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This topic comprises 4 pages: 1 2 3 4
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Author
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Topic: Experiment: Replacing Xenon lamps with a LED array in an old 35mm projector
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 08-26-2012 08:53 AM
quote: Gary Wachter
I actually experimented with a 100 element LED array last year. The results were less than spectacular. Pretty much of what the earlier posts state is true.
It is great to see somebody actually trying this in real life ![[Wink]](wink.gif) .
I have some questions though:
- It's a LED module of 100 x 1 Watt LEDs, is that correct?
- What was the specified color temperature of this LED array?
- How do you drive the LED? With a specific LED driver or just a regular DC power supply?
quote: Carsten Kurz
Also, with LEDs you don't need the mechanical shutter, and instead flash the LED. That also allows you to get closer access to the film-gate.
That's actually true, but you would need to find a reliable synchronization source for that.
quote: Carsten Kurz
The light output of an LED does not need to be even a fraction of a xenon if used properly - because with an optimal lightpath, a lot less light is wasted.
Well, I guess you could do with just a single condenser lens and a small reflector. Those LED arrays come in squares and rectangles. If you could find one that matches the aspect ratio of the gate, you could essentially focus it directly on the gate, this way your light loss should be at an absolute minimum.
But I'm curious if you would see some artifacts from the "dots" in the light source in the final projection.
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Lyle Romer
Phenomenal Film Handler
Posts: 1400
From: Davie, FL, USA
Registered: May 2002
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posted 08-26-2012 11:10 AM
quote: Monte L Fullmer
...along with the Kelvin temperature being up in the 8K area .. you'd have to color time your prints to compensate..
Not just the color temperature but you have to compensate for the spectral peaks and valleys.
quote: Scott Norwood
Do full-spectrum LEDs exist now? Last time I looked into such things, "white" LEDs did not provide reasonably consistent output across the entire spectrum of visible light, which would make color prints look odd.
No they don't. The closest I've seen is CREE makes some 95 CRI LED bulbs but they are nowhere near the lumen output of a 2k xenon.
quote: Marcel Birgelen
But, on the other hand, LEDs are being used for LCD/TFT backlight purposes too those days and that seems to work pretty well. The first LED driven backlights looked far to blue, but I'm currently looking at one and the color temperature is adjustable and OK.
The reason that LEDs (and flourescent for that matter) work as backlights for LCD displays is that the spectrum of the backlight is known so the LCD can be calibrated to produce the desired output. If you took an LED backlight and used it to light your room, the color rendering would not be nearly perfect.
Then again, we are being forced to used CFL bulbs that don't produce very "white" light even thought they are much better than they used to be. I guess at some point using the technique that CREE uses, LED bulbs might get very close to incandescent spectrum. They basically mix white LEDs with red in an array and combine the light together.
LEDs have a long way to go to be able to be used for high power applications. If they could do it, they would be using them in stadiums to replace the 1500 watt metal halide floods.
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Jonathan Bodge
Film Handler
Posts: 83
From: East Dorset, VT
Registered: Nov 2006
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posted 08-26-2012 01:19 PM
My boss man sent me this a few weeks ago...
Salem, NH—Apr 11, 2012
Laser Light Engines and SONY Present First Public Demonstration of
High Brightness Laser 3D Cinema on a Silver Screen
NAB technology preview delivers on promise of high performance
laser illumination
Laser Light Engines, Inc. (LLE), a venture-backed,
laser-illumination developer today announced the world’s first public
demonstration of fully-despeckled, high brightness 3D, high frame rate
(HFR), wide color gamut (WCG) laser projection on a silver 3D screen
And:
Christie Hosts Cinema Industry Heavyweights at First-Ever High
Frame Rate (HFR) Laser Projection Demo
Prototype Christie Laser-Engine Projector Shows James Cameron HFR
demo reel and generic clips
Christie® hosted a high frame rate (HFR) Summit yesterday at
Christie’s worldwide center for engineering, research and development
in Kitchener, Ontario, touring cinema-industry luminaries through its
manufacturing facility and providing the first-ever laser projection
of high frame rate cinema and alternative content on to a 30-foot
white screen.
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Alastair Bowlie-Evans
Film Handler
Posts: 34
From: South Wales, Swansea
Registered: Jun 2014
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posted 08-01-2014 06:27 AM
I know this is an old thread--but Ive been doing some experiments along these lines.
A few observations and experiments showed pretty early on that using a conventional lamphouse mirror and optics isnt going to work with an LED light-source, as they are designed for a point-source.
I believe the correct description of the LED lightsource is Lambertian, meaning it has a very wide field of view.
Couple this with the fact that the light is derived from a square say 20mm in size means some other different type collumation is needed.
Ive tried some cheap 80mm glass aspheric lenses from ebay with good results.
