Far Infrared LED possible?

[Melchior]

Infrared LEDs are quite common.

So how far along the Far-infrared spectrum can these be taken?

AKA can you turn a LED into a 'heat lamp'?

(not a normal epoxy led but something made out of ceramic and glass?)

Could LEDs someday replace heating elements as well?

[PhotonWrangler]

I regularly deal with IR laser diodes operating in the 1550nm region. Not quite in the "heat" region but pretty far away from visible light. BTW, a simple resistor works quite well as a heating element.

[jtr1962]

Could LEDs someday replace heating elements as well?

I doubt it as you can't exceed 100% heat conversion efficiency with an LED but you can with a heat pump. If you're talking solid state heating elements then my money is on current and in the future improved thermoelectrics. Efficiency is always at least 100%, and usually somewhat more.

[haserman]

I regularly deal with IR laser diodes operating in the 1550nm region. Not quite in the "heat" region but pretty far away from visible light. BTW, a simple resistor works quite well as a heating element.

do you sense the IR output as heat (*not* the heat generated at the device, but by absorption of the emitted IR)? thanks!

[haserman]

do you sense the IR output as heat (*not* the heat generated at the device, but by absorption of the emitted IR)? thanks!

sorry to put this one here again -- i haven't figured how to default to "send me notifications by email, so i'm just making that explicit.

[Benson]

Roithner has some pretty cool toys -- only mW/uW class output, but they do go out to 7um wavelength, and illustrate a key point. Look at the power output for LEDs in the same series with different wavelengths, and note how fast it rolls off past about 2um.

do you sense the IR output as heat (*not* the heat generated at the device, but by absorption of the emitted IR)? thanks!

There's a common misconception that IR in general and far IR in particular is somehow special about heating things up -- but reality just doesn't work that way.

1W of radiation absorbed (i.e. not reflected) will transfer 1W of heat, whether it's FIR, NIR, or visible.

Of course, it depends on what you're trying to heat -- because different materials are low reflectivity/high absorption at different wavelengths, and because in some applications (e.g. radiant heating of a room), absorption peaks may matter less, because whatever gets reflected at first will still get absorbed eventually.

So, if you don't care about visibility, you'd just pick whatever wavelength has the highest product of LED efficiency x target absorption -- which may well be visible. Keep in mind the previously noted rolloff of efficiency with wavelength, and that royal blue LEDs get the most R&D $$$*. If you do care about invisibility (maybe glare's an issue), then you select by the same product, but only among wavelengths >750nm or so -- which probably means NIR.

*This is because they're the basis of phosphor-type white LEDs. For reference, royal blue XT-Es get up to 55% efficiency at 350mA, whereas red XP-Es are about 35% and green XP-Es only 20%. If you can get ~2x the efficiency, that can make up for the target reflecting considerably more, while still using less power input, and having less waste heat to dissipate.


But the thing is, of the electrical power going into an LED, a fraction (55% is the highest I know of) comes out the front as photons, and the rest heats up the die -- and you have to conduct that waste heat away, because before an LED gets hot enough to transfer much heat by radiation, it dies. So they have low efficiency as radiant heat sources.

Whereas with an incandescent bulb, all the power goes to heating the filament, but most of that heat is then transferred by radiation (infrared or infrared+optical, it's all heat), and so gets into the beam, given a good reflector. Very little is conducted out the bulb base, so it's much more efficient as a radiant heat source.

Incidentally, in general the hotter the filament, the more efficient as a radiant heat source (since radiation scales with T^4, but conduction with T), so if you don't care about invisibility or matching wavelengths to absorption peaks, a hard-driven halogen is a better choice than a typical heat lamp.

p.s. WRT forum notifications -- you can click "Thread Tools", "Subscribe to this Thread..." to get notifications instead of posting again.

