason to market the pros and cons of CFLs for domestic lighting. There are many attributes to consider, as can be seen on some packaging (Fig. 1).

Luminous efficacy

A 100 W GLS emits 1370 lm (lumens) which is an efficacy of 13.7 lm/W. A typical 20W CFL emits 1371 lm which is 68.6 lm/W. Using GLS lamps will use about five times as much electricity for the same amount of light.

Lamp life

The 100 W GLS lamp will last on average for 1000 hours use, whereas a 20 W CFL might have a rated life of 12 000 hours. However, the life of CFLs has not been standardised, and lamps are available with lives from 5000 to 15 000 hours. It is therefore essential to check the claimed life when purchasing CFLs.

Through-life costs

The total cost of initial purchase, replacement lamps, cleaning, energy consumed and disposal will normally favour CFLs because electrical consumption is the major cost for lighting. However, where usage is only a few hours per annum the payback period will be longer. The initial capital cost is about or four times that of a GLS lamp. Currently the price of GLS lamps is increasing and subsidised and free CFLs are available, so economic comparisons can only provide a rough guide. However, energy costs are unlikely to fall in the short and medium term so reducing the lighting load will save money.

Size and shape

Most CFLs of equivalent light output are larger than the GLS lamps being replaced. There is a possibility the CFL will not fit or may protrude beyond the shade. The latest versions recently introduced are of the same size. With GLS lamps this is not a problem as the lighting fitting is labelled with the maximum safe wattage, and lamps conform to dimensions in a British Standard. With CFLs there is no guarantee that lamps of the same wattage will be the same size or shape.

There is a multitude of shapes for CFLs with different limbs and spirals, whereas the GLS lamp is a simple spherical shape. This means the GLS lamp is of roughly equal intensity in all directions except for the base. With CFLs, the light emitted is proportional to the projected area in any direction. When selecting a CFL for an existing fitting is important to pick not only the right size but also the right shape.


Colour is a complex topic, but there are two simple measures in common use. — colour temperature and colour rendering.

Colour temperature describes the colour appearance of the light emitted by the lamp. For full-spectrum lamps like tungsten filament it provides an accurate indication of the spectral composition. A GLS of 2700 K indicates a ‘warm’ colour with a high red content. Fluorescent lamps can be much ‘cooler’ with 5000 K and above, and natural daylight can be more than 10 000 K. The human eye has learnt to contend with changing colour temperatures, partly because logic suggests that the colours of objects are constant. This mental process is similar to the white balance used in modern digital camera and camcorders. Natural light all comes from the same source, and the process can break down when different colour temperatures come together. Colour temperature used for discharge lamps with selective spectra is only an approximation and is termed correlated colour temperature. A consequence is that two lamps from different manufacturers can quote the same colour temperature but will appear different. It is not good practice to mix suppliers of discharge lamps. In contrast GLS from different manufacturers will be indistinguishable.

The other measure of colour is colour rendering, which expresses how accurately the colours of objects will be revealed by a lamp. This characteristic is separate and independent of colour temperature. A general colour rendering index on a scale of 0 to 100 is based upon eight standard colour samples of mid hue and is called CRI or Ra8. The higher the value, the better the colour rendering. Because the calculation of CRI is an average, different lamps may show different colour shifts. If these are of the same magnitude, they will be given the same CRI. Equal CRI does not mean identical colour rendering. For interior lighting a minimum CRI of 80 is given in BS EN 12646-1 ‘Lighting of work places’. GLS lamps have a CRI of 100 because of their full spectrum.


Modern CFLs operate at high frequency (about 40 kHz), so there is no discernable flicker. Early CFLs with conventional mains-frequency copper and iron ballasts did flicker, as do GLS lamps. The exceptions are low voltage halogen (12 V) downlights and table lamps, where the heavier filament has sufficient thermal inertia to provide constant light output.

Power factor

Most CFLs are marked with wattage, voltage and current. Fig. 3 illustrates a lamp marked 7 W. However, it is marked with a current of 61 mA and a voltage of 220 to 240 V, suggesting 14.6 W — twice the 7 W. The reason for the discrepancy is that the current and voltage in discharge lamps are out of phase by 60°, so 14.6 VA has to be generated at the power station to enable to lamp to have a useful power consumption of 7 W. The power factor for this lamp is 0.5.

