So I was wondering if LED lighting can work in a PJ application. What if I used a 0.2 groove pitch fresnel in front of the leds and placed it very close. It just seems to me that if this works then it's the best of all worlds. It's energy efficient, low heat, and could be placed closer to the first fresnel decreasing the overall footprint of the PJ. I am extremely interested in doing this and would like alot of feedback on whether y'all think it would work.

I don't believe they are quite up to the task yet. They will work but nowhere near as much light will be on screen compared with a MH lamp.

DJ

But why? How many direct lumens does a MH lamp put off and how many lumens are lost on the way to the screen. Some companies manufacture LED arrays for street light use that they say are rated at 600+ lumens but I don't know how much will be lost on the way to the screen. Also is the quaility of light from an LED going to be better or worse?

well, lets put it this way..to get 400 lumens to your projection screen lets work backwards...
the triplet is 76% efficient(a clced value based on 3 lenses with some taken off for AR coatings I believe) so you need about 525 lumens hitting the triplet... most lcds are 6% efficient so you need at least 8750 lumens to hit the lcd screen... and if you use a fresnel in front of it(replacing the MH light engine with an LED array) , fresnels are about 92% efficiency so basically you need 9500 lumens to hit the lcd from whatever source you want(LED,Mh etc) ...now you can argue this a little , but that gives you the ballpark anyway...the biggest difference could be in the LCD trans percentage..... 6% is an average, 8.5% is doable with a few panels..... so figure 7-10,000 lumens into the lcd
edit: as for quality, if you use white LEDs, it depends on the spectrum they put out....

Someone on this board has posted a result for an 896 LED array powered projector. I think they were even selling PCBs for the light array.

Yes, it can work. The LEDs will cost you more than the MH lamp for that kind of a project, but it can work.

As to heat... MH lamps are more efficient in terms of lumens/watt than LEDs. That many LEDs WILL get hot, so be prepared to deal with that. The only advantage that the LEDs have is that it's (more or less) directional light, so you can get away with a lower wattage source and still make up the light.

It's a lot of work to solder up such a tightly packed array, and replacing any failed lamps (And you WILL have lamps fail when they're that tightly packed and there are so many) is difficult and time consuming.

Other than that, it CAN be done. What you will have to determine is if it's worth doing FOR YOU.

actually I was just reading that some company(maybe I saw it on digg?) has developed 135 lumens per watt white LEDs (not in production yet) so the LED parade is closing in on feasibility (MH is about 90-100 lumens per watt I think).... now , when they get the cost down...

Thanks for the great information! I guess it's just not bright enough yet. It never even occured to me that the LCD would be so opaque but I guess (DUH!) that's the whole point. Again thanks for the great breakdown.

well, they range quite a bit.... check out this thread for some lcd models we've tested...
http://www.lumenlab.com/forums/index.php?s...c=10164&hl=
however, manufacturers change panels in mid stream sometimes so consider these suggestions/probablilities rather than absolute fact

Coming from the guy who posted the original 896 led thread. "The projection will only be about 25-50 lumens"

Enough said.