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Author
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Topic: Outdoor screening illuminance
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Edgar Prass
Film Handler
Posts: 32
From: Tartu, Tartu county, Estonia
Registered: Mar 2013
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posted 05-18-2013 01:56 PM
Greetings,
In mid-summer we are going to do 3 outdoor screenings with our Ernemann 15 projector. As I'm quite new to the projection world I don't really know what to expect or how much lamp-power would I need. Projector has a 4kW bulb, (76,2mm) f1,9 lens for flat and 6 inch (152,4mm) f2,4 anamorphic lens for scope. Projection distance is about 28 meters. Screen is 7,8m wide. When in indoors the 4kW bulb for that screen seems more than enough. But the problem is that here in Estonia it doesn't really get dark at night in mid-summer due to being quite close to the polar circle. Sunset is at 22:15 at sunrise already at 5:00. In between is a twilight when the sun is just a bit below horizon.
Screenings can't start before 22:30 anyways. Is 4kW enough for 7,8m wide screen in such twilight conditions? Is there some reasonable way to do some calculations for it? Or does someone have personal experience?
For past few years I have been working for another outdoors film festival happening in similar twilight conditions, but not as a projectionist. There I know they use a 7kW bulb in a 35mm projector, but the screen is 20m wide and 90m away from the projector. Just a comparison. Really need some advice and tips.
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Frank Angel
Film God
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 05-18-2013 04:21 PM
Unlike indoor configurations where the environment is much more able to be controlled, outdoor setups encounter many more variables that are beyond your control. The amount of ambient light from the sky, and this not only included the position of the sun on the horizon, but also how much light from the nearby town or city, the road lighting, where the moon is at at showtime, etc., that is reflected from the atmosphere back down to your screening area. This can change from time of year to time of night, where the moon is at, none of which are within the theatre operator's control.
In our park setup the amount of light that reflects from our city is remarkably high. One thing you can do, is to make sure your screen faces away from the horizon. We were lucky in that the bandshell where our screen hangs does face away from West. But the reflected light from the sky is omnidirectional -- it is what it is. You have to compensate with the amount of light you can throw on the screen. But even then, blacks will never be black, and you need to understand that simply is the nature of the outdoor venues -- they are a beast.
As for the bulb size, there you don't have much of a choice. You have a 4.5Kw or a 7Kw upgrade. But the thing with that is, 7Kw will not give you the kind of light output you might image because the lumens do not increase linearly with the watts. Before going for a wattage increase, make sure your alignement is spot on, everything is clean and you are getting the most out of your lamphouse.
My suggestion -- the best way to know for sure what you are dealing with in the way of light in the "field" would be, if you can, set this thing up well before your first screening and just assess it in a real-life setting what your image looks like. This will tell you where you need to be...if you need a bigger bulb or if your 4K will do.
Depending on the seating -- are the isles wider than the screen) you could get better contrast using a silver screen like they use for 3D -- it might be a weapon against ambient light. Silvers will not reflect light that is not aimed directly at it (ambient sky light is not) and thus won't catch as much of the sky light as would a matte screen; you'll get better contrast. You can also play with tilting the screen downward so the projected light is aimed more accurately at the audience seating area, again giving more of the image light back to the audience. The trade-off with a silver screen is that there would be that hotspot which silver screens have as well as fall-off for any viewers sitting off to the sides.
We have a 55ft scope screen and we use 4.5Kw bulb in an area that has fairly high ambient light. It is a pearleasant high gain screen which has some directional reflective properties and that helps. We also hung it a number of feet back into the bandshell (which has a roof) and that does shield it from some ambience from above. In the end, I don't get blacks that I would like, but not distractingly so. The picture looks decent....I know, that's not a scientific measurement and sloppy for my liking, but I look at the image and am comfortable that the audience can enjoy a movie with our setup. I think you will know right away if you to a trial screening if you will be satisfied or not.
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Tomorrow is the day of first public screening. Did some testing yesterday night without proper screen.
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