# Culminating light from LED array



## abishur (Feb 11, 2015)

I'm in the research phase of building my own LCD projector. I'm using a 10.1" LCD panel, and want to use a split Fresnel design. So the arrangement of the projector would be


LED array ---> Fresnel -> LCD Panel -> Fresnel ---> Triplet lens


The issue is that the more I read about LED arrays the more I realize that rather than being one point of light, such as the Fresnel lens is designed for, they're 100 points of lights each shining at 1w or power. What can I do to minimize light loss from the LED array to the fresnel lens? I've been looking at placing a convex lens over the array in the hope that would help narrow the viewing angle of the LEDs down to around 40 degrees, but would that actually help?


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## idleprocess (Feb 22, 2015)

Sounds like you're doing a Lumen Labs style projector. Last I looked at their design a few years back, it involved a MH lamp _(I forget if it was 100 or 400 W)_ with a parabolic trough reflector that oriented much of the light in a semi-collimated fashion, then used another optic to get it all lined up for the trip through the LCD panel. I would think that individual LED optics would get the job done, although those would be rather inconvenient. Perhaps you should look at fewer, more powerful LED's such as Bridgelux or Cree arrays, which have aftermarket reflectors that can get the light down to < 90 degrees.


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## SemiMan (Feb 22, 2015)

Small high intensity array maybe even built from Luxeon Rebel ZES is best bet.

Its a fun project but keep in mind your results won't match even the cheapest off the shelf projectors so you may want to temper how much you spend.


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## abishur (Feb 26, 2015)

SemiMan said:


> Small high intensity array maybe even built from Luxeon Rebel ZES is best bet.
> 
> Its a fun project but keep in mind your results won't match even the cheapest off the shelf projectors so you may want to temper how much you spend.



Hi SemiMan, could you expound on this statement? There are a lot of cheap projectors out there that are low resolution and hard to see in any amount of light. For example I've seen a $30 projector that puts out 400 lumens at a 320x240 native resolution. 

But the panel I'm looking at is a full 1920x1080 native resolution, those kinds of projectors tend to start off near $700. So I think I can definitely top that $30 paperweight, and I'd like to think that I can match the lower end projectors that do a native 1080p resolution.

The limiting factor, from what I can tell, will be my light source. A bad light source will make this look bad, but if I get the light source worked out then I'll have a very decent projector. Ideally it would work as a low-mid grade media projector, but really I just need to to do presentations on at a true 1080p resolution to match the graphics being designed for clients. It gets really old to go show a client how their system will look at the resolution they've asked for only to find that they're projector is a 1024x768 projector! 

That's why I've come asking about these LEDs. If I get them done correctly then I could potentially make a nice projector for around $200


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## SemiMan (Feb 28, 2015)

You won't make anything decent for $200 and making anything decent period is tough. I don't consider anything for $200 a projector ... More a toy, but there are very good 1280*720 in the $500 range and even 1080p for $600 that are really quite good.
When
Getting good uniformity, contrast and even brightness is difficult. Commercial projectors have an efficient optical system, multiple panels for efficiency and or special color wheels and still only get 10% transmission. If you get 5% that would be good. 

If you temper your expectations and expect to spend more then no worries, but you will not be as good as the worst mid end projectors on pretty much any performance metric.


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## Anders Hoveland (Feb 28, 2015)

I was thinking about this subject, and realized that culminating the light from an LED array is geometrically much more challenging than conventional light sources. There are well established designs to focus a point-like light source, and fluorescent tubes are linear so that just involves bending a flat reflector to shape. LEDs are point-like, and inherently emit very directional light, but an LED array can involve a very large number of separated point-like light sources. This can be difficult to focus with any single or simple design. For higher power applications, LEDs tend to do better in arrays of multiple emitters with ample space between them. This is not very conducive to reflective culminating though.


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## abishur (Feb 28, 2015)

Anders Hoveland said:


> I was thinking about this subject, and realized that culminating the light from an LED array is geometrically much more challenging than conventional light sources. There are well established designs to focus a point-like light source, and fluorescent tubes are linear so that just involves bending a flat reflector to shape. LEDs are point-like, and inherently emit very directional light, but an LED array can involve a very large number of separated point-like light sources. This can be difficult to focus with any single or simple design. For higher power applications, LEDs tend to do better in arrays of multiple emitters with ample space between them. This is not very conducive to reflective culminating though.



