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Blacklight with green screen

CineCAS (IMAP)
 

Hey all,

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?  I would imagine I would have to keep the UV off the green screen.  If it "infects" the green screen would that be a problem?  If I light the screen with tungsten will that reduce the UV effect?  Should I light the screen with green tubes?  Thanks.

Charles Schner
Cinematographer + Camera Operator (not shooter)
New York at the moment

Patrick Blackard
 

I'd vote for green tubes, and flag or black wrap every light off the green screen.  Run a big teaser over their heads, to hopefully block the maximum front light. 

--Patrick Blackard
Gaffer, Austin, TX

On Sun, Mar 11, 2018 at 3:42 PM, CineCAS (IMAP) <charles@...> wrote:
Hey all,

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?  I would imagine I would have to keep the UV off the green screen.  If it "infects" the green screen would that be a problem?  If I light the screen with tungsten will that reduce the UV effect?  Should I light the screen with green tubes?  Thanks.

Charles Schner
Cinematographer + Camera Operator (not shooter)
New York at the moment

Mark Weingartner, ASC
 


On 11Mar, 2018, at 13:42 40, CineCAS (IMAP) <charles@...> wrote:

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?


A couple of thoughts that may guide your choices.
1. When we talk about lighting a scene with UV lighting it is worth remembering that we don’t record UV reflecting from the scene on our film (or sensor.)… or not primarily.
UV or Blacklights are used to energize  minerals that fluoresce.  Those minerals, whether part of makeup, paint,  the detergent that “makes whites whiter”  or the fluoride in the actor’s teeth, are essentially “powered by” the UV energy and that excitement causes them to spit out VISIBLE light which we then photograph.

So - the issue at hand is whether the UV light causes the green screen to fluoresce with any colors other than green… and in fact whether the greenscreen will even be affected by it.

A quick test will tell you - turn off all the lights except a blacklight and shine it on the greenscreen and see what happens.  It depends what dyes/pigments were used in making the greenscreen but my guess is that you will not have any untoward effect.   If you get a bit of green back who cares?  that’s what greenscreens do for a living.


Second issue is whether you should light the screen with green tubes.
Again, it depends…   Since the object of the game is to make as narrow a green on the vectorscope so that you can most easily replace it with the background you would think that green tubes would be the best thing…

…and in the film world, and “all other things being equal: with regard to spill control, etc, I would say “Yes - use green.”

However:  pretty much all digital cameras make better color channel separations than many film stocks, so the absolute purity of the green is less of an issue than it was in the film world.

If the screen, whether fabric or painted, is of decent quality and pretty monochromatic, lighting it with a broader spectrum tube won’t hurt you.

Some shows I have worked on have actually preferred  broad spectrum lighting on their blue and greenscreens and this may have to do with the super super saturated monochromatic green being “out of gamut” for the camera and maybe creating noise in other channels?  I don’t actually know, but on the greenscreens I have been shooting for features lately they have leaned towards full spectrum lighting.

One thing that people sometimes get wrong with narrow band lighting is that they make the screens too bright…   you don’t need as much total luminance on a green screen if the green is really pure.
Nowadays I would probably look at a scope and put the green a stop below middle gray - you will be able to see on the waveform monitor a significant difference in luminance on the screen in the three channels and that is, after all, the object of the exercise.

Hope this helps!

Mark



Mark Weingartner, ASC
VFX Photography and Supervision
Los Angeles based 


Tim Riese
 

Very thorough and insightful feedback Mark. I wonder if Digi-green vs standard chroma key green would have differences in how the UV light would react...

On Mar 11, 2018, at 10:07 PM, Mark Weingartner, ASC <vfxmark@...> wrote:


On 11Mar, 2018, at 13:42 40, CineCAS (IMAP) <charles@...> wrote:

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?


A couple of thoughts that may guide your choices.
1. When we talk about lighting a scene with UV lighting it is worth remembering that we don’t record UV reflecting from the scene on our film (or sensor.)… or not primarily.
UV or Blacklights are used to energize  minerals that fluoresce.  Those minerals, whether part of makeup, paint,  the detergent that “makes whites whiter”  or the fluoride in the actor’s teeth, are essentially “powered by” the UV energy and that excitement causes them to spit out VISIBLE light which we then photograph.

So - the issue at hand is whether the UV light causes the green screen to fluoresce with any colors other than green… and in fact whether the greenscreen will even be affected by it.

A quick test will tell you - turn off all the lights except a blacklight and shine it on the greenscreen and see what happens.  It depends what dyes/pigments were used in making the greenscreen but my guess is that you will not have any untoward effect.   If you get a bit of green back who cares?  that’s what greenscreens do for a living.


