Chroma Key Lighting

"How to properly light talent in front of a green screen" is one of the most asked questions that I get.

When lighting a green screen, the screen should be at least 4 feet behind the subject, the more distance the better, and lit with an even soft source. I have noticed that many stations have hard bright lights on a green screen which causes most of the problems they encounter.

I compare lighting a green screen to using a garden hose. If you stand near a wall and set the hose nozzel to get a hard spray, it will splash back drenching you with water. If you set the nozzel to a soft spray you can wash the wall without any of the spray reaching you. Light reacts much the same way, the harder you light the green or blue screen, the more splash of color you will get on your talent.

So, we need to diffuse the light hitting the green screen and light it evenly. You should then be able to light the talent any way you want as long as the shadows do not hit the background. This should not be a problem if the talent is far enough away from the screen. For exposure, the luminence of the green screen should render the same setting as your key light being usde for your subject or you can take the background down one half stop darker than the key if you prefer.

Just remember to avoid overexposing the background and you should have a nice clean key.

One of the most often asked questions is, “How do I get all of these little jaggies off of the edges of my Chroma-keyed subject?�

And the popular cure-all seems to be the proper lighting of your Chroma-key blue wall (or back-drop). Not just a wimpy little 100 watt light aimed somewhere in the general direction of the background, but a host of lights that literally flood the background.

We’re talking a minimum of two 250 watt quartz halogens aimed at the backdrop from high and off to each side so that they will not inadvertently cast any shadows from your key subject (who should be standing at least three feet out from the backdrop).

The key subject should be illuminated with it’s own key light (another 250 watt plus) that is preferably diffused with either an umbrella or a diffusion filter.

Every now and then, the background color of blue will reflect back onto the key subject. And this, of course, will cause background video to be keyed in onto the key subject, which you don’t want to do. This reflected blue can be mostly eliminated by placing light colored amber gels over your background lights (the two lights that flood your backdrop). More specifically, ask for “bastard amber� at your local photo shop when purchasing these gels. They’ll know what you’re talking about.

As for the key subject light, place it about three or four feet off to one side or the other of the camera so as not to cast any direct shadows onto the background.

Chroma Key Lighting Techniques Revisited

Obviously, lighting distances and other factors like ambient light will vary from one person’s studio to another. So, experiment with various lights, gels, diffusers, and distances until you obtain the perfect combination. Remember, Chroma Key is much more than just a “turn it on - turn it off� feature on a digital mixer. It is an art form and a science in itself that requires a lot of work on your part to perform effectively. And don’t forget to white balance the Chroma key camera. If it doesn’t see true Chroma key blue, neither will your digital video mixer.

Chroma Key Tricks

What if you want to key yourself in front of a very large background, such as a skyscraper or mountain, and your blue or green backdrop is only about 8 feet by 6 feet? Well, you could get a larger backdrop that would dwarf you in comparison. Or you could just miniaturize yourself electronically.

Begin by creating a Picture-In-Picture (about 1/9th size) with you and your blue backdrop inside of the P-I-P. On the outside of the P-I-P, select Background Color as the image that surrounds the P-I-P. Referring to the instruction manual, create a “custom� background color that identically matches the color of the backdrop in your P-I-P.

What you’ll see on the screen is a little tiny you in front of a very large backdrop. Now record this image onto a video tape and take that tape out and put it into your playback (source) VCR. With that tape now playing as the incoming source, set up your Chroma Key and let that gigantic backdrop be filled in with Mount Everest or the Empire State Building.

Shifting Backgrounds

Since most video mixers can lock onto virtually any background color as a backdrop, try setting up a multitude of colors on your studio wall and set up the Chroma Key to reveal the keyed image through all of the color (by moving the cursor about the screen and pressing “OK�). Now, with someone in front of the multi-colored wall, begin panning your camera from left to right and back again.

What you will see is portions of the background image revealing and un-revealing itself (even though it will not appear to be moving). What you’re doing is deliberately confusing the video mixer setup so that it will only reveal a background when it’s right on axis with the pre-registered color. Truly a bizarre effect.

Kill the “Jaggies�

If you’re picking up jagged edges off of your keyed in subject, it’s usually because the backdrop color is somehow bleeding onto your subject. The first thing to do is move your subject away from the backdrop. Then make sure that your diffused lights illuminating the backdrop are not creating shadows out of your foreground subject. And then make sure that you are not trying to key in a shinny object, such as a silver-colored model airplane (which will allow your backdrop to “bleed� all over it causing jaggies). For shinny things, such as metallic objects or bald heads, try some of the matting sprays available from art supply stores (i.e. Glare-Away) which should take out most of the shinny objects reflective qualities.

Another “color bleeding� problem solver for Chroma Key is the use of amber-colored gels over your backdrop illumination lights. These gels will have a tendency to absorb the bleeding characteristics of the color blue (which is the color most people use for Chroma Key backdrops).