Lamp Choices in Church Lighting
James Morse
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| St Bartholomew The Great Church, West Smithfield, where tungsten xenon uplighters were used to created an atmosphere similar to candlelight. |
In recent years the choice of lamp types available to the designer of lighting systems for churches has become ever more broad. It is because of this that careful consideration has to be given before the final choice of light source is made for the various lighting tasks within any worship space. No longer is it a case of the local electrician aiming a few standard (PAR38) spotlights onto the altar and tungsten halogen floods down into the nave. The criteria used in the decision making process, fall broadly into aesthetic and technical categories.
AESTHETICS: COLOUR 'RENDERING' AND 'APPEARANCE'
The main aesthetic consideration is colour, which is considered in terms of colour appearance and the colour rendering properties of the light emitted. There is often some confusion between these two somewhat interrelated terms. Put simply, colour rendering is how accurately one can perceive colours under a particular light source: lamps are rated against a percentage standard, with tungsten lamps as the benchmark of 100 per cent, and the monochromatic sodium lights still in use on motorways down as low as 30 per cent. Colour appearance, on the other hand, is the perception of the colour being emitted from a light source. This colour is measured in degrees Kelvin, and typically a standard light bulb running on full voltage has a colour temperature of around 2,700K.
The important interrelationship between these two criteria is that if for instance a very warm source of say 2,700K is used to illuminate a surface that is toward the other end of the visible spectrum, such as blue, the colour will appear rather dull and muddy. It follows from this that both colour rendering and colour appearance are important considerations when choosing a lamp. An example would be that if you wish to uplight a dark timber roof, a light source with an average colour rendering index of at least 85 per cent (Ra85) should be employed, coupled with a warm colour appearance of between 2,700K and 3000K to bring out the natural colour of the timber.
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A rule of thumb when choosing lamps would be to consider only those sources with a colour rendering index above Ra80 and, given the propensity of warm coloured surfaces in churches such as terracotta, oak and sandstone, with colour temperatures between 2,500K and 3,500K. The exception to this would be where a daylight effect would be preferable, with a colour temperature of around 6,500K. It is also important to remember that church interiors would have been viewed for many centuries by candlelight having a colour temperature of approximately 2,500K, and when a new lighting installation was required at St Bartholomew The Great Church, West Smithfield (illustrated on opposite page), replicating this quality of light was an important part of the brief.
Lamp types with suitable colour rendering and colour appearance characteristics include the following:
- Fluorescent colour rendering up to Ra90, colour temperatures between 2,700K and 6,500K. However, given the lamp shape and the light emitting geometry, they are really only suitable for some indirect applications such as uplighting and accent lighting.
- Metal halide colour rendering up to Ra80, colour temperatures between 3,000K and 4,000K. A very useful source, that is still unfortunately non-dimmable.
- Tungsten halogen excellent colour rendering, and a colour temperature of approximately 3,000K make this source ideal from a purely aesthetic standpoint.
- Tungsten xenon these low voltage capsule lamps are used in multi lamp linear arrays and are an ideal source for warm and soft indirect lighting with a colour temperature of 2,500K (see illustration of St Bartholomew opposite).
TECHNICAL CRITERIA
The important technical criteria governing lamp choice are energy usage and lamp life. Also of importance is whether a lamp is suitable for dimming.
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The table left illustrates the dilemma facing any designer when deciding upon what source to employ. It can be seen that if any luminaires need to be sited in inaccessible positions, the favoured sources are fluorescent, metal halide, or tungsten xenon as these have the longest life. However, the tungsten xenon lamp has such a low efficacy that it cannot be used for anything other than close, offset accent lighting. The choice then is between metal halide or fluorescent, and the decision is then based upon what type of effect is required and whether or not dimming is essential. Unfortunately, although fluorescent is dimmable, it is a totally unsuitable source for downlighting from lofty positions, and metal halide is not dimmable. A compromise is to bring the downlighting element of the lighting scheme down to a manageable elevation and employ tungsten halogen sources. The rational behind all this is that it is very desirable to have dimmable lamps for downlighting, but not really essential for uplighting.
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| Figure 2 St Mary's RC Church, Clapham, illuminated with metal halide uplighters mounted at a high level, and tungsten halogen PAR30 downlighters. Both lamps have similar aesthetic properties, but the metal halide lamp cannot be dimmed, making it more suitable as an uplighter, while the tungsten halogen lamp has a shorter life expectancy, making it more suitable for use where it is easily accessible. |
The lighting of St Mary's Roman Catholic Church, Clapham (see Figure 2) illustrates a typical compromise, with the nave downlighting effected by means of tungsten halogen PAR30 lamps at a reachable height, and metal halide asymmetric uplights mounted on the clerestory window sills to illuminate the roof.
A further advantage of mixing light sources within a church is that the most energy efficient and long lasting types such as metal halide can be used for security or cleaners lighting, in a similar way to performance spaces such as theatres and concert halls.
Dimming is always a desirable feature of any church lighting installation, not least because, like performance spaces, it is a tremendous asset to be able to stage manage either congregational acts of worship or other events such as concerts and plays. As well as modulating light output, dimming of lamps has two other effects; lamp life is extended considerably, and there is a warming of the colour temperature as the lamp voltage falls. The extension of lamp life is most welcome, especially in the case of tungsten candle lamps often used in chandeliers, where their life expectancy is around 750 hours. This figure can be doubled by running the lamps at 80 per cent of full voltage, and ramping up the output from cold to white hot over seconds rather than the milliseconds that occurs when lamps are conventionally switched. The warming of the colour temperature is also most often an advantage, as the colour begins to resemble that of candlelight, as mentioned earlier.
As with most design, compromises have to be made between aesthetics and the practical considerations of maintainability and energy usage. The advances in lamp technology have helped the designer, but it is still absolutely essential that any lighting design reflects the architectural intent of the church interior, and renders that interior in a way to maximise the unique ambience of a place of worship and contemplation.