Second issue is whether you should light the screen with green tubes.
Again, it depends…   Since the object of the game is to make as narrow a green on the vectorscope so that you can most easily replace it with the background you would think that green tubes would be the best thing…

…and in the film world, and “all other things being equal: with regard to spill control, etc, I would say “Yes - use green.”

However:  pretty much all digital cameras make better color channel separations than many film stocks, so the absolute purity of the green is less of an issue than it was in the film world.

If the screen, whether fabric or painted, is of decent quality and pretty monochromatic, lighting it with a broader spectrum tube won’t hurt you.

Some shows I have worked on have actually preferred  broad spectrum lighting on their blue and greenscreens and this may have to do with the super super saturated monochromatic green being “out of gamut” for the camera and maybe creating noise in other channels?  I don’t actually know, but on the greenscreens I have been shooting for features lately they have leaned towards full spectrum lighting.

One thing that people sometimes get wrong with narrow band lighting is that they make the screens too bright…   you don’t need as much total luminance on a green screen if the green is really pure.
Nowadays I would probably look at a scope and put the green a stop below middle gray - you will be able to see on the waveform monitor a significant difference in luminance on the screen in the three channels and that is, after all, the object of the exercise.

Hope this helps!

Mark



Mark Weingartner, ASC
VFX Photography and Supervision
Los Angeles based 


CineCAS (IMAP)
 

Thanks Mark.  I always light green screens with tungsten (kinos) so that if any of the direct light hits the foreground it’s just a tungsten backlight which might help separation.  My question is will any of the white spill from those kinos affect the contrast of the UV effect (glowing colors, etc.)?  I’m hoping I will have time to test.

Oh, and thanks Patrick for your input.

On Mar 12, 2018, at 1:07 AM, Mark Weingartner, ASC <vfxmark@...> wrote:

Second issue is whether you should light the screen with green tubes.

Charles Schner + Cinematographer + Camera Operator
310-399-9246 + 310-994-5429 + www.cinecas.com
New York at the moment

mat
 

Bingo, Tim! The answer is yes, there is a difference in how screens react. 

The digital green screen sold by Composite Components, (the company founded by Jon Erland who just won an Oscar for work such as this) has phosphors which will indeed emit green light of just the right frequency when hit by UV. 

As for the purity with which you light the green screen, Mark is absolutely correct that it has gotten much less critical.  Many of the algorithms that separate FG from BG can adjust for any combination of Red, Green and Blue in the screen.  The issue is getting good FG/BG separation and contrast.

In fact, I was informed by Black Magic's expert on Ultimatte that their new device prefers a screen lit with white light so that it sees enough of all three primary colors to key on that particular mix coming off the screen.

As for UV illumination, your challenge might be getting enough light coming off the foreground objects to read well.  In my experience we would have wack the set with enough UV to resemble a 70's disco to get enough exposure.  I would test it.

You might also want to to test a UV filter in front of the lens to make sure you are minimizing the UV getting into your lens where it can both fog and soften your image.

Hope this helps.  Good luck.

M

-- 
Mat Beck, ASC
DP, VFX Supe, etc. etc. 
Entity FX
11500 West Olympic Blvd. #400
Los Angeles, CA 90064
USA

Office 310-899-9779
iphone 310-804-7887



On 3/12/18 12:30 AM, Tim Riese via Cml.News wrote:
Very thorough and insightful feedback Mark. I wonder if Digi-green vs standard chroma key green would have differences in how the UV light would react...

On Mar 11, 2018, at 10:07 PM, Mark Weingartner, ASC <vfxmark@...> wrote:

On 11Mar, 2018, at 13:42 40, CineCAS (IMAP) <charles@...> wrote:

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?


Charles Taylor
 

It may be acceptable for me to offer some thoughts from the perspective of a compositing supervisor.

Regarding purity of the greenscreen, the critical thing is even-ness. The exact colour is less important, so long as it's green and distinct from the FG colours. The most important thing is that there are no wrinkles, creases, seams, patches, shadows, etc. on the screen. All keyers take a parameter for screen colour. What keyers can't do is distinguish between a wrinkle and a hair, or a wrinkle and motion blur. Ideally the screen looks like a perfect constant colour with tracking markers on it. The more even the screen, the better the key.

Regarding green tubes, I would worry that with an extremely "pure" screen colour that you'd end up getting a noisy red and blue channel, which could lead to a chattering key, but I haven't done a head-to-head test. Noise is a constant enemy to good keys, and anything you can do when shooting the plates to reduce noise is most appreciated in post.

Shooting a blacklit scene I would also be concerned about overexposing the screen relative to the FG subject, creating a lot of edge contamination and spill. Presumably, the blacklit scene will be quite dark, and if the proposed BG is also going to be dark, you can do the compositor a favour by keeping the screen exposure as low as you can without introducing noise. Dark subject, dark bg, bright screen is a pain - the despill algorithms are quite good at removing green contamination, but less good at dealing with bright/dark contamination.

