A concept: LED bulbs for front projection systems (1 Viewer)

[Francois Caron]

As many of you may know by now, white LED lights are beginning to replace traditional incadescent and even halogen light bulbs in everyday products such as flashlights. For the same amount of light or more, LED lights consume less power, generate very little heat, and usually last longer than the lifespan of the entire flashlight. So I was thinking. Can this technology be used as a replacement for LCD and DLP projector bulbs?

If you've ever changed a projector bulb before, you know it's basically nothing more than a high power light bulb mounted on a removeable cradle with two wires connected between the bulb and the connectors. For something costing hundreds of dollars, you'd expect a bit more for your money.

Now imagine if in that same amount of space, you could add a two inch square panel mounted with hundreds or even thousands of tiny white LED lights attached to a power regulator designed specifically for the projector being used. You MIGHT end up with a satisfactory replacement bulb.

I say MIGHT because I have no idea if such a concept is even possible. For now, the only white LED lights I seem to find on the Internet are the types that are already installed inside a clear plastic case. What I would need is practically the tiny LED element inside the casing, mounted on a board along with hundreds if not thousands of other identical tiny LED lights.

Can a two inch square board packed with thousands of LED lights work as well as a regular projector bulb? Can it give off the same amount of light or more while at the same time give off nowhere near the same amount of heat as a regular bulb? Are there any companies out there that have tried this out yet? What is the smallest size white LED light currently available on the market?

 

[Mike Wladyka]

Hey Francois,

LED's are awesome, and i think they could easily be used for FP's. However, they do generate lots of heat! I saw a demonstration of 6, 1 watt LED's with the casing off, and it was sooooooo bright that no one could even look directly at it, and about after 1 minute the circuit board was burning hot. I would tend to think that thousands of little LED's is not the way to go, but rather a couple big ones. There is 3, 5, and 10 watt LED's out there but they are expensive and can get very hot, and extremely bright. They do use very little power and will last a very, very long time. It is very cool technology that will eventually take over all lighting duty

Mike

 

[Philip Hamm]

Mike,

Incandescents and halogens, especially high intensity ones in FPs, also run extremely hot.

It would be interesting to see numbers on the number of lumens per heat unit (calorie?) for both technologies.

 

[Todd Hochard]

LEDs are MUCH more efficient than comparable wattage incandescants/halogens. By an order of magnitude. And, I mean that from both an energy and heat perspective (with electronics, the two are inextricably linked).

I think the problem with LED lights, in general, is that they just can't pack enough lumens into a given space. YET. The tech is improving, though.

 

[Mike Wladyka]

It would be interesting to see numbers on the number of lumens per heat unit (calorie?) for both technologies.

lumens per watt is usually used.

Why LEDs can be 10 times as efficient as incandescents in some applications but not in general home lighting!
UPDATED slightly 10/19/2003.
First, the figures:

The better usual modern white LEDs (as of late 2003) produce about 17-25 lumens of light per watt of electricity delivered to the LEDs when the LEDs are supplied "typical" current or that at which their characteristics are specified. Maybe just a little more for the highest ranks/binnings. Most such white LEDs will be slightly more efficient when moderately underpowered and will be less efficient when overpowered.

Nichia claims that white LEDs that achieve 60 lumens/watt will go into production in 2005.

Compare to 14-17.5 lumens per watt for standard "A19" 120 volt 60 to 100 watt incandescents, and typically 16 to 21 for most halogen lamps rated to last 2,000 hours or more.

i know that some companies are claiming up to a 100 lumens per watt

 

[Brett_H]

Francois,

The cynic in me believes you may have answered your own question:

If you've ever changed a projector bulb before, you know it's basically nothing more than a high power light bulb mounted on a removeable cradle with two wires connected between the bulb and the connectors. For something costing hundreds of dollars, you'd expect a bit more for your money.

If the bulbs in projectors lasted longer than the lifetime of the projector, a huge revenue source would dry up for the manufacturers and everyone else involved in the supply chain. Something tells me that these companies aren't going to be too hot on the idea of giving up this planned obsolescence.

-Brett.

 

[Mike Wladyka]

LEDs are MUCH more efficient than comparable wattage incandescants/halogens. By an order of magnitude. And, I mean that from both an energy and heat perspective (with electronics, the two are inextricably linked).

there isn't much heat from the light itself, but there is tons of heat created during the creation of the light. if a 1 watt LED is not cooled it will basically fry from getting to hot, i don't think that is much of a problem with 1 watt light bulbs

 

[Kevin P]

All the white LEDs I've seen are bluish-white in color, so color temperature would be an issue as well. I'm sure as the technology improves a high-efficiency 6500K white high-power LED (or array of LEDs) could act as a light source in a projector.

But if you're going to use LEDs, how about just having a screen consisting of an array of miniature red, green, and blue LEDs like they have in scoreboards and the huge screens used in concerts?

 

[Mike Wladyka]

But if you're going to use LEDs, how about just having a screen consisting of an array of miniature red, green, and blue LEDs like they have in scoreboards and the huge screens used in concerts?

i don't think that LED's are small enough for that to look good from close range, plus those screens use like 300,000 watts

 

[Mark Schermerhorn]

i think that is because there is actually no white LED's but rather it is a combination that will trick the eye into seeing white...not sure, but i think that is correct...and i think the color temperatures are around 6000K or greater, so they are close but not quite

That is incorrect. They are indeed white LEDs. One discrete package, one diode, with color temps around 5500 - 6000. All the white LEDs I've seen so far are using InGaN for the semiconductor, newer processes could be using something else, I'm not sure. I have seen LEDs that have a red, green, and blue LED within the same package, but that is a different product.

