Low energy lighting considerations – Part 2 of 3

In Part 1 we looked at the parameters of the different lighting types. Here we review each type in more detail and then draw a conclusion.

Incandescent lamps

I have chosen the most popular bulb wattages. Note that the lower the wattage then the lower the lamp’s filament temperature and the ‘warmer’ the colour. The warmer the colour the longer the lamp’s life, but the less efficient it is. Incandescent lamps do lose some luminosity over their life, but they fail because of a catastrophic failure of the lighting element before the drop in luminosity has reached a significantly low level.

Compact fluorescent

These lamps are miniature versions of the fluorescent tubes in their type and method of operation. They contain the necessary control gear within the enclosure. Because they are more compact then a tube design they run hotter and make more use of the phosphor to generate light so do not last as long nor run as efficiently. Many types exist and they are readily available in styles that will replace standard tungsten incandescent lamps. They do run a bit cooler than incandescent lamps, but not enough to make much difference to the lamp shade type.

Their failure mode is described in the next section.

Fluorescent tube

These operate using a special coating on the inside of a glass tube that has been filled with a small amount of a special gas. This gas is excited by the electrical current and this energy is used to make the coating glow. Modern tubes use electronic gear to make this happen at high frequency giving a rapid startup and a non flickering light. Because of the chemicals use to make the coating these tubes must not be disposed of in general refuse but recycled carefully. Being of a length, rather than compact, they run cool and can be readily used as concealed lighting.

As for incandescent lamps they lose some of their luminosity over their life, most noticeably with blackening near the end caps. They fail because of a catastrophic failure of the starting gear or within the lighting element before the drop in luminosity has reached a significantly low level.


Light Emitting Diodes (LEDs) have been around since the 1960s. Initially they were used as simple red indicator lights and then in numerical displays (also red). Now they are available in all sorts of colours and, through matching in sets, as white.

This group has been the main disappointment to me. The advantages of LEDs is being hyped up all over the press and internet, and their high efficiency and cool running hailed as a breakthrough.

However, the figures above paint a very different picture as their efficiency is no better than a decent fluorescent. Add to that the fact that the functional design of some of the lights compressed several LEDs into a space designed for an incandescent fixture and the temperature of these devices will still be fairly warm, even though less than an incandescent would be. The problem with this is that the life of an LED light is severely affected if the temperature rises above a critical point; and that point is relatively low. It is the temperature of the semiconductor junction that is critical and two LED fittings that look similar, but from different designs, can perform very differently.

The ‘failure’ mode of LED lighting is a lot different from any of the above as catastrophic failure is very rare. LEDs loose output steadily as time passes and eventually it gets to the point where the light has to be replaced. It seems to be agreed that this is somewhere between a fall to 70% and 50% of the initial output.

There has been some effort to standardise on the definition of an LED’s number of running hours being that for the output to fall to 70% or to 50% of the ‘nominal’ output, but the adoption of such methods is patchy at best. There are further issues with the measurement as the ‘nominal’ output can be determined at initial switch on or after a running time of 1,000 hours when it will have fully stabilised after a settling down period. Then, because the life is so long at 50,000 hours or more that the particular model of light will have been superseded by the time full figures are available and so projections are made based on the first 10m,000 hours of operation assuming ‘standard curves’.

Hence it is difficult to know how to calculate lifetime costs.

A further issue is that the light from each LED is directional. Sure some are being designed with angled arrays, but it is not clear if that is a real solution. It seems to be too early to tell.


From the above analysis it seems that the predominant light source should be either full fluorescents or CFLs. Both will give substantial savings compared to incandescent and their characteristics are well known.

For thoughts about lighting colours see Part 3.

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