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Old November 29th, 2007, 03:24 PM   #1
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Filters for greenscreen lighting

In the quest to get a more pure green greenscreen, I've been testing lighting gels from Rosco and Lee... The theory is, if you throw green light at the greenscreen, it has no choice but to reflect a greener green for the camera.

Since filters are subtractive, a good green filter will hold back most of the red and blue light, but allow most (but not all) of the green light to pass through. So for a relatively small decrease in exposure, you can almost eliminate the red and blue light which is corrupting your greenscreen.

I tested by gelling a flash unit and photographing a professional greenscreen from Composite Components. The winning gel was... (drumroll)... Lee #738 ("JAS Green").

Without the gel, the greenscreen is reading at:
R:31% G:92% B:36% (sampling the screen to the left of the "brown patch" on Macbeth)

The difference between our G channel and the R&B channel is 58.5%... This means our core color difference matte will only have 58.5% of the values between 0-255 to work with. This is the best we can do without help from the lighting. You literally can't buy a better greenscreen material than the CC Digital Green.

With the 738 on, we lose roughly 1/2 stop of light. Compensating by opening up the aperture, our reading is:
R:1% G:95% B:5%

(!!!) The difference jumps from 58.5% to 92%! Our matte can now utilize 92% of the values between 0-255.

I instantly ordered a bunch of T12 sleeves of 738 for my cheap fluorescents when I saw this.

If you only have access to Rosco, #86 ("Pea Green") is the closest to Lee 738, but not quite as nice. The reading from #86 was:
R:1% G:88% B:3%

Which gives an 86% difference between G and R&B. Still a great reading, but if you can get your hands on the 738, it has the edge. BTW, avoid Rosco's "Chroma Green." It's too blue, and too dark. You would have to play a lot of games with your white balance to get it to photograph as pure green.

I also tested a more subtle green gel, Lee 138 ("Pale Green"). It's nice because you don't have to compensate exposure at all. And if you're in close quarters and can't avoid having your greenscreen lights hit the subject, it wouldn't be a total disaster, because it isn't so insanely green. (Although you should always flag off your GS lights so they don't interact with the subject. Foamcore works well...) The 138 gave:
R:7% G:88% B:17%

..for a difference of 76%. Not too shabby, considering you don't have to compensate the exposure at all. Definitely a step up compared to not gelling the lights. Then again, if you have even half a stop to spare, the 738 blows it away.

I'm going to test more of these subtle gels in the near future. In particular, Rosco 388 ("Gaslight Green") looks good. Other contenders are Rosco 3304 & 88, and Lee 88...

I've attached three images -- one "clean" with no gel, one with Lee 738, and one with Lee 138. Notice how dark the Red and Blue patches are on the Macbeth in the 738 image! Crazy...

- ben
Attached Thumbnails
Filters for greenscreen lighting-flat-web.jpg   Filters for greenscreen lighting-lee-738-web.jpg  

Filters for greenscreen lighting-lee-138-web.jpg  
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Old November 29th, 2007, 03:35 PM   #2
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I should post the link to Composite Components... Their greenscreen material is incredible!

http://www.digitalgreenscreen.com/
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Old November 29th, 2007, 05:13 PM   #3
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Great info! Thanks for sharing.
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Old November 29th, 2007, 07:03 PM   #4
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Interesting... keep in mind though, its not as much about how "green" the background is, but much more about how evenly it is lit.
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Old November 29th, 2007, 08:53 PM   #5
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Ideally, it's about both...
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Old November 29th, 2007, 09:12 PM   #6
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Quote:
Originally Posted by Ben Syverson View Post
Ideally, it's about both...
Not really. By adding green gels to the lights that you're lighting the green screen with, you're trying to drive the color towards a certain tone of green. This could be solved simply by changing the background to that color, which is really moot, because that's the whole point of "Chroma Key Green" and "Chroma Key Blue", is that they are colors that are ideally as far away from skin tone as possible. Like I say, if the shade of green you're achieving by using gels is better, then they would just make chroma key backgrounds that color. No matter what kind of gels you use, if the lighting is uneven the key will not pull as cleanly. Check out this video, it explains it all:

http://www.digitaljuice.com/djtv/seg...how=all_videos
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Old November 29th, 2007, 10:02 PM   #7
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I write keying software for a living, and have been writing keyers for over 10 years now... So you may want to keep that in mind when reading my posts about greenscreen. :)

The greenscreen you see in the test shots I took is called Digital Green, and it comes from a great company called Composite Components. Jon Erland, the co-founder of CC, has written 15 papers for the SMPTE journal on the subject of traveling mattes. Their stuff is the gold standard in Hollywood, and they were rewarded with an Oscar for their screens in 1996.

So believe me when I tell you: this is quite literally the best greenscreen material you can buy. You should see it in real life -- it looks electric. In order to be any better, it would have to BE electric!

The reason why it doesn't photograph as absolutely pure green is that there is simply no material, dye or pigment that absorbs 100% of the red and blue in white light, while reflecting only green light.