[haserman]

thanks for the detailed reply, Benson. i am aware of a good part of that already. perhaps you can make sense of what i don't knows:

GOALS:
------
1. throw a hand-sized area of IR at 15-20 feet that can be **sensed** as heat using little energy.
2. i am only interested in energy thrown at a distance that can be sensed as heat. right now, collimated IR seems to be the way to go.
3. deliver about 5-10 mW/cm2


WHAT I DON'T KNOW
-----------------
IR LAMP DOESN'T HAVE RANGE. an IR heat lamp (e.g. a 250W) from GE prouces about 75 to 100W in front. i just need a measly 50-10mW/cm2 at 10-15 feet away. why don't i get this, even if i use lenses to concentrate the light outwards?

NARROW COLLIMATOR FOR LED? can anyone recommend a hi-power IR LED with a collimator that can do a 2-3 degree beam angle? the lowest i see are 15 or 20 degrees.

HEAT SENSATION. most ebay/alibaba vendors say that i cannot feel the throw of an 850nm LED as heat. very few say i can. any experiences here?


WHAT I KNOW:
------------

840nm LD THROWS HEAT. personal experience shows a laser diode at 840nm, 300mW (input?) produces sensible heat in front (*not* waste heat at the die). Even at 15' away. It's a small dot, but it can be sensed and objectively shown as well (by using a thermal fabric that responds only to IR near human sensible range). i can provide detail on this if needed.


840nm IS GOOD FOR HEAT. 840nm is a good wavelength for absorption and sensation of heat at the skin. longer wavelengths penetrate deeper but do not produce the sensation. this depends on the absorption of the water, fat, blood in the layers beneath the skin.

840nm IS ALSO REFLECTED. a good part of 840nm is also reflected, so i'm assuming that the absorption at the layer just under the skin, where the thermoreceptors are is the key.

THERMORECEPTORS. cold & hot thermoreceptors exist in more or less the same areas. the cold ones far outnumber the hot ones. if the goal is to feel "not-cold" then these are the ones to consider (not that i have had a reason to single them out just yet).

FOLLOWSPOT THROWS HEAT. a followspot (spotlight) at 750+W and an airplane landing light at 150W (spherical reflector, BIG) are able to produce light & heat that can be sensed at about 8' away.

HID GOOD BUT CAN BURN. videos of high intensity discharge flashlights on youtube show that there's enough power to burn cardboard with a simple biconvex lens. so this should be able to throw good heat, though the possibility of burning a sofa keeps me from thinking of this as a solution.

LD CAN BURN EYE. combinations of wavelength, intensity and time of exposure can burn your eye out. with IR, you won't even sense it happening AND you won't have the blink reflex which you'd have with visible light (same applies for UV, which is why it is better to have no sunglasses than ones without UV protection -- your eyes are open wider but don't know that they're getting fried by more UV than if there were no glasses).

THANKS SO MUCH!!!!

Roithner has some pretty cool toys -- only mW/uW class output, but they do go out to 7um wavelength, and illustrate a key point. Look at the power output for LEDs in the same series with different wavelengths, and note how fast it rolls off past about 2um.

There's a common misconception that IR in general and far IR in particular is somehow special about heating things up -- but reality just doesn't work that way.

[...]
.

[Benson]

GOALS:
------
1. throw a hand-sized area of IR at 15-20 feet that can be **sensed** as heat using little energy.
2. i am only interested in energy thrown at a distance that can be sensed as heat. right now, collimated IR seems to be the way to go.
3. deliver about 5-10 mW/cm2

Ah, that helps...


Unfortunately, I only have one "ebay special" IR power LED, and don't have proper test equipment (or manufacturer's specs) to know what sort of output power, so I'm afraid I won't be much use on the practical specifics...


I do know Osram makes an 850nm PD here which outputs 950 mW from a 1 mm^2 die, or 320mW/sr -- I can talk a bit about optics with respect to that, anyway. If you're looking at Chinese LEDs, they're probably lower output (for single-die), but you can pretty much scale it. That Osram emitter is about $10, though, so I'd probably go with it unless someone else can point you to something better.

IR LAMP DOESN'T HAVE RANGE. an IR heat lamp (e.g. a 250W) from GE prouces about 75 to 100W in front. i just need a measly 50-10mW/cm2 at 10-15 feet away. why don't i get this, even if i use lenses to concentrate the light outwards?

