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Grayhawk & foot lambert calculation  

post #1 of 7
Thread Starter 
In their AVS Special Guest thread, Don Stewart et al say that the Grayhawk is designed for projectors that put at least 11-12 foot lamberts on the screen. He also did a calculation for Lance, where he said that a Davis 450, with spec'd 500 lumens, would theoretically put 13.5 ftl (is that a real abbreviation?) on an 80" x 60" Grayhawk screen. He said realistically, you'd probably get about 10.

Now, I'm thinking about using a Davis DLX650 clone, which is rated at 800 lumens, on a 100" diag (89" x 48") 1.85:1 screen. I'm also considering using an anamorphic lens, vs. not using one. If I use an ISCO, I'll be using all 768 vertical pixels, whereas if I don't, I'll be using about 554 pixels, or 72%. So without the ISCO, I imagine I'll have 72% of the foot lamberts I would have *with* an anamorphic lens.

So now I'll try to get myself in trouble by scaling the 10 foot lambert number that Don quoted. If I take 800 lumens / 500 lumens * 72% * 10 foot lamberts, I get 11.5 foot lamberts. But I don't think that takes into account my wider screen... Do I adjust based on the linear width of the screen, or the increased area, or what? Or am I hopelessly lost? I'm sure it's the latter...

post #2 of 7
Mike here is what you need:

We generally follow the SMPTE standard that you need between 15-20 ft. lamberts of light reflecting of the screen to be acceptable. In order to figure this just plug the numbers into this simple equation.
(Projector Output / Sq. Footage of screen) x Screen gain = Foot lamberts
The Projector Output measurement must be in ANSI Lumens for this to work properly.
For example: Say your projector has a 1000 ANSI lumen rating and your screen is 6'x8'(48 sq. ft.) 1.0 gain. (1000/48)*1.0 = 20.8. If you were to use the new HC Da-Mat (0.8gain) you get: (1000/48)*0.8 = 16.6. Either one is acceptable for this application given there is no ambient light issues. If there are, the 1.0 or even 1.3 might be a better choice.

So in the case of a 800 lumen projector you would get:

without isco 800 x .75 = 600 lumen 600/30=20 20x.8=16 foot lamberts

with isco 800/30=26.5 26.5x.8=21.20 foot lamberts

hope this helps.
post #3 of 7
Thread Starter 
Thanks very much! Yes, that was very helpful.

So if ProjectorCentral says my Davis 650 has 800 ANSI lumens (I guess manufacturer's spec), but then I set up the projector for home theater (i.e. maximum contrast), what do you think my real ANSI lumens will be? Don seemed to take about 75% - does 600 lumens sound reasonable?

post #4 of 7
Noting that light sensitivity is a log function, wouldn't it be true that although 16 fl is ideal, 8 fl would be a barely noticeable difference?

It would definitely not seem like half as bright to the eye.

post #5 of 7
In a perfectly dark room, your eye will adjust so that an 8 f-l picture won't look half as bright as a 16 f-l picture. Add in even a little ambient light, and the 8 f-l picture will look bad compared to the 16 f-l image.

post #6 of 7
I agree with that wholeheartedly, the G90 at the Faroudja booth was putting out 8 foot lamberts (according to Don Stewart, in that same thread I believe) and it looked plenty bright to me in a fairly dark room. Not to mention very beautiful.


Kam Fung
post #7 of 7
Thanks guys, that's kinda what I was thinking. I do have complete light control.

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