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Backlit greenscreen

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Greg Sage
Backlit greenscreen
on Jun 8, 2019 at 7:43:20 am

I've built a few greenscreens, and it's always been a pain to light them in tight spaces battling flags, hotspots and all the rest.

Meanwhile, the light itself is generally coming from large softboxes whose purpose is to diffuse the light so it emits evenly across the diffuser surface.

But wait... that's exactly what we want from a greenscreen... for it to be evenly lit across the surface. It occurred to me today that perhaps the entire process is backward... that it's actually better to light a greenscreen from behind as led strips can be evenly spaced across the entire surface evenly, and can be diffused in multiple stages if necessary to get a completely even lighting across the entire surface that's just not possible with any sort of front edge lit approach.

I see only two downsides. First, it limits the choice of screen materials (no more painting directly on the wall). Second, it takes a bit of space. With modern LED's, though, I don't see why it would need to take more than a foot... which is considerably LESS space than is taken up by the multiple lights used to front-light a screen, and it completely eliminates the need to go so wide. So the second problem is actually a benefit.

A floor would be more problematic as it needs to hold weight and materials choice is already limited, but a backlit floorstanding greenscreen would seem to be an ideal solution. If not, I'd love to hear why, but from the perspective of space, evenness of lighting, etc it seems like a better approach.

Has anyone done this? I can't find any chatter about it.

I've never tried to build such a thing, and not up on the latest led strip details, but any thoughts on what the front sheet material should be? Green with white lights passing through? White (also then serves as white backdrop) that's lit by green LED's? Some other combo?


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Todd Terry
Re: Backlit greenscreen
on Jun 8, 2019 at 6:34:13 pm

I think it's a valid idea.

I did something like that a zillion years ago when I was in broadcast TV, a zero-budget version of the same idea.

I was directing a station promo, and I wanted these big freestanding "monitor-wall-like" set pieces that would be greenscreened, but because I had dark/dramatic/moody lighting in this spot and with the blocking putting the talent close to them, I couldn't use front-lighting.

We happend to have these two carpeted risers that lived in the studio, they were about 10" high by 6'x6'... portable and used various places on stage as set pieces, for seating risers during talk shows, etc.

So I turned these risers up on end, where they were basically empty plywood boxes. I put a bunch of lights inside, covered the faces with green, used gaffer tape to dress the edges and divide them up into nine squares each ("Brady Bunch" style)... and voilĂ , cheap-n-easy glowing greenscreens that served as my fake monitor banks.

The end results worked really well, but it could be done much better today. Remembering that this was almost 30 years ago, I just had incandescent bulbs in there. Also, because I had zero budget, the face was just cheap green "bulletin board" paper from the school supply shop. I had some hot spots with the lights shining through, but fortunately was still able to pull a clean key.

If I was going to do this today, yep I would probably use LED lights... maybe bunches of rows of those cheap "ribbon" LEDs, I think that would work well. For the face I would probably use green vinyl, a sign shop would be a good source of that and one of the more cost-effective ways to do that, and would look much more even than my green paper.

And yeah, while a floor would be do-able it would be much much more complicated and probably extremely expensive. You'd basically need to build a below-lit "disco floor," but with no seams. Tall order.

But as for a wall... give it a shot!

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Greg Sage
Re: Backlit greenscreen
on Jun 8, 2019 at 8:15:56 pm

Yeah, the "light went on" when the idea hit. I'm convinced now it's a better way to go (for walls, anyway.

Not sure whether it's best to use white/gray material or green, but I ran a couple of tests today since I'm in a music studio surrounded by huge mineral wool panels draped in white cloth. Would've been great if I could blast enough light through them to be usable as the greenscreen would just be a solid white while not in use, take up literally 4" of total space (including lights), and double as solid acoustic treatment (the other bane of the existence of vloggers).

The 4" mineral wool is too dense to be used for diffusion, but with enough green LED's, I might be able to use 2" panels to get the dual greenscreen / acoustic panel thing going.