I've used heatlamps, but never had one apart (or scraped the red filter off, if that's even possible) -- I assume they're designed like a regular PAR30 flood beam; the light coming from the reflector is already messed up beyond fixing due to the lousy reflector of a PAR30 flood, and the light coming direct from the filament (which could otherwise be focused with a good lens) is probably going through the rippled front glass (which I assume is there...).

So I'm guessing that's the issue; if I'm wrong, and the front glass is smooth, then you should be able to focus the filament to a parallelish beam with a lens, but the bigger the source, the bigger the lens needed. I'd guess around 1 foot diameter. And the ~75% of the light that goes by way of the reflector? wasted.

NARROW COLLIMATOR FOR LED? can anyone recommend a hi-power IR LED with a collimator that can do a 2-3 degree beam angle? the lowest i see are 15 or 20 degrees.

Well, I mentioned the best NIR LED I'm aware of above, so rolling with that... I'd tend to use an aspheric lens; let's consider a 4" diameter lens. You can figure that the LED's surface intensity fills the aperture, so we've got 320mW/sr, times the area ratio (2 in)^2*pi/(1 mm)^2, = 2500W/sr. At a range of 500cm, this is (2500W/sr)/(500cm)^2 = 10mW/cm^2 -- of course, after losses, it's less, but within your target of 5-10. The beam spread will be approximately 1mm/4in = 0.01r (0.6 degree), so at a range of 500cm, the 4 inch spreads by about 5cm, so something roughly like 6 inches across -- pretty much what you asked for.

So you should be doing something like a 100mm aspheric lens (optolife has one, some people around the forums said their smaller lenses were good), or the same size reflector, if that's how you roll. You'll need to go bigger if your LED is less powerful, or has a larger emitting area. But see below -- I'm not sure 5-10mW/cm^2 is enough (but as I said, I can't run a practical test here -- if you're sure it's adequate, go ahead.)

840nm LD THROWS HEAT. personal experience shows a laser diode at 840nm, 300mW (input?) produces sensible heat in front (*not* waste heat at the die). Even at 15' away. It's a small dot, but it can be sensed and objectively shown as well (by using a thermal fabric that responds only to IR near human sensible range). i can provide detail on this if needed.

OK, laser diodes are rated by output, and typically have about 25% efficiency IIRC -- if 300mW is the input, it would be perhaps 75mW output, but since you put that ? there, I'm assuming it's 300mW output.

As you say, it's a small dot -- I expect no more than 1 cm^2 at that range, although it depends on your optics -- so even if 300mW is the input, it's 10 times your target of 5-10mW/cm^2, and if 300mW output, more like 50x. Are you sure 10mW/cm^2 is really enough?

If you do need to get to 100 mW/cm^2, then you're looking at a minimum of 12" aperture with any IR LED I know of... maybe someone who does more IR stuff knows something better than the Osram emitter I'm looking at.

[...]

FOLLOWSPOT THROWS HEAT. a followspot (spotlight) at 750+W and an airplane landing light at 150W (spherical reflector, BIG) are able to produce light & heat that can be sensed at about 8' away.

Yep, I got a Q4509 ACL in a 6V lantern housing -- it was actually the first thing that came to mind when I read your requirements list above! But then you said "using little energy", and I'm afraid a 100W halogen running about 140W isn't quite it.

HID GOOD BUT CAN BURN. videos of high intensity discharge flashlights on youtube show that there's enough power to burn cardboard with a simple biconvex lens. so this should be able to throw good heat, though the possibility of burning a sofa keeps me from thinking of this as a solution.

Note that that's focusing it down to a point -- if you focus it to a parallel beam (as near as you can get), burning things will be less of a problem. HID (especially short-arc) with a high quality reflector (parabolic, or elliptical reflector + aspheric lens) is probably the most compact hot-sensation transmitter you can make. The bigger concern with HID (and it's not really a problem, as long as you deal with it) is to make sure there's a sheet of UV-blocking glass in there somewhere, as they deliver a hefty dose that can cause rapid sunburns and serious eye damage.

[haserman]

Ah, that helps...

