Martin Vlietstra, one of our frequent contributors, has written an article which he says deals with the metric system in use rather that looking at metrication per se.
In the last few years, we have seen a big change in the way in which units of measure related to lighting are expressed. Although they have been expressed almost exclusively in metric units for almost a century, there is a general degree of ignorance as to what these units actually mean.
Those of us who have been responsible for replacing light bulbs in our own homes for some time are familiar with the terms “100 watt bulb”, “60 watt bulb” (with lesser power bulbs for use in torches). What does “100 watts” means? Most people are aware that light bulbs get hot when they are in operation. A “100 watt bulb” consumes 100 watts of electric power. Some of this power is converted to light but most to heat. During the last decade, there has been a move to using alternative technologies for light generation with the aim of reducing the amount heat (and also invisible infra-red light) produced by the light bulb. This has in turn led to an expansion of the detail used in the bulb’s specification.
The headline specification of a light bulb is the amount of visible light generated (measured in lumens – symbol lm) rather than the amount of power that it consumes. As a rule of thumb, a filament or incandescent light bulb that consumes N watts will generate about 15 times N lumens while a light-emitting diode (LED) that produces the same amount of light will only consume a seventh N watts. The additional power consumed by the filament bulb is given off as heat. Dimensionally lumens are identical to watts, but unlike watts are defined with reference to the sensitivity of the human eye to different colours.
Many lampshades have a warning such as “Maximum 40 W”. This refers to the actual power consumed by the bulb in question rather than the amount of light given off. If you are using a 40 watt filament bulb (no longer available), the maximum light that you can safely get will be about 600 lumens, but if you switch to LED technology, you can use a light bulb that gives off considerably more light as long as the input energy (and hence heat plus light) does not exceed the limit.
Anybody who has watched a blacksmith at work will be aware that the iron with which he is working is “red-hot” (about 1000 K which converts to 727°C). The temperature can be judged from the colour of the glow of the metal. This phenomenon is the same as in a tungsten filament bulb except that the filament bulb, which uses a tungsten filament rather than an iron filament, operates at 3500 K, giving a yellow-white light. If you dimmed a filament bulb, you might have noticed that it turns more yellow. The reason is that it is consuming less power and is not getting as hot. A brilliant white light would theoretically be generated by a filament bulb running at 6000 K. In practice it is not possible to manufacture a filament bulb that operates at this temperature as all known metals melt well below this temperature, but other technologies exist that can create light of almost any colour that is needed. You will often see the “temperature” of a light bulb quoted on the packaging – this tell you whether you a buying a “soft” light or a “harsh” light, but does not refer to the actual temperature of the light. For the benefit of readers who have not studied studied physics (or who have forgotten what they learned), temperature can be measured in SI units using either kelvins or degrees Celsius. “Zero kelvins” is better known as “absolute zero”, the lowest temperature that can theoretically be attained. It is equivalent to about minus 273°C.