>> leds are more luminous and they consume little, but they emanate less
>> heat of the incandescent light bulbs.
>> And they do not melt the snow...
>> The article is in italian but the photo is enough!
>> http://www.zeusnews.it/index.php3?ar=stampa&cod=11596&numero=999

> LEDs do of course emit plenty of heat (though obviously less than
> incan), it's just that it tends to be extracted out of the back to
> keep the LEDs themselves cool rather than emitted out the front.

To a first order approximation ,yes. But it's interestingly more complex 
than that, and changing by the day as LED efficincies rise. At present about 
1/3 of the energy input to a very top efficincy white LED exits as light.

An incandescent bulb has an efficiency of about 15 - 20 l/W (lumen/Watt)
A good quality coiled CFL about 70 l/W.
A tube fluorescent about 100-120 l/W
A typical good quality White LED in the 1 W - 5W range about 100-120 l/W and 
somewhat more or less epending on what luminance "bin" you buy or are given.

So, to produce say 1500 - 2000 lumen you need.
100 W incandescent
 25 W CFL
 20 W tube fluro
 20 W LED

The 2000 lumen is equivalent to roughly 5 Watt of light energy
So the heat output of the various emitters will be about.

95 W incandescent
20 W CFL
15 W tube fluro
15 W LED

However, the incandescent radiates almost all the energy - both the 5W of 
light and the 95 W of heat energy (actually also light but not as we see it, 
Jim).So too the CFL and the tube fluorescent. In their cases the glass gets 
hot and acts as a radiator. I haven't worked out how much energy goes where 
and why, but a look in a modern CFL bulb shows that the switching 
transistors (often MOSFETS) have shunk from TO220 to something smaller, and 
that heat sinks aren't used. Electronic efficicny looks quite good. Only the 
LED relies mainly on conduction to remove its heat. In a decent calorimeter 
the radiated "light" and "heat" would all sum

So heating wise the ratios are about

100 : .25 : 20 : 5

While the LED consumes about 20% of the energy of the incandescent it 
radiates only 5% as much (which is what you said) - and what heating you get 
is ALL from "light" ! :-).

Interestingly, in the traffic light situation, a CFL would take about 20% 
more energy than a top LED, but produce about 5 times as much radiated 
heating output if all the output was absorbed (something you don't want in a 
traffic light). If youcan abosrb much of the radiated heat in the light 
structure you'd probably get 2 to 4 x as much radiated heating from a CFL 
than with an LED.

Presumably traffic light designers could arrange for the LEDs conducted or 
convected heat to be directed into the from sructi\ure to assist with snow 
melting.  It's 'funny' the requirements that can crop up along the way in 
apparently straight forward applications. Melting snow is unlikely to 
feature high on an LED designers checklist in many cases.


  Russell McMahon

YMMV / E&OE - sure to have made a rubbish assertion somewhere in there, but 
the general gist thereof should be OK.







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