That's why Composite Components also sells Digital Green (and Blue, and Red) fluorescent bulbs to illuminate their screens with.

Of course it's important to light your screen extremely evenly. Even lighting will improve your composites more than anything else -- if you're not using a live video analysis tool with scopes (particularly the RGB Parade) on-set to adjust your lighting, you should.

But once you master that, you'll start wanting a more pure green background. That's where the colored lighting comes in. If you have the cash, you can just buy the Digital Green tubes, which are purpose-made. But I'm always trying to figure out how to do things on a budget, because a lot of my customers are independent filmmakers, small video production shops, etc...

So that's why I'm testing all these gels. It's a way to go from a good screen to an amazing screen for about 6 bucks...
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Old November 29th, 2007, 10:27 PM   #8
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Quote:
Originally Posted by Ben Syverson View Post
Of course it's important to light your screen extremely evenly. Even lighting will improve your composites more than anything else
Whoa! Easy there big fella. What I quoted above was the only point I was trying to make. No matter what gels you put on the lights, that won't make up for uneven lighting. Just like you said "even lighting will improve your composites more than anything else". That's all I was saying. You need to relax a little bit, no one's attacking your expertise. I've shot plenty of chroma keys myself, there's no need for bragging about our credentials. Just a friendly forum and no one person here knows everything about a certain subject.

Cheers!
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Old November 29th, 2007, 11:20 PM   #9
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Sorry, I certainly didn't mean to come off as defensive, just wanted to place my comments in context... I even added a smiley! :)

Anyway, no worries -- I'm just trying to add to the knowledge collected here at dvinfo.net.
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Old November 29th, 2007, 11:28 PM   #10
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No problem, I think I misinterpreted your response and appreciate all the info you're putting up, its very detailed.

With the new chroma key material that you're talking about, are there any problems with it kicking back on the talent?

You're obviously on the right track since they sell lights that are green (or blue or red) to light key backgrounds. Thanks for sharing, and please continue!
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Old November 30th, 2007, 12:38 AM   #11
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Ha, that's the internet for ya, everyone sounds bitchy. :)

In terms of the screen kicking back light... It's still reflecting the same amount of light, but now that light is much more pure green, so it can cause problems. The key is keeping some distance between the subject and the screen, and being diligent about flagging off the screen lights to keep them off the subject.

Software can remove even high levels of spill and color casts very easily these days, but it can't handle full strength reflections, so it's always best to keep reflective materials in the subject from nearing a perpendicular plane with respect to the screen -- because of the Fresnel effect, reflections increase in intensity at those points. (Think of a reflective black sphere, where the reflections get brighter and brighter as they reach the edge.)

Hmm, that last paragraph would probably make sense with a graphic... I'll try to photoshop something up.
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Old November 30th, 2007, 03:31 AM   #12
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Do you have anything to do with Keylight? I was playing with your posted color chart image in After Effects, and the Keylight plugin worked great. I was watching the DV Creators video, and they talked about doing a Luma Key, so I convinced my wife to play along so I could give it a try. We were experimenting in my garage with a lighter green sheet and work lights, shooting with my XL2. Its a lot tougher in a small space like that. The other chroma key stuff I've done was in front of a very large screen in a huge room, so getting the talent away from the background was easy. Still, this one pulled really well as a Luma Key. The yellowy halo around her is actually something we added in post, as we were trying to imitate a little on-line ad that had the same effect. The spill is actually light green, as you can probably tell. Not too bad though for a bed sheet and a couple of work lights (she was light with an Arri 650w softbox and a 150w fresnel).
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Old November 30th, 2007, 05:50 PM   #13
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Hey Bert,

Cool! Nice shot!

I don't work on Keylight, but it's a nice little keyer. My keyer is called dvmatte.

If you wouldn't mind, could you post (or email) a frame from the greenscreen shot, and the background? I'd like to give it a whirl in dvmatte. If you also have a frame of just the greenscreen without your lovely wife in it, that would be a bonus, as I can use it to correct the screen.
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Old November 30th, 2007, 08:49 PM   #14
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As a filter company, for once I will tell you not to use an on camera filter. In my experience with green screen, I have found it most beneficial to use a back light with a 1/8 magenta LEE gel on it. This eliminates green fringing and artifacts that come about.

Otherwise, Kino Flo makes some really handi green bulbs.

Ryan Avery
Schneider Optics
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Old November 30th, 2007, 09:21 PM   #15
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Hi Ryan!

Yeah, I should be extra-clear in case there's any confusion. I'm suggesting Lee 738 as a lighting filter for the screen only, not as an on-camera filter... On the camera, unfiltered is definitely best -- unless you aren't able to get your in-camera sharpening low enough, in which case it might make sense to test a 1/4 or 1/2 Black Pro Mist or similar.

My experience with magenta backlighting hasn't been very promising -- I wind up having to struggle to remove the magenta cast in my edges. Can you elaborate about that technique a little bit, or even better, post an example frame?

Kino does make green bulbs... Hmm, maybe a Composite Components Digital Green vs Kino green shootout is in order. :)
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