Usually condenser lenses need to have their convex sides facing each other. It seems that this doesnt work out for these lenses and the LED.
Having both convex facing the screen, the LED at 1cm from the first, second lens practically touching the first, and the film plane at 3.5cm from the convex of the second lens gives the best brightness with no image of the light-source chips on the 100w LED.
The placement of the film at 3.5cm is pretty critical. This is the point (In my set-up) of maximum collumation (brightness) of the light-beam.
This however presents issues with many projector mechs due to the shutter distance from the gate and the way the projector is made.
Some machines--Like the Philips/Kinoton the shutter is pretty close--actually the furthest point away is 2.7cm, so for this we can easily mount the LED and improvised condenser and satisfy the focus requirements.
Ive done a fair bit of research (Googling!) into condenser design which describes the requirements of having the film plane at the focus of the second lens, and the image of the light-source at the Nodal Point Inside the Projection-lens for maximum light and even distribution.
Philips used a pulse-lamp in their SP20-S machine--It was a tiny quartz glass tube about 4mm diameter and about 5cm long,--the light production area in this tube was about 1cm long. It was held in a water-filled 'lamphouse' and current return for one lamp electrode was through the coolant water. --Good failsafe idea, no coolant, no lamp light/heat.
It was driven at 72 Hz and synchronised by three cut-outs in the Intermittent flywheel and picked up by a taco generator arrangement. This drove a 'rectifier' full of valves and spark-plugs that sustained the lamp at 800W in sync with the intermittent movement.
Doing something similar with a 100W LED will enable me to reduce heat dissipation--Or increase the light output, as its heat that kills the LEDs.
Theoretically I could double the effective drive due to the 50% duty-cycle, although in real life I could probably only manage around 1/3 more light with reasonable reliability
Something to keep in mind for driving an LED--Dont just connect it to the required voltage source, They are constant current devices, meaning that a tiny increase in voltage can mean a huge difference in current flow--which could well make the LED operate well outside its max ratings, with colour shift and very short life due to excess phosphor heating.
I plan some experiments along these lines, but will have only two 'on' cycles per frame/24th second, to maximise the light transmission.
My experiments so far at only a couple of watts dissipation from the LED while designing/messing with lenses etc show a fair colour rendition--Sure nowhere near 'Academy' Standard but reasonable for my home cinema....
At full power--the spectral response of the LED may change from what I observed at 2-10W. Its allegedly a 6K 10,000Lm 100W LED of cheap Chinese origin. If these experiments prove worthy, I can replace this device with one of the latest 5600K 98 CRI 100/150W LED's from a reputable manufacturer.
Experiments continue....
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 08-01-2014 06:55 AM
Most colour conversion substances ('phosphors') in these 'white' LEDs nowadays have some 'inertia'. While the LED itself can be clocked at very high rates and reacts quickly, the color conversion layer will not follow. It's a bit like with incandescent lamps, you can switch/drive them at 50/100 Hz (or 60/120 if you like), but due to the 'heat inertia' of the wire filament, it will give a near even light output.
Now, that doesn't prevent you from driving the LED with an electronic shutter, but when you look at the datasheets of these high-power LEDs, you will usually notice that the allowed current in pulsed mode is not much higher than in continuous mode. It still is a valuable goal to try it, even if it only yields some 10-20% improvement.
Interesting stuff about the condensor design. This is so far the best description I have seen. A friend of mine put a medium power LED source into his Super8 projector, and without any real adjustments, reached light output near a common tungsten bulb. So, there is room for improvement, and it also allows interesting applications like flicker-free 18fps or silent movie projection, still projection without the need of reduced lamp power, etc.
These 100W LEDs are available on ebay for very little money. Philips lighting has some very good 50W modules with CRIs of >90, and we're certainly not yet at the end of that development.
- Carsten
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Alastair Bowlie-Evans
Film Handler
Posts: 34
From: South Wales, Swansea
Registered: Jun 2014
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posted 08-01-2014 09:52 AM
Ah Carsten, Thanks for the compliments, much appreciated.
I know very little about Optics generally, all the new terms like 'Etendue' and 'pupils' I see in various optical tekkie stuff goes right over my head!
I did find a page intended for photographic enlargement,--which is basically a glorified projector anyway, which had some interesting stuff on it here--
http://www.durst-pro-usa.com/world_images/specular_light.htm
It gives perhaps the best info Ive found that is reasonably easy to understand, and will apply no matter what the source of light actually is.
As to the 'persistence' of the LED Phosphors, I hadn't considered that issue--but stands to reason there is persistence--seeing the phosphors are probably similar to those found in TV CRT's and oscilloscope CRT's....
Not sure if it works both ways, ie- time it takes to full brightness versus time to fully dark when switched...
I might need to build in some kind to 'timing advance' for the operation of the LED into the driver circuitry, or the mechanics. Shouldn't be difficult to overcome....
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