Power factor is not important to the end user as normal metering and tariffs are based on watts, not volt-amps. However, the power station has generated twice the electrical energy consumed, and the distribution system has to carry twice the current. Originally power factor was not considered important for low-wattage lamps as they represented an insignificant proportion of the total load. However, with Government policy encouraging the widespread use of CFLs domestically, power factor is important. There are 25 million homes in the UK with on average 23 fixed lighting points. The number of houses and the number of lighting points per house are both predicted to increase.


All fluorescent lamps contain mercury and are classified as hazardous under the Hazardous Waste Regulations 2005, so they can only be disposed of by recycling or at licensed landfill sites. In addition, fluorescent lamps (straight and compact) are included in Waste Electrical & Electronic Equipment Regulations, whereas GLS lamps are exempt. The good news is that modern fluorescent lamps contain less mercury and last longer.

Research in the Netherlands showed that the extra electricity from gas-fired power stations needed by GLS lamps to produce equivalent light output to CFLs emitted more mercury into the air and the ground than is used in the lamps, so GLS lamps are indirectly a greater source of mercury pollution. Coal-fired stations can also emit mercury, the amount varying with the quality of the coal. With our dependency on fossil fuels, the use of electricity is a much greater problem than fluorescent lamps.


Fluorescent lamps are relatively large area and low-brightness light sources and thus cannot create the refractive sparkle for crystal chandeliers. Until LEDs are developed further, clear filament lamps are the only solution. The lamp industry is aware of this and is introducing new tungsten-halogen packages for retrofit, offering equivalent light output but lower wattage (Table 1).


Table 1. Where light sources are required to provide ‘sparkle’ halogen lamps provide an energy-efficient alternative to standard filament lamps. This comparison is for lamps from Osram.


Table 2. Compact fluorescent lamps are available that can be dimmed. These figures are for the Dial or Switch (DorS) dimming series from Megaman. The energy saving is by comparison with a 100 W GLS lamp.


Contrary to widespread reporting, CFLs are available that can operate with conventional wall dimmer. Also there are CFLs with sequential switching to provide four lighting levels (Table 2).


GLS lamps deliver full output as soon as they are switched on. For CFLs there is a short delay of less than a second, but if the lamp is cold the initial light output is only about 30%. Full light output can take one or two minutes. If a CFL is warm, the light output on switch on is much higher. This is an irritation, as up to 20% of domestic situations require lighting in a room for less than a minute and 50% for less than 11 minutes.

Temperature sensitivity

Filament lamps operate over a wide temperature band and are found in refrigerators and ovens. CFLs are low-pressure lamps; the example in Fig. 1 is typical, with a range of only -10 to 40°C. When the temperature varies from 25°C ambient the light output falls. CFLs may not be as effective outdoors or in compact enclosed lighting fittings.


Fig. 2: The spectrum of light from GLS lamps (left) and CFL (right) is very different


Fig. 3: Compact fluorescent lamps are marked with their power consumption, current consumption and voltage — from which it can readily be worked out that this lamp has a power factor of about 0.5.

Building Regulations

Part L of the Building Regulations for England and Wales provides recommendations for both new buildings and the refurbishment of existing building services.

Efficient lighting is defined as using lamps with greater than 40 lamp lumens per circuit watt for dwellings and 50 lamp lumens per circuit watt for non-dwellings. For dwellings, at least one in four fixed lighting sockets should be of this type. For non-dwellings, it is the average for the total installed general lighting.

The recommendations for sustainable housing increase the proportion to 75% of the fixed lighting. In addition the lighting should have dedicated sockets that will not allow BC or ES type lamps to be substituted. One advantage is that control gear can be separated from the lamp and does not have to be replaced with every lamp failure.

Stricter enforcement of these recommendations is likely to come with UK implementation of Energy Performance of Buildings Directive (EPBD) and inspection prior to certification.

CFLs are clearly not the same as GLS lamps, so changing light sources requires care.

GLS lamps are familiar to all and are manufactured to internally established standards.


High-power LEDs and specially designed optics reduce the number of Orbik Elled emergency-lighting fittings need to illuminate escape routes.

CFLs have not been harmonised, and there is a bewildering choice. But is this sufficient ground not to change? Most interior lighting is by fluorescent lamps. Domestic furnishings, clothing and food are all purchased under fluorescent illumination, so why is not acceptable in the home? Surely installing energy-efficient lighting is a lifestyle change preferable to giving up golf trips to Spain, long-haul holiday destinations and exotic fruit at Christmas.

Richard Forster is lighting adviser with BSRIA.

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