This is definitely the crux of my dilemma. It's why I was hoping someone here knew of some device that took an array of LEDs and focused them to a point (or as close to a point as one could get it). Honestly one of the things I've thought about is taking a third Fresnel lens, acting as a collector and placing it in front of an LED array. The idea being that the collector would take all the disjointed LED sources and bring them down to a single point. So it would look something like

LED array ----> Collector Fresnel ----> Collimator Fresnel ---> LCD Panel ---> Collector Fresnel ---> Triplet Lens

But I don't know enough about the actual math behind these Fresnel lenses to know if this would work like I think it would, or if I'm being silly.


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## Anders Hoveland (Feb 28, 2015)

As long as you are not using cheap Chinese LED chips, many LED emitters (from reputable brands) are already fairly directional (though more expensive of course). They make 10 Watt LED emitters with narrow (45-60°) beam angles, thanks to the silicone dome, and sometimes additional lens optics built onto the board.


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## idleprocess (Feb 28, 2015)

Anders Hoveland said:


> I was thinking about this subject, and realized that culminating the light from an LED array is geometrically much more challenging than conventional light sources. There are well established designs to focus a point-like light source, and fluorescent tubes are linear so that just involves bending a flat reflector to shape. LEDs are point-like, and inherently emit very directional light, but an LED array can involve a very large number of separated point-like light sources. This can be difficult to focus with any single or simple design. For higher power applications, LEDs tend to do better in arrays of multiple emitters with ample space between them. This is not very conducive to reflective culminating though.



Could always use 2D trough reflectors with lines of LED's for a similar effect to floro tubes.


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## SemiMan (Feb 28, 2015)

abishur said:


> This is definitely the crux of my dilemma. It's why I was hoping someone here knew of some device that took an array of LEDs and focused them to a point (or as close to a point as one could get it). Honestly one of the things I've thought about is taking a third Fresnel lens, acting as a collector and placing it in front of an LED array. The idea being that the collector would take all the disjointed LED sources and bring them down to a single point. So it would look something like
> 
> LED array ----> Collector Fresnel ----> Collimator Fresnel ---> LCD Panel ---> Collector Fresnel ---> Triplet Lens
> 
> But I don't know enough about the actual math behind these Fresnel lenses to know if this would work like I think it would, or if I'm being silly.




Sorry, but such a device as you are thinking does not exist.

If you have an array of LEDs, then you have a bunch of points --- spreading light everywhere. The only way to collimate with with a huge lens. There is no way to get around that The best results are with as much power in as small a space as possible.

Learn about Entendue.

Semiman


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## Anders Hoveland (Feb 28, 2015)

Depending on how tight a beam you want, you might investigate higher power laser diodes. These are surprisingly similar to LED emitters.
Contrary to the idea you may have when you hear the word "laser", these laser diodes generally do not, by themselves, emit in a parallel culminated beam, but rather just have a narrow angle of light emission.
(And yes, they do make white laser diodes) Not sure how practical this would be for you, though. High power visible laser diodes are still fairly expensive (though the prices have gone down over the last few years).
You might have to wait a few more years on that.


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## more_vampires (Feb 28, 2015)

Anders Hoveland said:


> (And yes, they do make white laser diodes)



Link? I thought it was multiemitter entangled to hack white. I'd really like to order a discrete white laser diode. What wattage?


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## Anders Hoveland (Feb 28, 2015)

Many LED spotlights use a clear plastic or acrylic collimator, something like this:
http://www.ledtuning.nl/en/shop/pro...es/led-lense-collimator-45-degrees-triple-way
http://www.sunshineopto.com/china-2...lens_for_3_1_watt_bridgelux_chip-1982174.html


Individual reflectors can also be used on high power LEDs
http://ru.aliexpress.com/item/5pcs-...L-XM-L-LED/1070520218.html?recommendVersion=1


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## SemiMan (Feb 28, 2015)

Big difference between a simple lens and something suitable for projection though.

Posted by really crappy Tapatalk app that is questionable wrt respect of personal data.


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