According to this article many energy saving lighting bulbs have a design flaw which means they start to become dimmer over time. Whilst it is advertised that the bulbs can last as long as five or six years, it appears that many will need replacing long before that. A good quality bulb is likely to lose about 20 per cent of its light over its estimated 8,000 hour lifespan with cheaper alternatives emitting just 60 per cent as much light as a new bulb. The problem is with CFL bulbs which give off light when a current is passed through a gas filled tube. The gas glows with ultraviolet radiation which lights up a coating of white phosphor on the inside of the tube, over time, however, this coating starts to lose its ability to light up. Other low energy bulbs such as halogen lights and LED bulbs are more reliable and more likely to stay bright throughout their life. With conventional bulbs being withdrawn from sale it appears we will soon have little alternative but to purchase the slightly dimmer alternative.