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Old September 16th, 2005, 10:52 PM   #1
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Lighting greenscreen with green light- does it help?

Random green-screen lighting question:
If you hit the green-screen with green light, then the light hitting the green screen should be more saturated.
Why: If you were hitting the screen with white light, then it'll reflect some red and blue light. This makes the green screen less saturated.

So if that's true, why not light the green screen with green lights? Use plant lights (typically with more of a green component than other florescents I believe) or other extremely low CRI florescent lighting (which should be green). Possibly gel the lights so that the gel gets rid of lots of red and blue.

Last edited by Glenn Chan; September 17th, 2005 at 12:31 AM.
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Old September 17th, 2005, 12:01 AM   #2
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Yes. This is standard practice.
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Old September 19th, 2005, 09:34 PM   #3
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That's good to know Charles. I don't see many people recommending lighting a green screen with green light.
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Old September 20th, 2005, 06:58 PM   #4
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Not to be contrary, Charles, but I have never seen a greenscreen lit with green light. Don't see why you couldn't do it, other than losing output by putting a gel over your source, and usually you want all the output you can get from your greenscreen lights. For example, if you used something like tough plusgreen, you would lose about 1/4 of your output.

Obviously, this would all apply to bluescreen as well.

But hey, I'd be happy to learn a new trick.

Wayne
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Old September 20th, 2005, 07:38 PM   #5
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I suppose someone can test this.

You'd have to figure out what usage to test it for...
A- Professional usage, where you pay $45/gallon for chroma green paint.
B- Amateur/hobbyish/Scrooge usage, when you just use green paint.

And probably then just key the results using "normal" keying methods (software or hardware). Nothing fancy, because that would just hide the difference.

I don't think I have the resources (lights and/or gels) to test this out.
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Old September 21st, 2005, 12:16 AM   #6
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No need to use green light. Yes lighting greenscreen is about hue not luminance but you don't need much light to make a green screen green. Regular light will do fine.
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Old September 23rd, 2005, 04:16 PM   #7
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Is it a good idea to backlight your subject with the complementary color (I think it's auburn?)

Just to create a seperation, and to get rid of any bounced green light that your subject might have on it?
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Old September 23rd, 2005, 09:39 PM   #8
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Quote:
Originally Posted by Bryan Mitchell
Is it a good idea to backlight your subject with the complementary color (I think it's auburn?)

Just to create a seperation, and to get rid of any bounced green light that your subject might have on it?

You backlight your subject based on what he backplate will be. If it a moon shot then a blue backlight simulating the moon.
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Old September 24th, 2005, 12:47 PM   #9
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I used this discussion as an opportunity to research the issue, and although I was basing my knowledge on the experiences I had had both working on large feature sets where the greenscreen lights were gelled green and from successfully employing the same technique, I see that there are different schools of thought on this (read this for a taste, on both the green light for greenscreen question and the complementary color for backlight question). What I think we can all agree on is that the more saturation in the green (up to a point), the easier the key can be pulled. Adding green will indeed knock down the intensity of your lights, but this is only a problem if you are working with units that are just up to the task to begin with or have limited number of units (which well may be the case with a DV scaled lighting package).

As to whether this is necessary or not may depend on the compositing software you are using. I also wonder about the application of a color correction filter just before the key that is targeted to specifically increase the saturation and/or adjust the luminance of the green information in the image to achieve the same result...?
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Old September 24th, 2005, 02:08 PM   #10
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Also curious:
I notice on some DV cameras, increasing the iris increases overall saturation levels.
You can look at the image on a vectorscope and the trace/blob/whatever will actually get bigger and things will move around.
*I have no idea why this happens. Gonna do a little more experimentation when I have time.
Some information at http://www.glennchan.info/video/exposure/exposure.htm

If you want to light the green screen dark (to minimize spill), some extra saturation in the green may be useful??

I'm not sure if that makes a practical difference. I certainly need to test it out.

2- If you can get a really saturated green screen, how dark could/should you light it?

(Ambient light hitting the green screen may be a factor, as it would lower the saturation of the screen.)
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Old September 24th, 2005, 04:32 PM   #11
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Of course there is a bit of wanking in this thread, since dv is not the optimum format for green/bluescreen work. That said...

I talked to a few people about the use of colored lights, and the general consensus seemed to be that colored lights are more popular in film compositing, but not really useful for video composite work.

So, for dv work, the overall saturation level isn't as important as even lighting on the background, preventing spill, and a proper ratio between subject and background.

To respond to Glenn, I don't know what the advantage is to increasing your iris to increase saturation. Your iris setting should be based on the look you want on your subject and your background level. There are different schools of thought on the ratio between subject and foreground. Ultimatte suggests a 1:1 ratio, but the more popular settings would be to light the background at 1 to 1.5 stops below key. Certainly never have the background hotter than the subject.

Charles link has some excellent suggestions, although I am not a fan of skypans, as one poster suggests. If you have that kind of money and power available, go the Kino Flo route. But I luuv the suggestion of the Chimera china ball. Simpler is always better. A string of photo lamps will do a similar job, but you have to make the string.

Wayne

There are additional sources for information in lighting for composites at
http://www.ultimatte.com/
http://www.kinoflo.com/product/b_g_s...eenscreen.html
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Old September 24th, 2005, 04:39 PM   #12
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Sorry, to clarify:
I didn't mean that you should increase iris. It's then when you increase iris, saturation goes up. It seems to indicate things that are at lower light levels will be less saturated.

So if you light the screen darker, it will be less saturated when it gets on tape. Possibly.
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Old September 27th, 2005, 01:58 AM   #13
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When lighting a green screen with a green light, you might run into the risk of having some of the green hitting your subject.
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Old September 29th, 2005, 01:26 PM   #14
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A few things from my television days and lighting, and relighting weather walls.

First, the light needs to be even. Cheap trick, use your cameras zebra. Shoot the area of the wall you will be using, clear out props, etc for a moment. Turn on the zebra, doesn't really matter what level it is, 70%, 100%, etc. Drop the iris. slowly bring the iris up until the zebra goes off. At this point you will probably notice there are areas where the zebra is not showing. Either those areas need more light, or your zebra areas need less light. The point is, the zebra will go off at a specific point. If the wall is well lit, we're only talking level here, the zebra should pop on for the majority of the wall all at once. A half stop or so from where the zebra comes on and the rest of the zebra shows is normally good.

Second, on the green vs white light issue. Here's my thing. The wall is a saturated green and should only reflect saturated green. If you hit it with a solid blue light, technically, it won't light up at all. So, it shouldn't make any difference if you use green or white. Only the green freq of the white light will get reflected anyway.

So that's my input on this one.

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Old September 30th, 2005, 09:13 PM   #15
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http://www.film-and-video.com/broadc...eenscreen.html
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