Things that glow
Physics Narrative for 14-16
Lighting is a kind of accumulating
Even up until the end of the First World War, electric lighting was only enjoyed by rich householders. The advantages over lighting with gas or oil were significant, but the early adopters developed into a significant market only after the perfection of the metal filament lamp (1911). Books were written offering advice on lighting your home, praising the advantages of rapid control of mood, achieved simply by throwing a few switches.
The slowly developing understanding of the new technology led to some companies installing signs mounted next to these new-fangled appliances, such as:
Do not attempt to light with a match. Simply turn the switch.
Teachers of electric circuit theory at that time probably faced rather different challenges.
A choice in how to accumulate
Nowaways the choice in any lighting retailer is near-overwhelming, and reasonable sized towns may have several shops. There are many different (incompatible!) fittings: bayonet (BC and SBC), screw in (MES, SES, ES, GES…), simple push in, push and lock (GU10…), and end mounts for fluorescent tubes. MES (miniature edison screw – yes, Edison the pioneer of electrical lighting) and SBC (small bayonet cap) fittings are common in school labs. Then there are a variety of mountings: desk lamps; floor lamps; ceiling roses; spotlight arrays; recessed spotlights, to list only a few.
Significant efforts are made to persuade us to part with our money to establish a mood in the home.
Much of this mood is established by the
bulbs that provide the light. These were originally glass bulbs enclosing carbon filaments. In many designs there is still an exoskeleton that protects the active part of the light from the surroundings, but often does not protect us from the lamp, as the outside of many kinds of bulb still gets hot. This warmth is not good, and better lights do more lighting and less heating. Typical powers in the pathways vary enormously with the construction of the lamp: filament, fluorescent, compact fluorescent (CFL) or light-emitting diode (LED).
What you want is luminous power: a measure of the brightness of the lamp as seen by us. All too often what you get is radiant power: a measure of the power radiated at all frequencies – whether you can see them or not. The more efficient the lamp, the higher the ratio: luminous powerradiant power.
Power and colour
Different lamps also emit different qualities of light. A room lit by candles looks very different from one lit by filament lamps, or by fluorescent lamps, or by the Sun. These sources emit different ranges of frequencies, and that's how they affect the mood. The
colour temperature is a simple description of how the eye notices these differences. Take a special kind of radiator, a
black body (more on this in the SPT: Radiations and radiating topic), and heat it to this temperature, and the light emitted will be indistinguishable from the corresponding source.
Here are a few sample colour temperatures: incandescent filament, 2700 K ;halogen filaments, 3000 K ;LED, 3200 K ;CFL (daylight), 3500 K ;moonlight, 4100 K ;daylight, 6000 K .
Each bulb is a typical value only: engineering prowess can alter these significantly to suit different environments.