I was looking at some of the data over on the Lumileds site. I was wondering
how die temperature affects light output.
also interested in how efficiency l/w is affected as the current is lowered.
Not looking for exact figures, just in general how the luxeon LEDs perform
considering the above.
The datasheets for the various Luxeon products have curves showing light
output as a function of temperature and as a function of temperature.
The curves are "normalized" to 100% at "characterization current" and at
25 degree C junction temperature.
In the "electrical characteristics" table earlier in the datasheet is a
figure for thermal resistance, in degrees C per watt. MUltiply this by
power input in watts, add this to the temperature of the heatsinkable
surface of the LED, and that is the junction temperature.
In general, Luxeons of all colors will have a little more luminous
efficacy when mildly to moderately underpowered than at full power. Same
is true of most white, InGaN blue and non-yelowish green LEDs of any
brand. All LEDs whose temperature characteristics I have seen have
efficiency varying inversely to some extent or another with temperature.
Also, what is the typical output of a 12 volt .5 amp incandescent bulb
(likely argon gas fill) for comparison.
This will vary with design life expectancy and to a little extent with
other factors such as filament shape.
First order of approximation - somewhat less than half the approx. 180
lumens of the "93" 12.8V 1.04 amp lamp, and with a little less overall
luminous efficacy (lumens out per watt in) if life expectancy is the same
(750 hours) - I would pull 60 lumens out of a hat.
Although at this moment I do have the urge to mention a rather
efficacious low power incandescent lamp - Osram's HPR50, a flashlight
lamp. It is a halogen lamp with xenon as the main fill gas ingredient and
has a rated life expectancy of 25 hours, but is designed to produce 89
lumens from 4.42 watts.
I would want it noted that incandescent lamps have some "economies of
scale" where efficacy largely increases with wattage and is also usually
favored by higher design currents (thicker filaments). Because of
this, 12 volt incandescents tend to be more efficient than higher
voltage ones of the same wattage and life expectancy. Other factors
also matter - such as low voltage design (generally near and especialy
under 5 volts) resulting in very short filament length compromising
efficiency due to heat conduction through the ends of the filaments.
- Don Klipstein (
[email protected])