Anyone else given this a shot? With proper diffusion, I'm just not seeing any downside. I'm assuming, of course, that there's some trial and error with materials and light to get the proper dull finish and chroma green color.

Just came across the retro-reflective / green lens ring approach today while searching this too. Very interesting. Not ideal for larger shoots, but probably the best approach for someone in a tiny room where they can (and should) get right back against the screen while shooting.


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Todd Terry
Re: Backlit greenscreen
on Jun 8, 2019 at 8:35:23 pm

Yes, those reflective systems have been around for quite a while. I've never personally used them, but I've heard some decent things about them.

And yes, they are appropriate for fairly close stuff, talking heads and such. For big wide shots, not so much.

The biggest downside is the reflective material... which is extremely expensive.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Greg Sage
Re: Backlit greenscreen
on Jun 8, 2019 at 11:22:45 pm

Never used such a system, but it certainly looks like thery're just repackaging 3M ScotchLite fabric for an insane markup... and there's even Chinese knockoffs of that:

https://www.ebay.com/i/222893880826?chn=ps

Here's a whole breakdown on how to do it for dirt cheap:

https://www.youtube.com/watch?v=rJKLEZnsVsQ

I think there's a followup on his channel with more details too.


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Greg Sage
Re: Backlit greenscreen
on Jun 9, 2019 at 4:20:07 pm

Definitely interested to hear from anyone who has done a backlit system, or maybe due to their use of some of the materials in another context has insight into elements like:

1) White or green LED's: Type, rating, brightness, etc.

2) Diffusion material: Fire resistance, proximity to lights

3) Framing: Portable or fixed setups, collapsable frame for transport, rollup led's, etc. Just a thought here, but if you're doing a whole wall or near in a house or other place you have to live with the setup, might just be better aesthetics to frame the entire wall floor to ceiling and end to end... even if you only do the led's in one section. That way it's fully stealth. You just have a cloth wall 2" to 6" in from the actual wall. with no visible frame. Could even paint a design or something on the portion not being lit.

4) Screen material: White, gray, green, fabric type, resistance to shadows, reflections, etc.

5) Possibility of serving dual purpose as sound absorption panel (use of mineral wool, fiberglass, or other sound deadening material for diffusion. Just a thought too, but they could coexist. If it is, for instance, not feasible to use mineral wool in front of the led strips, if it was behind them, and they were on a mesh of some sort with 2" of diffusion in front of them the entire thing could still be both a greenscreen and sound absorption panel... and still be only 6" deep with a (potentially) white surface.


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Mark Suszko
Re: Backlit greenscreen
on Jun 10, 2019 at 7:56:03 pm

Yeah, its not a new idea: there are photos in an old CINEFX magazine in my collection of the Original-Star-Wars era ILM shooting motion controlled models against blue and orange back-lit chromakey backdrops. They used fluorescent tubes for theirs, which may make for fewer single hotspots than LED's even COB LED's on a ribbon...

You will still need some separation distance from the screen and the actor to reduce not just spill, but light wrap with the key color in it.

They seem like a great idea if you have no need for portability.

What exact color the LED's might get you isn't nearly as important as controlling saturation and overall evenness of the source. So the right diffusion material would be vital. Getting it in an affordable, suitable seamless size might be a problem.

That ring-light and beaded screen stuff was popular back in the day but like Todd says, it was pricy. It also had a limited amount of coverage it could do, because of the cone angle of the LED ring, and the fact the retro-reflective bead fabric maintained hight brightness by having a *narrow* return angle to work with.... and the ring light also sometimes interacted with eyeglasses and reflective jewelry on the talent, no to mention prompter mirrors...

NFL Films used it for those tight bust shots of players used in the intro /stats b-roll packages with great success for some seasons. I don't know if they still use it. But you can't key a whole car with one. Or anything bigger than up to about 2 people - if - they stand hip-to-hip.

The upcoming breakthru keying tech is light-field based, and won't need any kind of backdrop or color factors. You just dial-in the distance to the lens of the subjects you want to see, and z-depth controls what's visible and what's not. I think it's working in a lab somewhere already but won't be commercially available for a few more years.


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