For what it's worth, among compositors, I've never heard the complaint 'if only they'd used green lights on the green screen!', but the complaints 'why are there so many seams/wrinkles/patches/gaps' and 'why is the screen so bright for a night shot' are constant refrains.

If at all possible, of course, shoot a test and discuss with the compositing supervisor (if any is attached already, or if one is available for consulting). Ideally with the actual screen you will use and a FG representative of what the actual FG will be. Bonus points for having a representative BG plate, too. They'll be able to toss your stuff in Nuke and quickly assess noise levels, screen exposure, colour separation, etc. etc.

Thanks,
Charles A Taylor
Compositing Supervisor
Toronto

On Mon, Mar 12, 2018 at 12:29 PM mat <mat@...> wrote:

Bingo, Tim! The answer is yes, there is a difference in how screens react. 

The digital green screen sold by Composite Components, (the company founded by Jon Erland who just won an Oscar for work such as this) has phosphors which will indeed emit green light of just the right frequency when hit by UV. 

As for the purity with which you light the green screen, Mark is absolutely correct that it has gotten much less critical.  Many of the algorithms that separate FG from BG can adjust for any combination of Red, Green and Blue in the screen.  The issue is getting good FG/BG separation and contrast.

In fact, I was informed by Black Magic's expert on Ultimatte that their new device prefers a screen lit with white light so that it sees enough of all three primary colors to key on that particular mix coming off the screen.

As for UV illumination, your challenge might be getting enough light coming off the foreground objects to read well.  In my experience we would have wack the set with enough UV to resemble a 70's disco to get enough exposure.  I would test it.

You might also want to to test a UV filter in front of the lens to make sure you are minimizing the UV getting into your lens where it can both fog and soften your image.

Hope this helps.  Good luck.

M

-- 
Mat Beck, ASC
DP, VFX Supe, etc. etc. 
Entity FX
11500 West Olympic Blvd. #400
Los Angeles, CA 90064
USA

Office 310-899-9779
iphone 310-804-7887



On 3/12/18 12:30 AM, Tim Riese via Cml.News wrote:
Very thorough and insightful feedback Mark. I wonder if Digi-green vs standard chroma key green would have differences in how the UV light would react...

On Mar 11, 2018, at 10:07 PM, Mark Weingartner, ASC <vfxmark@...> wrote:

On 11Mar, 2018, at 13:42 40, CineCAS (IMAP) <charles@...> wrote:

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?




  

George Hupka
 

I've never had complaints about using green tubes, but then again I usually use them because when matched with an appropriate green screen paint or fabric, you can light a much larger area with fewer green tubes than with white light... Overexposing the green has never been a good idea...


--
----------
George Hupka
Director/DP
Saskatoon, Canada
Listmum, Cinematography Mailing List

CineCAS (IMAP)
 

Thanks everyone for your input!  As it stands things are evolving and the green screen might now totally go away leaving just a blacklit scene with a translucent stage lit underneath by LED.  My new question now is this:
If I am lighting a 40 foot by 40 foot space how many Wildfire Viostorm 120’s would I need to shoot 360 degrees around the set.  I am shooting with the Varicam LT and will most likely be in the 5000 ISO mode shooting at a 2.8-4 stop.  Also, are there any alternatives to the Wildfire lights if the producer balks at the rental cost?  Thanks again!

On Mar 12, 2018, at 1:34 PM, Charles Taylor <c.angustaylor@...> wrote:

It may be acceptable for me to offer some thoughts from the perspective of a compositing supervisor.


Charles Schner + Cinematographer + Camera Operator
310-399-9246 + 310-994-5429 + www.cinecas.com
New York at the moment

Tim Sassoon
 

I haven’t, but I don’t think it would be a huge problem. You could put a standard UV cut filter on the lens to be sure. The question is whether the fabric or anything else in the scene would fluoresce that you didn’t want to? Cotton certainly does. I’d have talent put on sunblock, and be careful of their eyes if there’s a lot of exposure. Low visible light with high UV is dangerous.

The point of a green screen is of course to have as narrow-band a color and exposure as possible. I like to look at a WFM/VS or camera histogram to see if I’m getting a good clean spike.


Tim Sassoon
Venice, CA 90291




On Mar 11, 2018, at 1:42 PM, CineCAS (IMAP) <charles@...> wrote:

Anyone out there ever shoot green screen composite shots where the foreground is lit with UV lighting (blacklight)?  I would imagine I would have to keep the UV off the green screen.  If it "infects" the green screen would that be a problem?