Getting Optimum Chromakey Results

Chroma key works by recognizing a single video color (blue, green, or black) and replacing those pixels with a background image. We understand that everyone wants to do this in the space of a small closet. You can and the results will show it. Or you can use the least amount of space for an optimum setup.

Options for Materials:

Paper
Paper is the easiest, least expensive, and fastest temporary solution. In the US, the primary manufacturer is Savage. www.savagepaper.com

Paint
Paint is the most labor intensive, but also the easiest permanent solution. The main US manufacturer is Rosco. Home supply store paint is not optimized for low reflection.
www.rosco.com

Fabric
Regular cloth and foam-backed cloth are available. The regular cloth is a smaller package but requires that you remove the wrinkles before use. The foam-backed material does not show wrinkles and is easy to use.

Cyclorama
A formed background that curves between the wall(s) and floor -- this is a more permanent solution. It is simple to use but more expensive.

Retro-reflective Backgrounds
Using the same theory as traffic signs, this material reflects light back toward the source where it came from. With a ring of LED’s surrounding the camera lens, you can use a small amount of light to illuminate just what the camera sees. Usually it will help if you “back light“ the subject(s) to make it easier for the chromakey to distinguish between background and foreground.

Lighting
You can quickly get the best results by being able to measure and control your lights. In general, we suggest that more light is better than less light, but it must be well-balanced, i.e. a bright background and dim subject will not give optimum results. Your foreground subjects should never be brighter than your background.

What to use for Lighting

Use lights that are reliable and consistent. Matching the color temperature of the lights will make things much easier.

Use diffusion, such as white gels or soft boxes, for all your lights to spread light evenly across your background. Avoid Frost diffusion for background lighting. Frost diffusion does not soften the light but spreads out characteristics of a hard light.

Chroma03How to Light your background
You should light the background separately from the foreground. Do this first. You don’t want the background light hitting your talent as it will distort your talent’s overall color and brightness.

Place lights at least 5 feet from the background. First angle the lights so they are facing perpendicular to the Green Screen. Five feet is ideal for dispersing light softly and evenly. If the lights must be closer, watch for hot spots. Then angle the lights inward towards the background until the key is even.

The talent should stand at least 6 feet away from the background. Place your key and fill lights at least 6 feet in front and 2 feet above your foreground subjects, angled downward. This will allow your talent/subject and your background to be lit separately and evenly.

Use an orange/3200k gel or a negative green/Magenta gel, for the backlights on your subject. This will help make a more defined edge and improve the aesthetics of the image. Have the backlights feet behind the subjects at an angle at least 2 feet above their heads.

Adjust your backlighting, key, and fill lights for your talent to match the scene element your talent will be placed in. If it is a virtual studio with a news set, there may be virtual lighting, so you can have backlights spot heavily into their hair, shoulders, etc.

The more you do to enhance the illusion, the more believable your key will be!

For wall to floor green screen set ups, it is best to light from above. Overhead lighting is the best way to light the background wall and floor. Key and fill lights can still be mounted on stands.

We suggest a portable truss, super clamps, and safety cables. This approach is cost effective and simple.

How do you measure lighting?
You can use a light meter or you can purchase and download the “Cine Meter” App on your Apple or Android Tablet. The Waveform and spot metering are functional enough to see adjustments you need to make to get an even key. Chroma02

The idea is to get even illumination on the background. Do not use your eyes to judge the evenness of illumination. Once you have the best illumination you can get, then you can adjust the Chroma key to compensate for differences.

Camera distanceChroma04
You will need at least 10 feet for a talking head shot and 15 feet for a full standing shot between your subject and the camera. This will help avoid any wide angle lens distortion and give you depth of field, meaning your background can be soft focus and your subject in focus, further separating the background for an even key.

Wide Angle Lenses - Yes or No?
Chroma01Camera lens focal length will affect the size of the background you must use. A wide angle lens requires a larger background. Some wide angle lenses will distort your subject.

With more light, the camera electronics will work less (theoretically) and the signal has less noise. Less expensive cameras can look better with more light. More light also means that the lens is stopped down more than the depth of field is greater i.e.- more is in focus.

Light Vocabulary
One foot-candle is the luminance at a point on a surface which is one food from, and perpendicular to, a uniform point source of one candela.

One lux is the luminance at the same point at a distance of 1 meter from the source.
One lumen uniformly distributed over one square foot of surface provides an illumination of 1 footcandle.

If you work in feet, your results will be in foot-candles -
(1 foot-candle = 1 lumen/square ft.)

If you work in meters, your results will be in lux -
(1 lux = 1 lumen/square meter)

The Edge that Separates the Foreground from the Colored Background
We suggest that you experiment with the illumination ratios using your particular screen and setup.
If the green/blue screen is brighter than the subject, what does the edge look like?

If they are about equal, what does the edge look like? How does lighting the back of the subject affect the edge?