We are looking to add a second projector and screen. The problem is that is would be located near a stained glass window. Can anyone make any recomendations on a projector that will work well in a fairly bright location? Also what screen surface would work best?

Thanks for any input you can provide.
God Bless
Chris

With a large amount of ambient light, using a rear screen projection setup would be best but if that is not possible then there is a mathimatical way to determine your minimum ANSI lumens need.

1) Determine the square footage of your intended screen (ie. 10' h x 12' w, assuming a 4:3 projection format) So 10' x 12' = 120 sq ft.

2) You need approximately 20 lumens per square foot, so 120 sq. ft. x 20 lumens per sq. ft. = 2400 lumens.

All done? Not yet. The 20 lumens per square foot is assuming a TOTALLY PITCH BLACK environment. Oh... so now what?

3) Time to measure the ambient light level at the screen using a light meter (ft candles). And for the sake of the example we find that there are roughly 5 ft candles hitting the screen. Multiply that number times 5. So 5 ft candles x 5 = 25.

Now we have the number of lumens per square ft (25) needed to cancel out the 5 ft candles of ambient light on our screen.

4) Take THAT new number (25) and plug it in to our first equation (120 sq ft of screen area). 120 sq ft. x 25 lumens per sq. ft = 3000 lumens.

Done. Well, that's the simple version. There's also contrast ratio of the projector and any screen gain to consider, but this will get you headed in the right direction.

Be blessed.

PS There are also formulas for determining screen size as well, so holler back if you need those.

Peace.

OK, so Pete can cut and paste faster than I can. At least you have the screen formula as well.

These other gentlmen gave you the formulas you need. I'm curious but you wrote you were adding a second screen and projector. How will this new one be set in relationship to your first one?

Actually, we were needing more lumens than we an afford.

Thank goodness for refurb!

__________________
Joel Osborn
Milton SDB Church
My Blog

What kind of supreme being would condone such irony? - Burt in Tremors 3

We will be moving our current projector/screen. Now we have one very large screen in the middle of the santuary and we would like to have 2 smaller screens, one on each side so it is less distracting.

Thanks to everyone for all the info and formulas. I have a question about how to measure foot candles. I am a photographer by trade and use a light meter alot. We measure Exposure value. Is this the same as foot candles?

Nope, and you really can't get there from here...

So if your getting an Exposure Value then you must be taking a reflected light reading?

Can your meter be changed to take an incident light reading? Those are usually measured in foot-candles (LUX in the UK (metric)).

If not then let's work off of an assumption...

Light Value (LV) is equal to EV at ISO 100.
A change in LV of 1.0 = a one stop change in brightness; and a change in EV of 1.0 = a one stop adjustment of the camera. Confirming LV=EV.

From there use the following chart to convert your EV reading to foot-candles. (I'm giving your more numbers than you need, but you'll be able to work out the relationship between values for fractional readings.) Just remember that every +1.0 change in value doubles the number of foot-candles.

__EV_____Foot-Candles

0.00_____0.0097
1.00_____0.0195
2.00_____0.0390
3.00_____0.0781
4.00_____0.156
5.00_____0.312
6.00_____0.625
7.00_____1.25
8.00_____2.5
9.00_____5.0
10.00____10.0

Hope that helps you further.

"if all else fails read the directions..." Well I did just that. I use my light meter in the incident mode most of the time and in the directions they have a chart that converts the EV to LUX or FC. I'll take the measurements this weekend and apply the formulas and see what I get. Can any recomend a screen. We currently have a da-lite brand screen. I do not know what the surface is, but see that there are several options - how do I choose?

I went to Da-lites website and went thru their check list and it recomened the Cinema Vision screen. Does anyone have any positive or negetive experience with this product?

No experience other than looking at the viewing angle numbers. The Cinema Vision has a viewing angle of 45 degrees which given that you're moving from a single center mounted screen to dual-side mounted ones shouldn't present any problems, and getting a bit of gain from the screen is a plus.

I think this thread has given the best answer to the how many lumens do I need question and also filling in the blanks on using a light meter. I don't know if I should feel sad or happy for everyone that I was unable to log on sooner to throw an attempt in the answers.