Radiant Heat from LEDs?

[hotfoot]

And I'm not talking about IR LEDs either folks.

I was playing around with a 5W blue luxeon mod and for the heckuva it, shone it point-blank onto my lips. The flashlight made no physical contact with my skin, but I *swear* I could feel warmth radiating out from the flashlight and onto my lips.

Next, I took my probe thermometer and did a similar experiment. After switching on the thermometer and letting it stabilize to room temp, I took the same blue 5W LS mod and shone it on the probe, again without making any physical contact between probe and the flashlight or me (and my breath..etc..etc). The temperature shot up by at least 3 degrees C almost immediately!

In both cases, I was using an NX05 optic and at the time of both experiments, a stone-cold flashlight was used (ie. LED was allowed to cool down).

Lumileds and other LED manufacturers have been touting the fact the radiant heat is not emitted when using LEDs, but my 2 experiments above seem to indicate otherwise! This raises a few questions:

- What is the source of this radiant heat? If LEDs are fairly monochromatic and I was using a deep-blue LED, surely there could not have been any infra-red, right? Or was the near-UV energy strong enough to cause similar heating phenomena?
(BTW, I tried the experiments with overdriven white 1W LEDs with similar, albeit less dramatic, results.)

- If we start using clusters of high-powered LEDs (eg. a cluster of 6x5W LSs), would radiant heat once again become a problem, akin to that faced by users of halogen lamps?

Anybody have any ideas to this? I'm stumped...

 

[Doug S]

Leo,

The source of the radiant heat is the LED. Any object with a temperature greater than absolute zero radiates heat. It has been 30+ years since I studied thermodynamics but I believe that the rate of radiated energy varies with absolute temperature raised to the 4th power. Two objects with different temperatures will have a net energy transfer between them via radiation from the warmer to the cooler. This is what is happening with your lips and LED. Apparently the collimator is effective in collimating the IR and well as the visible. At room temperature, radiation is typically only a minor factor in cooling LEDs but since high power LEDs can reach die temperatures in excess of 100C [373K] radiation can become significant due to that 4th power relationship of radiated energy to temperature.

 

[milkyspit]

I'm a newcomer here and not sure if this is relevent or not, but I use lithium AA's in my Streamlight 4AA LED flashlight, and with those in place the flashlight head gets noticeably warm after several minutes of continuous use. Would this be due to radiant heat becoming significant due to the increased output, or is it due to some other factor?

 

[INRETECH]

At present, no device is 100% eff at converting energy to its design output - as in the case of LEDs, no LED is 100% eff at converting the electrical energy into light

As with any device, the lack of eff is ALWAYS converted into HEAT

I worked at Intel for 4yrs designing high eff switching power supplies, and I used a $64,000 thermal camera to check the conversion of the power supply

http://users.easystreet.com/n7dkq/pictures/heat.jpg

 

[INRETECH]

Thermal and IR are different wavelengths, so many people confuse them often

 

[James Van Artsdalen]

I found one reference that says that 1 watt equals 683 lumens (at 555nm).

When you consider that a *5* watt Luxeon only produces 120 lumens of output it's pretty clear that the vast majority of energy is "wasted" as heat.

That in turn gives you an idea of just how inefficient incandescents are: an LED is far better than a bulb but still nowhere near ideal.

 

[Jonathan]

Okay, my turn to jump in, but the nit-picking is just clean fun.

Minor Nit: Inretech, I can think of at least 1 device that is 100% efficient at converting input electrical power to desired output power...a resistance heater

Major Nit on Lumens: The _definition_ of a lumen is supposed to account for the difference in the eye's ability to perceive different colors of light; so that 1 watt of 640nm red photons will have a different lumen value than 1 watt of 555nm green photons. At the wavelength of peak eye sensitivity (555nm for _bright enough_ light), 1 W of photons will have a lumen value of 683 lumen. For the mix of photons that would appear _white_, 1 watt of photons would be about 200 lumen. A 5W Luxeon with its 120 lumen output is about 12% efficient.

Minor Nit on efficiency of incandescent lamps: A properly driven 5W incandescent lamp will have an efficiency in the 20-30 lumen per watt range. A white LED will have an efficiency in the 20-30 lumen per watt range. For efficiency at bulk white light production, LEDs are not appreciably different from properly driven incandescent lamps. However _small_ incandescent lamps (0.5W) are much less efficient, while LEDs maintain the same 20-30 lumen per watt efficiency. LEDs also maintain efficiency when dimmed below rated power, which is very good for flashlights where the batteries drain.

Finally, _all_ light is radiant energy. If you had a 100% efficient green LED, one which took 1W of electricity in and produced 1W of green photons, with no heat production and no IR production, and you put a thermometer in front of it, the thermometer would register heat. This is because _green_ photons carry energy just like IR photons. LEDs do _better_ than incandescent lamps in terms of radiant heat in that the _unwanted_ radiant energy is produced at a low temperature (100C) rather than a high temperature(3000C), so that the total amount of radiant energy (both desired visible light and wasted IR) is much smaller for the same amount of desired radiant energy.

-Jon

 

[Empath]

The energy contained in a beam of light will be equivalent to the energy producing it, minus the energy lost to the inefficiencies of the generating system. If the beam of light strikes something, all of the non-reflected light will be converted to another form of energy, usually heat.

In the case of a white LED, the original blue light strikes the fluorescent coating, converting the energy into white light. A broader spectrum of white light reflects off more surfaces, thus a smaller percentage of the energy is usually absorbed and converted.

 

[Empath]

Originally posted by Jonathan:
Okay, my turn to jump in, but the nit-picking is just clean fun.

Minor Nit: Inretech, I can think of at least 1 device that is 100% efficient at converting input electrical power to desired output power...a resistance heater

<font size="2" face="Verdana, Arial">Minor Nit of a minor nit: A resistance heater would also be producing wasteful magnetism and RF energy.

 

[dy5]

So, shining white light from a fiber optic light pipe onto a black object would cause the object to heat slightly - is that right?

This discussion reminds me of a demo publicised in the '50s or '60s where a subject, wearing a white dress shirt and a black tie stands in front of a large laser. The experimenter sweeps the laser beam across the subject's chest, which cuts the tie in half but leaves the shirt (and subject) unscathed.

 

[Don Klipstein Jr.]

I think the detected thermal effects in the original posting are attributable to the blue light. A 5 watt blue Luxeon could put out perhaps 400 milliwatts of blue light, which is roughly as much power as 4 square centimeters or .6 square inch of direct sunlight.

- Don Klipstein ([email protected], http://www.misty.com/~don/index.html)

 

[Rothrandir]

i was wondering the exact same thing hotfoot...

the other day i finished my first 5w mod (royal blue) and i noticed the top of the led actually gets *hot*. touch it and go "ouch"

how hard are you driving this? i am currently using 6c's but will go down to 5 d's as soon as i can go buy some.