Light and heat in the front room

kw: analysis, light, technology

The way to make light, that is, to create photons, is to accelerate electrons. The oldest technology for doing this is to burn something. The hot carbon atoms in a candle flame emit light because the molecular collisions due to the heat knock electrons about. Also, carbon particles in the same flame emit light because the heat energy in the particles is carried by electron interactions, some of which produces "incandescent" photons. Neither process is particularly efficient, which is why lighting a room with candles, kerosene lamps, or gaslight creates a lot of heat. All the technologies of making light are driven by the need to do so at lower cost in power consumed, that is, in heat emitted.

I got to wondering how little heat I can produce while producing the light I want in my front room, by means of various technologies. There are two scenarios: (1) I am reading by myself; (2) We have guests. In the first case, I do quite nicely with one 18-watt CFL in a lamp that used to have a 75-watt tungsten bulb. The amount of light is equivalent, about 1100 lumens. The bulb's efficiency can be expressed as about 15 lumens per watt; the CFL's is about 62 lm/w.

In the second case, we add a stand light that has three 13-watt CFL's and a torchiere fitted with a screw fixture and a 23-watt CFL. That total of 80 watts of CFL's at 62 lm/w produces about 4,950 lumens. The same amount of light once needed 330 watts of incandescent bulbs.

But what if I had to use gaslight or candlelight? The former produces about 0.6 lm/w, the latter about half that. These watts are now counted as heat of burning. At 0.6 lm/w, 4,950 lumens requires more than 8,000 watts of heat. And candles? 16,000 watts seems an inconceivable amount! A heat pump type furnace consumes 3,000 watts to produce 10,000 watts of heat transfer. Even in winter, you'd drive everyone from the room in short order. This is why in the days of combustion-type lighting, the parlors were lit quite a bit more dimly than they are today.

And what of the future? Will LED's do better? The present state-of-the-art LED (so far only in the lab) produces 90 lm/w. I could light my room with 55 watts. That is a little better than today's 80 watts. LED's produce light by more cleverly accelerating electrons within certain semiconductor materials. The "white" ones in LED flashlights use a blue LED and a yellow wideband phosphor that converts part of the blue to green, yellow and red.

The theoretical maximum efficiency for an LED, using multi-colored emitters rather than fluorescent conversion, is either 200 or 250 lm/w, depending on the color mix, whether "flat white" or "center-hump". At 250 lm/w, I could produce 4,950 lumens using only 20 watts. Now, that's a goal worth shooting for.