8 Energy efficiency

The efficiencies of technical devices and processes are normally rated as percentages. Just with light this does not really match, since with respect to the perception of brightness the human eye is differently sensitive to different colours. Therefore the sensitivity of a standardised average eye has already been integrated into the unit for assessing the brightness of light sources. This unit is called lumen (plainly the Latin word for light). Hence, the efficiencies of lamps and luminaires need to be given in lumens per watt. This and only this indication is appropriate to measure and compare which technical device generates most light per unit of drawn electrical power. Theoretically an efficiency of 683 lumens per watt (lm/W) can be achieved. This, however, is only valid for mono-chromatic green light with a wavelength of 555 nm, where the human eye has its greatest sensitivity. So the greenest assumable lamp is indeed green. Irrespective of any political opinion, however, it remains more than questionable whether we really want to illuminate streets, squares, halls, offices, supermarkets or even living rooms in this way. For white light – or what we consider white when mixing all colours from 380 nm to 780 nm wavelengths – yields a theoretical maximum of 199 lm/W. Setting this equal to 100% brings fluorescent lamps already considerably closer to the desired 100% ideal than a modern diesel engine is. Speaking in these terms, an incandescent lamp could merely be compared to an ancient steam locomotive. The European Commission set out to support such trends towards such efficient lighting techniques and in June 1999 released the first draft of a directive with the objective to accelerate the transition of the Community industry towards the production of electronic ballasts and the overall aim to move gradually away from the less efficient magnetic ballasts and towards the more efficient electronic ballasts which may also offer extensive energy-saving features, such as dimming. This sounds as if it went without saying that an electronic ballast is

• always dimmable
• and always the more energy efficient choice.

Back to the latter item in section 8.4. The misconception of the former has already been clarified in section 3.3. Adding to this: