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# How to Predict Screen Brightness

There are a lot of postings in this forum asking how bright their screen will be with a given screen size, projector and screen gain.
Screen brightness is measured in Foot Lamberts. For those of you who want to know here is a guide on how to do it. To give you a bench mark of what a foot lambert is think of it this way. A commercial movie theater runs at about 12 to 13 foot lamberts. A direct view 27 inch TV is about 32 to 36 foot lamberts
.
Its fairly easy to calculate your set up if you know the ANSI lumen output of your projector. The formula is: Projector LUMENS divided by screen SQUARE FOOTAGE multiplied by screen GAIN equals peak white FOOT LAMBERTS.
For those of you who need a little help calculating the square footage
size of your screen here are the formulas.

16:9 or 1.78:1
Diagonal x .87275 = Width
Width x .5625 = Height

4:3 or 1.33:1
Diagonal x .8 = Width
Width x .75 = Height

To figure the foot lamberts in above formula make sure you convert any dimensions that you may have done in inches to feet.

Most experts consider the bottom threshold of a "watchable image" is minimum 8 foot lamberts in a dark room.

Use Caution.....The foot lamberts in this formula are theoretical and in the real world you must consider that the lumen output of your projector will decline with CRT and Lamp aging. If you have a gain screen the maximum Foot Lamberts will be only in the sweet seats.
If you have a lot of ambient light, say 40 Foot Candles on the screen,
A peak white Level of even 80 foot lamberts would give only a contrast ratio of around 2 to 1.

Good Luck

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Don,

Great post! Keep them coming! With a little teaching on your part, we can finally understand how a screen works and what will be the right screen for a specific application.

Kudos!

Lee
Don,
that is a great shortcut for calculating screen brightness. This type of calculation appears easy with fixed panel displays, but is confounding with CRT displays secondary to the large difference between ansi and peak brightness (usually at least a 5:1 ratio). What number would you use when advising an installer on a screen selection?

Thanks
Don O
Dear Don.
I am not sure that I am clear on understanding your question.
I think you are referring to the lumen specifications that have traditionally been stated on CRT projector brochures.. like an 8 inch CRT Projector having 950 peak white lumens. The proper name for that useless specification is called "Marketing Lumens". They are of no use in the science of light measurement.
A quick, but somewhat loose, rule of thumb for CRT projector ANSI lumens are:
5" CRT Projectors equals 110 to 140 ANSI lumens
7" " " " 150 to 180 " "
8" " " " 170 to 230 " "
9" " " " 210 to 270 " "

Of course these numbers will vary from manufacture to manufacture and also will vary from one set up to another.
I do realize that there is a recent model 9" CRT projector that states 350 Ansi lumens but we have not had one in our Lab to test and verify.

Regards,
Don
Don,
thanks for the answer. You understood me completely, and your conclusion is succinct.

Don O
I'll be setting up my first HT over the next couple months in the home I'm in the process of renovating. I'm planning on selecting and installing the screen toward the end of the process after my projector (G-11) is in place so that I can fit the screensize exactly to the maximum projected image I can squeeze out of the space.

I've just begun to familiarize myself with the factors that will govern my screen choice and am trying to get a handle on what constitutes the best image. I found this post very helpful in that it allows me to begin to quantitatively evaluate my options based on information I have. Unfortunately, I don't have the experience to make any decisions based on these numbers. : )

Is there an optimal target number for Foot Lamberts?

My rough calculations suggest my screen will be just about 80x60 or 33.34 sq feet (probably slightly smaller). Given a perfectly black room and the G-11's Lumens, the variable I multiply by screen gain to arrive at Foot Lamberts is ~30. Based on Don's examples above, that seems like perfectly adequate brightness on a 1 gain screen. Is there any reason to go any higher than that? Lower?

There are two windows in the room where I will be setting up over which I plan to mount lightblocking shades. It won't be perfect ambient light control... any guess as to how much that might effect my image brightness?

Would one of these gray screens I hear so much about be something to consider under these conditions? (If it matters, I'll be using the setup as much for doing graphics work with my PC as watching movies) Unfortunately, my understanding of theory hasn't quite crystalized into understanding yet and I haven't quite gotten a handle on how gray screens work... if anyone has a pointer to a post that explains how gray screens with different gains effect the projected image I would appreciate it.
tdemelle,

As Don Stewart said in his post, "commercial movie theater runs at about 12 to 13 foot lamberts". That is a good starting point for a dark room where you have almost total light control. Your light blocking shades will definitely help a lot, at 30 ft. lamberts you've got a REALLY bright image. That's regular tube TV bright! You could conceivably watch it in daylight (although your black level would suffer for it...). Your problem may be that it is TOO bright when you close off ambient light, I had that problem with my current RPTV before calibration. Bright scenes were literally painful to my dark-adapted eyes.

You might consider a 16*9 screen of the same height, you G11 should be able to handle it easily. Even without an anamorphic lens, you could get as much as 16 ft. lamberts out of the G11. With an anamorphic lens you could get more than 20 ft. lamberts even with the loss of brightness from an added optical element.

At 30 ft. lamberts, your black level will also be rather...un-black. If you get your projector calibrated (to gain some additional contrast) and throw a black border around your screen this may not be noticeable. A gray screen will also help by making your "blacks" darker, this will similarly lower your white level as well (but you have plenty of that...). I would also think that using a gray high gain screen would be counter-productive, the two would work against each other and you would also get hotspotting!

Hope this helps!

Kam Fung
Kam,

The 80x60 screen is at the maximum throwdistance for the G11 and my understanding is that the limiting dimension is the width... so I wouldn't be able to get anything any wider than that at 16x9 as far as I know. 80x60 fits nicely in the space anyway, and as I am using the system primarily as a computer display, it makes the most sense.

Once I have the projector installed, I think what I may do is experiment with different shades of gray (painted on a piece of masonite or something) to see how much I can effectively drop my black level in the space without bringing my whites down too much. It'll be a couple months before I can do that... and in the meantime I'll be watching this thread for info and products.

Thanks again!
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