Low energy lighting considerations – Part 3 of 3

In Part 1 we looked at the options for improving the energy efficiency of the current lighting. In Part 2 we examined the options in more details and came to the conclusion that the only real option was to change to either fluorescent tubes or compact fluorescent lamps (CFLs).

In this section we look more at the way the light is delivered and the type (mainly colour) of the light.

But before we do that there is one interesting ultra low energy fitment to be called into play, and that is the sun. This dwelling is a bungalow with a relatively large footprint. It has, therefore, two hallways. One, the entrance area, is m x m with a window on one side. The other, which links to the bedrooms, is entirely internal and the only natural light comes from some glazed doors. For safety reasons they will have to be replaced making it a dark place. The kitchen has only one small window and, currently, a glazed door. Again, to reduce heat loss and improve security that glazing will be reduced. If we are not careful we will be needing to use artificial lighting 24 x 7.

One solution would be to put a roof light in the ceiling, but that would be a bad idea. Such roof lights let heat in during the summer when it is not wanted and out again in the winter as, even if well glazed; windows have a poor U value and are a serious source of heat loss. Flat, or south facing, roof lights are particularly bad in this way.

However, there is a device called a Sunpipe that can help. It is a 300mm/ 12” diameter silvered tube connected to a light gathering dome on the roof and a flat light outlet at the room’s roofline. It’s modest size minimises heat loss, it does not have to open (unlike a roof light) and so can be well sealed, and it will illuminate ‘up to 14 m2’. It has a U value of 1.8, just fine for what we need. It is intended to fit one of these to each of the spaces identified.

Light colour

We all know that white light has different ‘colours’. You can see this when taking photos. If you use the same camera settings for an indoor scene lit by incandescent lights then it looks as if it has a red/ orange tinge compared to the same subject outside on a sunny day. His is because the temperature of the radiating body giving the light is less for the indoor scene that from the sun. In both cases it is what is called ‘black body’ radiation, that is radiation derived purely form the temperature of the radiant body. For full daylight the nominal temperature is from 6,000 to 6,500K. An incandescent lamp is about 3,000K and so appears redder. A candle, with an even redder light, comes in at below 2,000K.

The light from a fluorescent lamp is produced by exciting a chemical coating inside the tube, not heat. So its colours are defined by the types of chemicals used. In general there are two major colours used, referred to as warm white and cool white. The warm white simulates a temperature of around 3,500K; that is a bit whiter than an incandescent lamp, and the cool white has more blue light in the colour and is rated at around 5,500K, much closer to daylight.

Most domestic lighting will be done using the ‘warm white’ colour as we are used to associating the redder tinge with the evening indoors, candlelight and firelight. The cool white is much better for task lighting, e.g. for hobby spaces, work spaces, cooking and reading. This light gives increased contrast for improved visibility, and truer colour rendition.

For more information see Colour temperature (Wikipedia reference)

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