LEDs for light seem to come up a lot - I very seldom see an incandescent
red stop light, for example. But are they really more efficient, like
lumens per watt, or however that's measured? 'Cause if so, it seems like
a no-brainer, although the one "white" LED flashlight I've seen was very
blue, and would have been icky for general room lights. Maybe a reading
lamp...
LEDs are now getting more efficient than incandescent lamps, but there
are extra factors in traffic lights, flashlights, and some other
applications.
Traffic lights: The incandescents in those are not regular ones, but
ones designed to last (typically) 8,000 hours. These are something like
30% less efficient than the usual household incandescents.
But the real kicker is that a red filter or a green filter blocks about
2/3, maybe about 70% of the light. You probably get only 3-4 lumens out
per watt in, as opposed to around 11-17 lumens per watt with the usual
"standard" household incandescents. (Slightly less for 230V versions in
part because their filaments are thinner and have to be run slightly
cooler for the same life expectancy).
But they make red LEDs and trafic signal green LEDs with about the same
overall luminous efficacy as white LEDs - 20 lumens per watt, even more.
Lumileds makes some red ones that get 40 and traffic signal green ones
that get 30 or more. Nichia makes traffic signal green LEDs that get 25
lumens/watt. Similarly efficient ones are available from ETG.
Flashlights: The main problem with incandescent is that their efficiency
decreases greatly as the battery weakens. On the other hand, white LEDs
usually have slightly increased efficiency when moderately underpowered.
Their efficiency is usually maximized with current somewhere around 20% of
the "typical" current in the datasheet, and as current decreases below
that they lose efficiency but not nearly as rapidly as incandescents do.
I mention more in
http://www.misty.com/~don/lede.html
BTW - Many LEDs actually have some color change as current is varied.
- Don lipstein (
[email protected])