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Topic: 2000 watt Lamphouse lumen rating.
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Ian Price
Phenomenal Film Handler
Posts: 1714
From: Denver, CO
Registered: Jun 99
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posted 04-26-2003 04:09 PM
quote:
Definitions:
Foot-Candles
Light a candle. Get a ruler. Stick the candle on one end of the ruler. Turn out the lights. One foot-candle of light is the amount of light that a candle generates one foot away.
That's a neat unit of measurement. Why? Say you have a lamp. You are told it produces 100-foot candles of light. That means at one foot from the lamp, you will receive 100-foot candles of light.
But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light! A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights.
Now, lets get to LUMENS.
A LUMEN is a unit of measurement of light. It measures light much the same way. Remember, a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area.
So, if we take your candle and ruler, lets place a book at the opposite end from the candle. We'd have a bit of a light up if we put the book right next to the candle, you know. If that book happens to be one foot by one foot, it's one square foot. Ok, got the math done there. Now, all the light falling on that book, one foot away from your candle equals both…….1-foot candle AND one LUMEN!
Ok, what comes out of the lens or video projector isn't important. It's what is happening on the screen that is important. A movie screen should have 16-foot candles on the screen. A movie screen can be any size and any distance from the light source. All this affects the light.
I have four 2,000-watt lamphouses from two different manufactures. Most of those screens are 10' x 24'. Using the above definitions and the fact that SMPTE asks for 16-foot candles on the screen. I conclude that for those screens I need a video projector of at least 3,840 lumens. (10x24x16=3,840)This is born out in the fact that a brand new video projector with a 4,000-lumen output seems to light up my screens nicely.
My calculations and assumptions may be wildly off base and I'm no expert. But there you go.
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Dave Macaulay
Film God
Posts: 2321
From: Toronto, Canada
Registered: Apr 2001
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posted 04-26-2003 08:35 PM
Light measurement units give me a headache.
A Lumen is a typical one. It is a measure of light power, in a way. 1 lumen is about 1.5mW. It is (paraphrased) defined as the amount of light falling into one steradian of the spherical area around a uniformly radiating light source of one candela. (my head is starting to hurt)
A candela is the fancy term for what was once a real genuine standard candle one could light with a real match.
The steradian measures solid angles - a floodlight might illuminate 1 steradian but a spotlight only 1/10. The standard candle would light the entire sphere surrounding it (OK, ignoring its own shadow).
A sphere contains 4pi steradians. The area of a sphere is 4pi(r**2) so the area of one steradian is exactly one square radius - the one foot radius defining a footcandle gives one lumen as the light falling on one square foot ... of the sphere's surface though. (throbbing pain now)
The brightness of a perfectly reflecting matte surface of one square foot area which is illuminated by one lumen of light is one footlambert.
This has very little relevance to the question of light output from a 2kW lamphouse, eh?
If I didn't need aspirin, I might be able to figure the usable light output... given the largest screen that can be lit to 16 Ft-L with a 2kW lamphouse, effective lumens should be calculable from the screen area, brightness, and gain. This is not the total lumens from the lamphouse, just the ones used to light the screen after all inefficiencies in projection are applied. Most projectors immediately waste 50% of the light when the shutter is closed, for example.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 04-27-2003 11:46 AM
Distance has no direct effect on illumination brightness.
However there are other issues relating to distance that might affect the efficientcy of the system.
Most lenses are 70.6mm in diameter. As such, by definition, above 141.2mm the f-number (speed) of the lens will be getting progressively slower (and you only get to 141.2 if you could magically have the exiting lens element use all of the barrel diameter...so lets call 140mm the max).
As your distance increases, your focal length for a given format will increase and, in the case of drive-ins, will probably go beyond the 140mm focal length. For Scope, they often use reverse anamorphic lenses to go over 200mm effective focal lengths (EF). Thus, a drive-in using "modern" lenses will be less light efficient.
In the olden days, lens manufacturers had 4" (101.6mm) diameter lenses with fast (f/1.7) optics for using in long throw theatres and drive-ins.
When one is evaluating light and anticipating the results...everything is a factor...starting with the xenon lamp and its design...the closer the arc resembles a point source, the better the efficiently of the system...so all 2KW lamps are not alike. Then, moving on to the reflector...how well it is designed and how well it works with the projection lens affect efficientcy.
As others have mentioned...shutter design can affect how much light reaches the lens. 50% is nominal but there are those that are more efficient and those that are less (3-blade).
Then there are the lenses...there are ones with faster and slower f-stops and those that are more efficient at receiving the light (like ISCOs Ultra-Star PLUS) lenses. However even a lens design can be foiled by a incompatible reflector design.
When it comes to video...we have effectively a shutterless open gate...generally a video system at 12fL will look identically bright to a film system running at 16fL since we measure the film system with no film...at clear film and you will find that the measured light will drop to 12fL, on average.
Steve
[ 04-27-2003, 03:22 PM: Message edited by: Steve Guttag ]
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