CRI When Lighting a Chromakey??

[Terry VerHaar]

Quick question: It seems like having the absolute highest CRI when fighting a chromakey background wouldn't be absolutely crucial. Is that the case or am I missing something? Would 98+ yield any benefits that you couldn't get from 90??

I am, of course, assuming that there is no light from this source falling on talent.

Thanks!!

 

[Ulf Krentz]

From my experience it also helps to have green gels on the lights for the green screen. I don´t see any benefits of a higher CRI for this purpose. Cheers!

 

[Ryan Williams]

Ulf,

Are the gels just to keep the color/hue closer to green instead of having lighter (whiter) greens? What are the green gels doing?

 

[Joel Voelker]

Ulf,

Are the gels just to keep the color/hue closer to green instead of having lighter (whiter) greens? What are the green gels doing?

Green gels make the green more green and more different from your foreground. The greater the hue/sat difference between foreground and the green, the easier/better/cleaner the key.

 

[Terry VerHaar]

Thanks very much for the information, Ulf. One can always count on the RED community!!

Now - while I am on a roll - another lighting question. (I think I have this one,, too, but not sure I know the science to confirm it.)

Regardless of the falloff characteristics of a given light source, if that light provides X amount of light at a given distance and I add another identical light source right next to it, will I now get 2X light, measured at that same given distance?

Thanks, again!!

 

[Ulf Krentz]

Ulf,

Are the gels just to keep the color/hue closer to green instead of having lighter (whiter) greens? What are the green gels doing?

Nearly every object is also reflecting some light "from the surface" as nothing is 100% matte. If you gel your lamps green, this part of reflecting light will also be green resulting in a cleaner overall color, also what Joel already said, you add some saturation that helps separation.

 

[Ulf Krentz]

Thanks very much for the information, Ulf. One can always count on the RED community!!

Now - while I am on a roll - another lighting question. (I think I have this one,, too, but not sure I know the science to confirm it.)

Regardless of the falloff characteristics of a given light source, if that light provides X amount of light at a given distance and I add another identical light source right next to it, will I now get 2X light, measured at that same given distance?

Thanks, again!!

Hi Terry, yes you got that right, too It is 2x the light, so 1 stop brighter, unfortunately if not going through heavy diff it often results in an unpleasant or even two visible shadows. Of course if that´s meant for lighting the green screen it is even better to have multiple smaller lights to get an even illumination.

 

[Terry VerHaar]

Haha - lucked out a second time! Thanks again, Ulf!!

Now I have to think of a really good third question and go for the trifecta.

 

[Ryan Williams]

Green gels make the green more green and more different from your foreground. The greater the hue/sat difference between foreground and the green, the easier/better/cleaner the key.

Nearly every object is also reflecting some light "from the surface" as nothing is 100% matte. If you gel your lamps green, this part of reflecting light will also be green resulting in a cleaner overall color, also what Joel already said, you add some saturation that helps separation.

Thanks Joel and Ulf! It makes sense. I just never really thought about it though. Good things to keep in mind should I ever do green screen work.

 

[Marc Wielage]

I think there's a danger in lighting a green background with green gels, because of the problem of spill and reflection.

You're a lot better off just following the detailed rules for lighting green screen in one of these books:

American Cinematographer Manual

VES Handbook of Visual Effects: Industry Standard VFX Practices and Procedures

Every time I've seen somebody try to tinker with the tried-and-true method for lighting green screen (like adding magenta fill to the foreground or something), it winds up making things worse. Just stick to the normal rules and you won't go wrong.

The three biggest things to keep in mind are generally:

1) use a really, really big green screen that's evenly-lit all the way from the top to the ground

2) keep the actors as far away from it as possible to avoid spill

3) use the correct shade of green as recommended by the keying application.

Rosco ChromaKey Green Paint is very good:

http://us.rosco.com/en/product/chroma-key-paint

 

[Ulf Krentz]

I think there's a danger in lighting a green background with green gels, because of the problem of spill and reflection.
snip

Marc, the reflection from a green screen will always be green, it´s not getting worse with gels and of course you have to keep it under control, it does not have to hit the talents and I can easily imagine that trying to improve things with a magenta front light starts a nightmare. Flags do work very well for this purpose. I also agree I would not try to alter the hue or saturation and keep the amount of light in the proper range, but using green tinted gels surely helps to achieve an evenly lit screen, especially when not in a full equipped huge studio.

 

[Ryan Williams]

2) keep the actors as far away from it as possible to avoid spill

How do you deal with this in a confined space, say, like a small house or room, where you can maybe on get a few yards/meters away from the green screen?

 

[Hrvoje Simic]

How do you deal with this in a confined space, say, like a small house or room, where you can maybe on get a few yards/meters away from the green screen?

You avoid shooting chromakey shots in a confined space. Because you cannot make a proper light separation.

 

[Ryan Williams]

You avoid shooting chromakey shots in a confined space. Because you cannot make a proper light separation.

Then how are you keying so that you can still use the green screen?

 

[Ulf Krentz]

I won´t say it is impossible, but it is definitely asking for trouble as you will not only have to take care of the green spill to the foreground but also your front lighting will spoil your even illumination of the green screen itself, creating shadows etc. In the end it probably boils down to more post work which can be a real PITA. So yes, Marc´s rule No 2 could actually easily be No 1

 

[Ryan Williams]

So besides "not doing the shot" is there some other way that you execute a close quarters green screen?

 

[Ulf Krentz]

So besides "not doing the shot" is there some other way that you execute a close quarters green screen?

Ryan,
I did not suggest "Not doing the shot" as it is obviously no option. Not sure what else can be said though. If you have to work in confined space place the talent as far away from the green screen as you can, place flags left and right and may be even on top behind the object to minimize the spill of the background. Use multiple lights to get an even illumination. Use your front light in a way that primarily lights your object and don´t hit the background. (For example if possible use a less brighter source closer to the talent, light will decrease in the square of the distance) But still build a light situation that suits the final composition and you should finally get away with an acceptable result

 

[Ryan Williams]

You avoid shooting chromakey shots in a confined space. Because you cannot make a proper light separation.

Ulf, I wasn't implying that is what you said. It is however highly suggested in this post which is what I was referring to. Thanks for explaining how you would handle it, though I agree that it sounds like a lot of work and huge PITA if you have to do things in CQ.