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.

Yes. This is standard practice.

That's good to know Charles. I don't see many people recommending lighting a green screen with green light.

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

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.

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.

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?

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.

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 (http://www.cinematography.net/Pages%20GB/greenscreens_bluescreens.htm) 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...?

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.)

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_screen/bluegreenscreen.html

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.

When lighting a green screen with a green light, you might run into the risk of having some of the green hitting your subject.

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.

Sean McHenry

http://www.film-and-video.com/broadcastvideoexamples-greenscreen.html

I did some work in a studio the other week where they had it set up to just throw green lights at a white wall and use that as a green screen. We weren't doing any of that, but I did ask the DP. He said that he personally was against they approach, but that the studio used it pretty often.

Wayne - are you sure that you would lose 1/4 of your green output? Certainly you lose a significant portion of the overall light, but I think that it will mostly be the blues and reds which would not make the green wall any brighter anyway. Do you lose green output with a green gel? Hmmmm, not sure.

You shouldn't loose any green using a green gel. Look at IR photography. You can't get a deeper red filter than that. It effectivly blocks all visible light totally, but the IR wavelengths pass through and are illuminated quite nicely. The same is true for IR light emmiting panels. You can't see the light from the IR led in your remote controls (your camera quite often can - interestingly) and they are filtered heavily too yet they get enough of the right light to work fine.

When you gel a light source, you are adding a material in the light path that reflects, or absorbs the colors you don't want passing through. Blue gels let a certain wavelength through as do any color gel. Saturation of the gel color is a key to how well it blocks all the colors you don't want.

In those swatch books, and I haven't seen one for a while, they used to have a chart for each color explaining what portion of the spectrum gets passed with each gel. If it isn't getting passed, it's being absorbed so, white light with a green gel will absorb all those excess colors and allow your green to pass through.

The only reason I can see NOT to use a green filter is the splash and spill off the green gelled lights. You don't want that green light hitting anything it's not supposed to. If it's controlled right, I don't see anything particularly right or wrong with it.

I will finish by saying that green paint, or any color will not be perfect. If you use white light, a small amount of the other colors will still get reflected as that green paint, no matter how expensive a version you use, will still reflect other colors in minute amounts. You could minimize that by using predominantly green lighting.

I was thinking more about this just last night.

Sean McHenry

Based on my practical experience. I usually keep the green screen far away from the subject and lit with kino flo. As Walter expalined, use te corresponding backlight for background you would be keying in.

All does matter is the seeing it on the production monitor. If you can afford just do a sample testing using a NLE(FCP) right away, do it. Which i did at home. You don't have to get stressed about this and i wont use a white short for any of our artist period.