LED Projector Post #1
Hi,

I have searched this forum for any recent comments on this subject, but I couldn't find much. Sorry if I overlooked some.

Here is my idea:

Using 8mm WIDE ANGLE 40° 70kmcd White LEDs(sold on ebay)
and

from http://www.3dlens.com/shop/fresnellens.php

to make the lights source of a lcd projector.

I thought of getting 3 such fresnel-lenses, cutting the larger fresnel of, keeping the small ones. Then combining them to a plate of 7x10 little fresnels, about 35x35mm each. That would match my 17" screen pretty well.

Behind each little fresnel, I would place a group of 4 of the mentioned leds(for higher luminance, reducing danger of failure and widening the angle a little: as the focal point of the fresnels is 40mm). Each little fresnel should collimate(condense) the produced light from the group of leds.

Each led would produce ~26.5 lumen. (Using this calculator to convert candela into lumen). Now using 7*10*4 leds, that would make ~7500 lumen. Which is quite nice.

Advantages:
-Low costs: these leds aren't very expensive($7 for 50), 1fresnel plate = $2
-Low power consumption: each led uses 0.1W, even with 300leds, that would still only use 30W
-Less spacy: the leds are placed 4cm from the fresnels
-Less heat production: no more trouble cooling the light bulb, saving even more money



I'm fairly new to building projectors, so my view may be very idealistic . Please correct my starter errors, and give me some advice on the feasibility of this idea.

I could use some help on wiring and powering the leds, as well as on the parts for the rest of the projector(collector fresnel and triplet lens).

The little fresnel lenses are negative 40mm focal length, they make things look smaller. Regular positive focal length fresnels magnify.

Since the little fresnels have a negative focal length, they will spread the light, not condense it. They will have an effect just the opposite of what you want.


Tgreenwood

Hmm good point, damn...

so this is how the little fresnels work?



Would it bypass the problem to put the leds on the corners of each fresnel pointing inwards(towards the focal point)? Or can this be solved in another way... I can't find any similar fresnel panel with little ones that have a positive fl?

Unfortunately that fresnel diverges light (as you understand) but is the exact opposite of what you would want.

I am working on a 10.6" LED panel, and am worried about loss of contrast and washout due to scattered light. I've contemplated a pegboard of sorts that only lets collimated light through, but the light would still have pin hole diffusion so it would be useless I think.

Anchor Optics has something that might work for your idea. The have a 7"x6.75" multi-element fresnel. It has 25 1.5" lenses per sheet. It doesn't say the focal length so it would be worth asking about. They are small so you would need a couple to do a 17" monitor, but they are only $5 each. http://www.anchoroptics.com/catalog/product.cfm?id=24

Maybe that'll help you out.

I mailed and they don't know LOL, not taking the risk though