Originally Posted by Shagg
My understanding is that the mathematical calculation from FL back to lumens is not always reliable, because a screen's advertised gain and it's actual gain are not always the same. Unless you really know what your gain is, just plugging in the advertised gain into an equation will frequently give you the wrong lumens. Not to mention that uniformity from the screen is another variable. IE, depending on how/where you measure the FL, you may be introducing other factors from the screen that will impact the calculation.
That's why it's much better to measure lumens directly from the lens, when comparing projectors. FL introduces an extra variable, which isn't always a known quantity.
When comparing the entire system, IE lamp to eyeballs, then FL is the better number. However, this discussion has been primarily talking about the projector as an isolated component, not the end to end results.
You make an excellent point. In addition, even if the manufacturer's gain was as claimed, the actual gain delivered to the viewer's eyes varies based on many factors. Forum member FLboy has developed a screen gain calculator to calculate the actual delivered gain. Listed below are the factors he uses to calculate the delivered gain.
What is the type of screen? Specify A for angular reflective or R for retro reflective.
What is the manufacturer-published on-axis centerscreen gain of your screen?
What is the published maximum viewing angle (to one side in degrees off-axis)?
What is the centerscreen gain at the published maximum viewing angle?
What is the minimum gain of your screen at large off-axis angles, e.g., 60 degrees?
What is the horizontal width of the viewing area of your screen?
How high above the room floor is the center of your screen?
How high above the room floor is the center of your projector's lens?
What is the distance from the front of the lens to the screen? (Measure perpendicular to the screen.)
How far back from the screen is the viewing position? (Measure perpendicular to the screen.)
How far to the right or left of the screen center is the viewing position?
How high above the floor are the viewer's eyes when seated at the viewing position?
How far is your projector offset to the right or left of your screen center?
Trying to back into the value of projector lumens by first measuring foot-lamberts will actual involve a bunch of other variables not just one extra variable.