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Topic: Formula for video lumens

John Pytlak
Film God

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

posted 05-06-2006 10:28 PM

No, footlamberts is a measure of the light reflected from a surface (like a screen). It is measured with a "spot" luminance meter, as specified by standard SMPTE 196M.

You may be thinking of how distance affects the light from a "point" light source, with intensity (e.g., footcandles) following the inverse square law. With focused light sources, the inverse square law does not apply.

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Tim Reed
Better Projection Pays

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

posted 05-06-2006 10:33 PM

Yes, but when the light is reflected from the screen, it is no longer focused. It is incident.

I know if you take readings at different distances, you will get different values of fL. The specification states (or, at least, it used to in my day) that readings be taken from the center of the viewing area.

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John Pytlak
Film God

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

posted 05-06-2006 10:37 PM

The reason you get different values has more to do with the uneven gain distribution of some screens, rather than distance. Hence SMPTE 196M specifies measurement location.

Measured from the same angle, you should get the same "footlambert" reading regardless of distance, if you are measuring the same area of the screen.

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Tim Reed
Better Projection Pays

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

posted 05-06-2006 10:47 PM

Well, this is a new one on me. Where can I find more info or a dissertation? Obviously, I have missed something somewhere in my early education.

Btw, is that in the old National carbon books?

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Steve Guttag
We forgot the crackers Gromit!!!

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

posted 05-07-2006 10:24 AM

The key here is the same area of the screen. The light meter is supposed to read the same area of the screen but in fact will read more or less depending on the distance since 1-degree, is 1-degree...the further away you go, the more screen you are going to read. As the screen surface is almost never matte-white in theatres (based on actually measuring the screen, not on the manufacturer's names) explains why the measured location can indeed affect measured reflected light.

About the only screen manufacturer that I've measure that hits their screen gain is Stewart which is 4-5 times the cost of conventional screens from Hurley, MDI, Technikote or Harkness. I remember measuring a "matte-screen" at 1.2 gain...not very matte to me. The manufacturer explained it was a "high-gain matte white"...which is like saying it is a bumpy smooth surface. In truth, everyone wants more light and by having a brighter screen they were going to get more sales.

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Tim Reed
Better Projection Pays

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

posted 05-12-2006 04:28 AM

quote: Steve Guttag
the further away you go, the more screen you are going to read

I was mulling this over as I was out on the truck this week. Thanks, Steve.

John, any comments to add? Not that I can't take your word for it, but I guess I would like to know more of where this is "written", if you will (i.e., technical literature). [Smile]

Most of what I learned in this area as a teen was from the aforementioned texts. Maybe I have mis-interpreted other things.

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John Pytlak
Film God

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

posted 05-12-2006 07:53 AM

Some links to light measurement tutorials:

Electro-Optical Tutorial

quote:
The diagram gives just the candela values emitted from the lamp. The designer needs to translate this into light energy falling on an object at any distance from the lamp. The energy density striking an object is given in lumens per square meter, generally known as lux.

International Light

quote:
Online Inverse Square Law Calculator
Use this calculator to find out how an irradiance/illuminance measurement taken at a particular distance from a source will change as the distance to the source increases or decreases.

Oldenburg Tutorial

quote:
Quantity Measures

Luminous flux is commonly called light output and is measured in lumens (lm).
Illuminance is called light level and is measured in footcandles (fc).
Luminance is referred to as brightness and is measured in footlamberts (fL) or candelas/m2 (cd/m2).

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Tim Reed
Better Projection Pays

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

posted 05-12-2006 08:47 AM

Jeepers, you're good! I was hoping you'd come back with something like that.

Thanks, John! I appreciate your time. [Smile]

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