I do know Osram makes an 850nm PD here which outputs 950 mW from a 1 mm^2 die, or 320mW/sr -- I can talk a bit about optics with respect to that, anyway. If you're looking at Chinese LEDs, they're probably lower output (for single-die), but you can pretty much scale it. That Osram emitter is about $10, though, so I'd probably go with it unless someone else can point you to something better.

thanks! i will look into that. this is very useful stuff, btw, thanks much again.

I've used heatlamps, [...] I assume they're designed like a regular PAR30 flood beam; [...] is probably going through the rippled front glass (which I assume is there...).

So I'm guessing that's the issue; if I'm wrong, and the front glass is smooth, then you should be able to focus the filament to a parallelish beam with a lens, but the bigger the source, the bigger the lens needed. I'd guess around 1 foot diameter. And the ~75% of the light that goes by way of the reflector? wasted.

PAR64, no rippled front glass. i've tried using smoothly rolled Al foil, extended out like an antenna and throwing the light out through the aperture on the other end. the heat _does_ travel that far, but not having much luck it throwing it much farther thru air after that point. i would like to see a 1cm2 area at 15' get heated. can't even seem to do that and i don't know why. i have other options, but i wish i knew why a 100W output (250W consumption) can't do that. i have a couple of 5x6 fresnel lenses i harvested from an old LCD projector that i haven't tried yet.

Well, I mentioned the best NIR LED I'm aware of above, so rolling with that...

thanks, i will look into this & the math when i get home. fyi, by my calcs, 5-10mW/cm2 is what i need, 100mW/cm2 is way too much. see http://hyperphysics.phy-astr.gsu.edu...bodrad.html#c1 for a heat loss calculator and put in 0.0001 m2 as the area.

Yep, I got a Q4509 ACL in a 6V lantern housing -- it was actually the first thing that came to mind when I read your requirements list above! But then you said "using little energy", and I'm afraid a 100W halogen running about 140W isn't quite it.

compared to using a 1500W heater in a small room, 140W isn't much :-).

Note that that's focusing it down to a point -- if you focus it to a parallel beam (as near as you can get), burning things will be less of a problem.

good point, no pun intended. i should look into this. in any case, the heater should probably be used with supervision, not left unattended.

HID (especially short-arc) with a high quality reflector (parabolic, or elliptical reflector + aspheric lens) is probably the most compact hot-sensation transmitter you can make. The bigger concern with HID (and it's not really a problem, as long as you deal with it) is to make sure there's a sheet of UV-blocking glass in there somewhere, as they deliver a hefty dose that can cause rapid sunburns and serious eye damage.

[sonrider657]

I doubt it as you can't exceed 100% heat conversion efficiency with an LED but you can with a heat pump. If you're talking solid state heating elements then my money is on current and in the future improved thermoelectrics. Efficiency is always at least 100%, and usually somewhat more.

Can you please explain to a layman how efficiency can be greater than 100%?

[CarpentryHero]

This is an area where I know very little but find interesting because IR LED's are what night vision cameras use, which may come in handy when your aiming and focusing the beam. interesting topic

[fyrstormer]

Can you please explain to a layman how efficiency can be greater than 100%?

What he means is, the wattage of the heat transferred is greater than the wattage used to power the heat pump, because most of the heat supplied by a heat pump is existing heat transferred from one location to another, instead of heat directly generated from usable energy.

Think of an air conditioner. It pumps heat from inside the house to outside the house, producing some heat of its own in the process, but overall it transfers much more heat than could be produced if the energy used to run the air conditioner were used to run a heating coil instead. A whole-house air conditioner can run on the power used to power a cooking oven, but the cooking oven produces far less heat than the air conditioner can transport.

[sonrider657]

What he means is, the wattage of the heat transferred is greater than the wattage used to power the heat pump, because most of the heat supplied by a heat pump is existing heat transferred from one location to another, instead of heat directly generated from usable energy.

Think of an air conditioner. It pumps heat from inside the house to outside the house, producing some heat of its own in the process, but overall it transfers much more heat than could be produced if the energy used to run the air conditioner were used to run a heating coil instead. A whole-house air conditioner can run on the power used to power a cooking oven, but the cooking oven produces far less heat than the air conditioner can transport.

Got it. Thanks. Very helpful explanation.