Measuring Ambient Light For Required Projector Brightness

Bubby4j

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Dec 10, 2012
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North Texas/North DFW
I'm trying to determine how bright of a projector we need in our church's auditorium.

Right now we have 2 Panasonic PT-DW5000U projectors per screen with the image from each projector stacked on the other (we have 2 screens). Each projector is supposed to be 4,500 lumens so we'd have a theoretical brightness of 9,000 lumens per screen.

The screens are about 220" diagonal (16:9 or 16:10), and I think they're Da Lite Tensioned Large Cosmopolitan Electrol screens. I'm not sure what the surface gain is, though I'd guess it's either 0.9 or 1.0. It appears white and not a high contrast gray.

This gives a surface area of roughly 144 square ft.

The projectors are ceiling mounted somewhere around 25ft from the screens. If we got new projectors we could mount them at whatever distance is best, though I think the distance they're at now is pretty good.

The problem we have is that the image quality isn't all that good, and they're not bright enough unless we have most of the lights off. Video is particularly problematic, text is okayish. The lower portion of our stage is wood, and it likes to bounce the stage lighting onto the screens a little bit.

TL;DR
I need to figure out what the ambient lighting on the screen is like in order to properly find out how many lumens we need. To do that I need a light meter, but the decent ones seem expensive ($200 ish).

Is there a place I could rent a light meter for a day, either online or in North Texas?

Right now my feeling is that we need somewhere between 6-10k lumens per screen, depending on the ambient light.
 

Bubby4j

Active Member
Joined
Dec 10, 2012
Location
North Texas/North DFW
There are some free phone apps that should be good enough for this purpose, Luxmeter comes to mind.
This article provides some other choices and some caution about relying on a phone for results:
https://www.dial.de/en/blog/article...artphones-suitable-for-measuring-illuminance/
Good thought. I actually tried using my phone, the problem is that at the lower light levels of our auditorium my phone isn't accurate. It said something like 8-10 lux, which I don't think is correct, and that's with my phone looking directly at the house lights.
 

MNicolai

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Sarasota, FL
Based on what you said, I'm assuming you're talking about that you have dual 4500lm projectors on each screen and they're getting washed out. If that's not the case, please clarify what you have existing.

If you're already at 9000lm per screen, going up to 10,000lm will not produce a noticeable improvement. If it's as bad as you say, you may have to go as high as 15,000lm just to move the needle on this and even that will not cut through that amount of ambient light.

My recommendation before you go investing in large projectors for what is not that large of a screen --
1) Do something with the floor to reduce the lighting reflections onto the screen. Paint it, scuff if, carpet it -- whatever.
2) If you can't mess with the floor, move the lights so they don't reflect onto the screen.
3) Get your room lighting off of the screens if it's shining there at all.
4) Get any sunlight away from the screens -- add shades if necessary.
5) If none of those work for you, move the screens somewhere more favorable where they'll have fewer conflicts with other lighting in the room.

You can spend a lot of money on beefy projectors for something that can be fixed with the room.

Also, before you go down any rabbit holes check your lamp age. Lamp brightness deteriorates over time. If your lamps are anywhere near the end of their rated life, they may be much darker than fresh lamps would appear.

Running some quick numbers. 9000lm per screen would be appropriate for 5.5 foot-candles at your screens.

upload_2016-9-16_9-40-29.png



Moving up to 10 foot-candles would put you at 17,000lm.

upload_2016-9-16_9-42-29.png



This is the kind of thing that gets expensive fast if you try to solve a problem with your room lighting by purchasing larger projectors.
 

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Bubby4j

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Dec 10, 2012
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North Texas/North DFW
I'm not sure that there's really 9,000 lumens on the screens right now - we have a little dinky EIKI projector for our confidence monitor screen which is 4,500 lumens, and it almost appears brighter than our main screens, though the screen it's on is a little smaller and 4:3 aspect.

We change bulbs regularly - more regularly than the life they're rated for. Even when the bulbs are all brand new, it still isn't bright enough. It makes me wonder if there's something wrong with the projectors.

This is another reason I'm looking at new projectors, as the bulbs for our existing projectors are super expensive. Each projector takes 2 bulbs, and each bulb is around $500. That's $4,000 each time we change bulbs!
One option I was looking at is the Epson Pro G7905U - the bulbs are only $200 each, and there's only 1 bulb per projector. Even if we needed 2 per side, the projector cost would only be $20,000ish and replacing all the bulbs would be only $800 - plus we'd have HD projectors with what I assume would be a far superior image quality.

We were actually considering moving the screens, simply because we don't like where they're at right now.

Room lighting and sunlight aren't issues on the screens - the primary ambient light on the screens is the stage lighting.

All of this is why I'd like to actually measure what we have right now - I'd like to measure where we'd move the screen also. It'll help me convince my superiors that we need to move the screen.
 

Morte615

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Sep 21, 2011
Location
Clermont, FL
Look around for camera or film rental places. They use light meters often and you may be able to find one to rent from there.
 
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sk8rsdad

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Location
Ottawa
Instead of the projector, try to get a demo of an Ambient Light Rejection Screen. They're designed to be good at bouncing directed light from a projector back at the eyes of the audience while scattering or absorbing off-axis light.
 

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