There is no way to "trick" the eye into seeing white, either the photons being emmitted from the light source have a uniform frequency distribution across the visible spectrum or they don't. If they don't, you will see something other than white.

 

[Mike Wladyka]

http://www.trainweb.org/girr/tips/ti..._led_tips.html

There are other types of "white" LEDs that are made from several different LED chips of different colors assembled into one package. These have not been particularly successful as they tend to change color depending on viewing angle and their color balance is not real good at best.

 

[DaveF]

"White" LEDs typically are a blue LED with a phosphor covering. The phosphor absorbs part of the blue light and re-emits yellow light. The yellow plus blue appears white. This is a problem for viewing colored materials, since a lot of red is lacking and the appearance of colors will be different from what people are used to (compared to incandescennt bulbs or daylight). I don't know if this would be an issue for direct-view images (video).

There are white LEDs in work which use a UV LED to excite multiple phosphors to emit blue and yellow light. There is also research to develop devices which emit a broader color spectrum, for improved color reproduction.

And yes, LEDs are far more efficient than incandescent bulbs. But cooling is a problem because all the heat is generated at the backplane, rather than being emitted over a spherical area as with a lightbulb. They are also substantially more expansive than incandescent bulbs for now.

(I know these few things because the October 23 2003 issue of "OE Magazine" had a few articles about LEDs. It's a publication of "SPIE The International Society of Optical Engineering" and is aimed at the general, educated audience, if you're interested.)

 

[Francois Caron]

http://www.theledlight.com/led-specs.html#White_LEDs

They advertise a 6500K white LED, but I have no idea if the information is accurate. At least someone is trying to make them.

As for cooling the LED board's backplane, a standard heat sink should do the job nicely. Projectors are already designed with an efficient cooling system. As long as the fan blows or sucks fresh air through the heat sink fins, overheating should not be a problem.

What I'll need are LEDs mounted inside SMT type casings. This way, I can pack them in tightly on the same two inch square board. But the question still remains. Will this design produce enough light for the projector?

Another idea would be to use a conventional external light source which transmits the light inside the projector casing via a bundle of fiber optics cables. They don't even have to be data grade fiber optics.

Another possibility. How hard can you push compact fluorescent technology before it explodes?

I'm still thinking here!

 

[Dave Poehlman]

I'll have to look but there was a website (I think I found it someplace in the AVS forum) about a guy who had attempted to build a LED based projector.

The light output wasn't enough to make a decent picture through a LCD panel. And he had used the super bright white LEDs.

I'll see if I can still find the website.

 

[Mike Wladyka]

As for cooling the LED board's backplane, a standard heat sink should do the job nicely.

i wouldn't be so sure about that. for projector sized light from LED's you will probably need a pretty big heat sink plus some pretty good thermal interface material...with both of those plus the projector's fan you should be good to try it out, if you want

try www.lumileds.com for more LED's

 

[Dave Poehlman]

FOUND THE SITE!

It's a very long thread (181 pages!!!) where this guy (vdi_nenna) assembles a LED array and, last I read, it didn't turn out bright enough.

Apparently, there's some new information, however, since the thread is still going on. Perhaps I'll sit down and read it tonight.

 

[Francois Caron]

Dave, great link! That guy did what he could to make it work, but the final result did come up short. My SMT LED based design MIGHT do a better job, but only if the device can output as much or more light than a standard projector bulb. If his initial design was already very dim on a small screen, producing enough light for a screen that could easily be ten times his screen's surface area will require ten times the number of LEDs. That means 480 LEDs at today's standards. Not good.

I'm still studying the possibility of using compact fluorescent technology. Already the bulbs can output a lot of light at a very low wattage level. I have a 13 watt bulb in my desk lamp that outputs as much light as a 60 watt bulb. And it produces so little heat that I can actually TOUCH the bulb without burning myself. Inside a projector, that saving in heat will be of great benefit for the electronics. But how hard can you push a CF design?

One poster mentioned that metal halide bulbs need a 400 to 600 spike to activate the spark in the element's gap. Do projector bulbs work on the same principle? Feeding off the projector's power supply for the bulb might fry the new bulb's electronics if the design is not quite right.

 

[Eric Kahn]

incandesent and flourescent bulbs produce the same heat per watt, 3.414 btu's per watt I think, but since flourescent bulbs produce more lumens per watt, they are more effecient and cooler

I thought most of the newer front projectors used a metal halide bulb, not a halogen bulb, metal halide is an arc light, about 4 times the lumens per watt as a flourescent bulb, but lots of heat due to it being a plasma arc

metal halide bulbs change color over time and small ones explode randomly when used beyond their usefull life which is why the projectors have time counters on them, the bulb exploding would do major damage to the projector

 

[Mark Shannon]

So I take it doing this with LED's is a difficult thing to do. What about Francois' suggestion a couple posts back. Could you use a bundle of Fibre Optic wires with powerful light coming from a seperate source?

 

[Philip_G]

metal halide bulbs change color over time and small ones explode randomly when used beyond their usefull life which is why the projectors have time counters on them, the bulb exploding would do major damage to the projector

from experience they start to do funky things before exploding. In my 2 years as a projectionist I never saw one explode, but I did see them fail to ignite, flicker like mad, and require massive amounts of juice to function.
I don't think they'd damage the lamphouse if they blew. My employer was too cheap to replace them based on the hour counter, which didn't work on half the lamphouses. I think I was the only one in my town that could actually replace a bulb, I had to go to the other theater in town because the MANAGER didn't know how to replace a bulb, and they had nicer lamphouses than we did.

I also run MH bulbs over my fishtank, they're very similar to those used in a theater, the MH bulb is contained within a glass envelope.