Light not-so fantastic

Brian Hammond posed a valid question in his correspondence 'Can you shine a light?' (Letters, 10 December 2007).

Higher voltage bulbs require a higher resistance for comparable wattage, so it follows that the white-hot tungsten element is thinner.

To give 220V bulbs a comparative cost-effective life, they are manufactured to run a tad cooler, so are less efficient — that is they have fewer lumens.

Conversely, a low 110V bulb is more efficient. This life-enhancing manufacture is particularly vital when the spread of voltages outside North America is 220-240 and especially so for the UK 240V system used in several countries. (Although EU voltage is nominally 230, plus or minus 10 per cent, 240 or 220 voltage systems are still actually used).

Our 240V is eight per cent higher [than 220] creating 27 per cent more light, and takes our lumens and efficiency to US voltage standards. But we pay a price; 240V usage drops bulb life by 30-40 per cent, say from 1,000 hours nominal to 600.

There is only a small envelope of optimum voltage for incandescent light bulbs. If the voltage is increased slightly the light-efficiency rockets, because radiant heat and light from resistive elements is proportional to the cube of the voltage. However, the life expectancy is drastically reduced.

On voltage reduction, life expectancy rockets exponentially but the light drastically reduces. Hence it is assumed 'efficiency' means the best cost-effective compromise, which has been well researched.

With global trade and a spread of 220-240V worldwide (outside North America) most manufacturers will optimise design nowadays at 230V on most goods.

Nothing in the world will stop a typical, Chinese-manufactured 230V filament bulb of a specific nominal wattage from radiating nearly 30 per cent more light in the UK compared with the continent.

PH Field

St Albans

Herts