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Author Topic: Experiment: Replacing Xenon lamps with a LED array in an old 35mm projector Alastair Bowlie-Evans Film Handler Posts: 34 From: South Wales, Swansea Registered: Jun 2014 posted 08-01-2014 01:06 PM                     The LED I'm using is rated at 3.1A and 33-34V, so a little over 100W--allegedly. I noticed above, that Gary was running his 100W LED from three lead-acid 12v gel-batts in series at 11A..... No wonder it had Horrible light chroma! It was being run at 396W! Wonder it survived that heavy overload!  |  IP: Logged Lyle Romer Phenomenal Film Handler Posts: 1400 From: Davie, FL, USA Registered: May 2002 posted 08-01-2014 02:28 PM                     quote: Alastair Bowlie-Evans the LED is square--so is the gate..... Most high power LEDs (and even the low output) are actually led arrays on a chip. The shape of the LED is square but the light pattern is not. The light pattern is whatever the pattern of each led in the array is with a very small offset for each one.  |  IP: Logged Alastair Bowlie-Evans Film Handler Posts: 34 From: South Wales, Swansea Registered: Jun 2014 posted 08-01-2014 04:18 PM                     Yes--Thats right, The LED is square, and composed of 100 x 1W led chips within the phosphor covering stuff. --The light-guide/pipe arrangement by its nature diffuses this effect, according to the white-papers Ive read. Correct implementation using suitable glass lenses for a condenser and you'll not have any patterning of the chips on the screen, Ive proved that already with the arrangement previously described..... Its only when the implementation is incorrect--like you try and use the wrong focal-length lenses or position of lenses/film/projection-lens, or use Lamphouse mirrors to focus the light that you get the image of the chips on screen and very unsatisfactory light output..... Focus and lens position is everything, just as the projection-lens must be focussed accurately to the film for a sharp image, the condenser lenses and their position need careful alignment for optimum light coupling to the picture-gate, with minimum artifacts.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 08-01-2014 04:42 PM                     the lens that Ziess Ernemann used was called the "Bildfensterlinse"  |  IP: Logged Alastair Bowlie-Evans Film Handler Posts: 34 From: South Wales, Swansea Registered: Jun 2014 posted 08-01-2014 05:17 PM                     Hmm--Picture-window-lens..... In German, its common--it appears, to join words together..... Guess we would call the same part, an Aperture-Lens, although I think the German translation is more descriptive....  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 08-06-2014 04:53 AM                     I don't really get how a lightpipe is supposed to focus the light correctly onto the gate, because the light that enters the pipe at an angle, will also come out at the other end at an angle. Sure, you could capture almost all the light from the LED array with it, but a light pipe itself doesn't focus the light. It might help to sufficiently blur the individual dots of the LED array and you can probably extend the distance between the gate and the light source to a more workable length. But, before you put it trough the light pipe, wouldn't you still need some kind of condenser to "straighten" the light so it comes out as a somewhat straight bundle on the other side?  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 08-06-2014 07:36 PM                     Rather then a light pipe I always thought an integrator as used in a digital projector would be well adapted to use in a film projector. Not only does it direct the light but it also mixes it and keeps the illumination even across an area. Expensive it would certainly be however it should work very well. Mark  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 08-06-2014 11:42 PM                     ^ and this would be a good reason for film to come back into theatres as its medium.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 08-07-2014 02:07 AM                     quote: Mark Gulbrandsen Rather then a light pipe I always thought an integrator as used in a digital projector would be well adapted to use in a film projector. Not only does it direct the light but it also mixes it and keeps the illumination even across an area. Expensive it would certainly be however it should work very well. I've looked at those in the past, but it's almost impossible to get them, unless you take them out of an old projector and they will never match with your condenser lens(es). You may try to work the other way around (get a condenser lens that somehow matches your integrator lens array, if at all possible), but getting hold of the exact focal length of the lenses in such an integrator isn't really easy for someone that lacks the proper equipment.  |  IP: Logged Alastair Bowlie-Evans Film Handler Posts: 34 From: South Wales, Swansea Registered: Jun 2014 posted 08-07-2014 06:33 AM                     This Integrator.... Are you thinking of the flys-eye lenses often found in LCD projectors....? I tried messing round with those in combination with lenses and on their own, but couldnt get much sense out of them. I trashed an old LCD projector for all the glassware inside, but couldn't get anywhere with any of it.... The light-pipe idea--while good and would certainly combine any hot-spots and image of the light producing device would definitely still need a collimating lens/lenses after it.... I thought of improvising one with some mirrored strips, but thought how am I to make them aligned to each other accurately enough--maybe I'll still try this for a larf..... My current experimental set-up uses two large aspheric lenses supplied by the cheapo Chinese LED suppliers, and does a pretty reasonable job, considering. I'm currently waiting for a larger lens of a similar type and F/L, as I'm losing some light at the second lens as its a bit small in diameter, rough estimate of around 30% light-loss at the moment due to spillage. We see how it looks with two 88mm lenses as opposed to one 88mm and one 65mm..... When collimated, --even though the LED is a 'cold' source of light and while not hot enough to burn film, it certainly hot enough to burn your hand,--and thats only running 60W into the 100W LED! (Power-limited at the moment due to bench-PSU voltage-limit, 31.5V--Good enough to prove concept)  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 08-07-2014 02:35 PM                     Some digital projectors indeed use fly eye lenses instead of light pipes. But D.I.Y, a classic condenser is the best you can achieve. Fly eyes need to be computed and made exactly to match a specific projection path. And light pipes are just too expensive. Even with a light pipe, you need a set of mirrors or lenses to couple the light in and out. - Carsten  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 08-07-2014 04:41 PM                     The integrator's in cinema DLP are typically made of pyrex or quartz glass and there are several "mixing chambers" within it that even out the light passing through. I was told that Christie integrator's are actually made of fused glass fiber otics. The Xenon lamp actually gets focused on the input end of this integrator on metal input aperture that is just slightly smaller than the integrator cross section itself. So for an optical system like this to work efficiently the reflector, integrator and lamp have to be designed as a system. This wouldn't be for use with LED but it would work with laser sources. Mark  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 08-07-2014 05:42 PM                     quote: Alastair Bowlie-Evans Are you thinking of the flys-eye lenses often found in LCD projectors....? Yeah, those "lens arrays". The whole process is called "Köhler integration" if I remember correctly. Usually, you'll find two of those lens arrays in any given system. You can see a very rudimentary explanation of it right here. Many modern digital projectors use those, but as Carsten has already explained, they're designed specifically for the given application. A year or two ago as a hobby project, I actually fiddled around with a set of them I got from an old DLP venue projector. I tried to use them in a DIY LED projector retrofit, but I wasn't able to reliably figure out the focal length of those arrays, so I gave up on them rather quickly.  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 11-22-2015 06:19 PM                 It has been 472 days since the last post.  |  IP: Logged Lloyd Bumm Film Handler Posts: 1 From: Norman, OK, USA Registered: Oct 2015 posted 11-22-2015 06:19 PM                     I know this is a an old thread, but I am very interested to learn if anyone has had any success. LEDs are improving so I am hopeful. I can tell you that a light integrator is probably not needed because the LED array has a larger area than a short arc xenon lamp. Because the xenon arc lamp is basically a point source, the light intensity on at the film plane will be brightest at the center fall off toward the edges. The light integrator is used to scramble the light in a specific way to flatten the intensity across the field. A piece of frosted or opal glass will do that too, but at the cost of a huge intensity loss. This would only be needed for an LED if the gaps between the array elements was a problem. And then there are simple types of diffusers that can help. For an LED array the real issue seems to be is there enough light? And how much can be captured and directed toward the film without over filling the projection lens. That is to say, the light from a large LED array can be tightly focused on to the film to achieve high intensity, but if the light is so tightly focused that the diverging light cannot be accepted by the projection lens, it is wasted. (This is the well know etendue problem.) Really one wants to work backwards from the lens to determine what the LED characteristics need to be. I see a number of members have tried this. I am curious what the results were? I have some Eiki SSL-O projectors I would like to retrofit. We are competing with the ELC lamp. Also now that liquid coolers CPU coolers are readily available, it would be pretty easy to incorporate that into the design, if heat were an issue.  |  IP: Logged

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