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Topic: Formula for video lumens

Richard May
Phenomenal Film Handler

Posts: 1057
From: Floral Park, NY USA
Registered: Aug 2004

posted 05-04-2006 08:57 AM

What would the formula be to determine lumens for indoor video presentation? Would it be the same as 35MM? The formula I mean. [Big Grin]

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 05-04-2006 09:06 AM

With a matte white (gain=1) screen, to get 12 footlamberts on the screen, you need about 12 lumens per square foot of image area. But take "lumens output" ratings with a grain of salt.

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Richard May
Phenomenal Film Handler

Posts: 1057
From: Floral Park, NY USA
Registered: Aug 2004

posted 05-04-2006 09:36 AM

Thanks John. So in general, is there a way to find out how many lumens you would need using an XGA LCD projector?

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 05-04-2006 09:49 AM

If you want a 10 x 8 foot image to have 12 footlamberts on a matte white screen, you need at least 10 x 8 x 12 = 960 lumens. But again, the "lumens" are not aways as stated, and decrease as the lamp ages.

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Barry Floyd
Phenomenal Film Handler

Posts: 1079
From: Lebanon, Tennessee, USA
Registered: Mar 2000

posted 05-05-2006 11:09 AM

So I would need 18,000 lumens to light a 25x60 screen? How to lumens convert and / or compare to a wattage comparison?

Example: 18,000 lumens = ??? watts?

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 05-05-2006 11:32 AM

quote: Barry Floyd
So I would need 18,000 lumens to light a 25x60 screen? How to lumens convert and / or compare to a wattage comparison?

Example: 18,000 lumens = ??? watts?

Yes, 18,000 lumens will light a 25x60 foot gain=1 matte white screen to 12 footlamberts.

The relationship between lumens and watts is not quite that simple. Lots depends on the efficiency of the optical SYSTEM: reflector and its alignment, heat filters, shutter, aperture size, lens, port glass, etc. For example, the f/number of the lamphouse and the lens are critical, whether you are using a two or three blade shutter, and the angle of the shutter opening, do the lenses and port glass have anti-reflection coatings to improve efficiency, etc. For digital projection, great strides have been made from the first models of a few years ago, such that luminous efficiencies are finally approaching that of film projectors.

Remember the old "rule of thumb" regarding lamp wattage for a typical 35mm setup: about 5 watts per square foot of "scope" image area to light a matte white (gain=1) screen to 16 footlamberts in the center. For a 25 x 60 foot screen, you are in 6000 to 7000 watt territory.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 05-05-2006 07:38 PM

If you want 15 fL, does the number in the formula simply change to 15? The other way around, if you divide the (theoretical) lumen by square feet of the screen area, do you get the (theoretical) fL reading you should expect?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 05-05-2006 09:34 PM

The current target is 14fL for Video, BTW.

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Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 05-05-2006 11:48 PM

How come? I thought the reason they chose 16Fl for film was because any brighter and flicker becomes obvious and annoying. There was some testing done a few years ago with brighter images, I seem to recall up to 22Fl, with good results -- the observers thought a brighter picture looked better, except I don't remember how they said they dealt with the excessive flicker that would cause.

So since video has no shutter flicker, you would think they could push the screen brightness as high as they could go, limited only by the amount of brightness the video projectors can produce without setting the nano-mirror modules ablaze. Why did they settle on a figure lower than that of the film specification?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 05-06-2006 08:57 AM

Orignally they went for 12fL since it was compared to a film projector with exposed film running in the gate...that is the brightest scene on a DLP running 12fL equalled a film projector running the same scene if the film projector projected open-gate white light at 16fL.

To my eyes they didn't look equal with the film projector having the edge. I think they moved a bit too quickly to come up with a single measurement that would explain it. How does one's eye integrate the 48-flashes as compared to a meter? Does your eye see the film projector as two 32fL flashes where as the light meter will average it to 16fL? I'm not convinced the eye sees it as 16fL but a bit higher.

So, in a sense when the target was raised to 14fL, I think in perception, the two images started to look more identical. I would consider 12fL to be a minimum spec when the lamp is at the end of its life.

Another limitation in raising the video white light level is that the black level rises too so as technology improved, the ability to put more light out also improved without negative consequences. People really should realize these absurd contrast ratio specs video people put out are just that absurd...there is no 2000:1 video projector out there. Sure, they may be able to put up a black screen and then a white screen in a controlled environment and claim that...try putting up a checkerboard patter that is roughly 50% black and 50% white and read it...it will be down in 100s (and not high 100s either).

If you have the black levels to do it, raising ones white level is the only way to increase contrast since it sets one of the numbers used in the ratio.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 05-06-2006 11:06 AM

I don't think we see two 32fL flashes because of the inertia of the visual systems we have. I don't know what the "official" reaction speed of the eye is, I think it is actually a little faster than 48Hz - otherwise we wouldn't see any flicker at that freq -, but I think it is actually light level dependant, among other factors. I guess at those levels it's somewhere around maybe 60Hz because you really don't see much if any flicker on 3-blade shutter projectors at 72Hz.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 05-06-2006 11:31 AM

quote: Michael Schaffer
I don't think we see two 32fL flashes because of the inertia of the visual systems we have. I don't know what the "official" reaction speed of the eye is, I think it is actually a little faster than 48Hz - otherwise we wouldn't see any flicker at that freq -, but I think it is actually light level dependant, among other factors.

You're right ! You might see if anyone is still around at Showscan. They did psychological studies regarding all that to help them determine what frame rate/light levels were required for the most realistic looking images. I'd say they were dam successful at achieving that!

Mark

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 05-06-2006 12:04 PM

I think it is much more complex than that. 3-wing shutters even give the appearance of more resolution and contrast when in fact the film is less stable with 3-wing shutters.

My perception of brightness changes with screen size. I can accept lower light levels on very large screens yet find low light levels very objectionable on smaller screens.

Just as with Mr. lines/mm I don't think a single number is sufficient to qualify the appearance...it is but one piece to the puzzle. I think more study really should be done one how people perceive projected images. A big objection I have with the current digital images versus film based images is that digital images always appear very lacking in depth...film based has an inherent depth to it. Lines/mm alone nor illumination explains that.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 05-06-2006 12:21 PM

quote: Steve Guttag
I think it is much more complex than that. 3-wing shutters even give the appearance of more resolution and contrast when in fact the film is less stable with 3-wing shutters.

All the high speed pull down movements I've worked with over the years give a similar effect to that. An apparent increase in sharpness not to mention the oodles of extra light. Its very apparent in the ending titles in a film.

Mark

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 05-06-2006 10:25 PM

quote: John Pytlak
With a matte white (gain=1) screen, to get 12 footlamberts on the screen, you need about 12 lumens per square foot of image area.

Measured at what distance from the screen? Footlamberts are not a measure of intensity on the screen, but rather of light reflected from the screen. By definition, it involves distance from the reflecting surface (or am I missing something). Without factoring in distance, I don't think a formula can be used to calculate reflected light readings.

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