Jerry Maxey wrote on Thu, 27 September 2007 17:20 |
First of all ANSI Lumens are not a direct measure of the light output of the projector, but rather a measure of the light reflected from the screen. From Hiretec "Choosing an LCD Projector": "ANSI lumens is an average Lumen measurement around the screen. This is accomplished by dividing the screen into 9 rectangles (three vertical coulombs and three rows). Take a measurement of light using a good spot light meter 1 foot away from the screen pointing at the center of each of the nine areas. Then you take the average of the nine areas and that is your average (ANSI) Fl (foot-Lambert) light output."
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I agree that it is important to have a correct understanding. However, based on the information I have, what you presented seems to be an incorrect method for determining ANSI lumens, especially as the result is ft-L not lumens, and lumens are actually a measurement of the direct output (luminous flux) and not the reflected brightness (luminance).
According to IEC 61947-1 (the original ANSI standard was actually retired in July of 2003, but the current IEC standard uses the same process), you are measuring the projector's direct light output. According to the standard, you measure the illuminance in lux at the plane of the readings. You do measure at 9 points and you do account for the area of the image at the measurement plane as we want lumens (a measurement of luminous flux) and lux are lumens/square meter. So you measure the illuminance in each of the nine areas (3x3), add them and divide by 9 to get the average illuminance in lux. Then multiply that by the plane image area in square meters to get the luminous flux in lumens. In US units, you measure foot-candles and multiply by the area in square feet.
The important thing here is that you are measuring the light coming from the projector at the plane, not the light reflected off a surface. Measuring the reflected light off a screen would be pointless since it would also include the characteristics of the screen rather than just measuring the projector. I think they maybe your source just got mixed up as foot-Lamberts (fL or ft-L) is a measure of luminance and is the proper measurement for the reflected light, but it seems that it has nothing to do with determining ANSI or IEC lumens.
Edited to cover up, I mean correct, my poor typing skills.