Creating Affordable Green Screens

Historical Overview

There has always been a desire by producers and directors with vision to want to improve what is before them. That includes the use of action or scenes that don’t exist, or allowing actors to appear to be in any location without leaving the studio. Such was the case with the 1940 movie The Thief of Bagdad, which called for Jaffar, a massive genie, to play a prominent role in the film. Special effects coordinator Larry Butler rose to the challenge and is credited with invention of blue screen compositing, for which he won an Academy Award for Special Effects. This was a very tedious and precise process involving several layers of film which had to be precisely aligned when creating the master negative.

Decades later, advancements in computing brought the technology into wider use, including television production. One of the most common examples today is the TV weatherman, who is standing in front of a green wall. A special computer is instructed to replace all regions of a particular color with satellite animations and weather graphics originating on a second computer.

Replacing a color with an image is keying or “keying out” the background, which refers to its
historical roots as a hole, or “keyhole” in one of the film layers.

Blue or Green?
Why did The Thief of Bagdad use a blue background, but you’re local TV station opts for a green background? In most circumstances, the compositing is placing a person in another environment, and either blue or green is the furthest color from human skin tone. A decision of blue or green may be decided by wardrobe styling and colors used on set.

 


Figure 1

Lighting a Chroma Key Set
The technical challenges to creating a realistic-looking chroma key aren’t limited to color choice, since the weatherman isn’t standing in a dark room. The on-screen talent must be properly lit. This presents a new set of challenges.

o 2 and 3-Point Lighting
Whatever background image you choose to key over, pay particular attention to how the
light and shadows fall in this image – you must now match the direction and angle of lighting on your talent. Is the strongest light coming from the right? Then that needs to be where you place your primary light source, known as the key light. Mount another light above and behind the talent as a separating hair light, and you have a minimal setup: two-point lighting.

 


Figure 2
This is beneficial in the field when there is limited set-up time or you don’t have the space to set up numerous lights.

This backlight can be a problem shining back toward the lens, which introduces flaring, lens artifacts and reduces contrast (all of which will interfere with a successful key).
The solution to our problem is to block the light from shining straight into the lens. This can be accomplished with black foil or a barn door to shield the lens. You can also place a grid in front of the light which reduces its angle to a narrow beam pattern. The illustration below shows a 20-degree grid, which converts a broad, scattered wash of light into a tight spot.

 


Figure 3
A more common and improved setup is 3-point lighting. Add another light on the other side
of the camera but with an output that is 1 to 2 f-stops lower than the key light. This is the fill light, which balances tone and fills in shadows on the other side.

 


Figure 4
o It’s not how bright the wall is
Grabbing every available light in the facility and pointing it at the wall will not create an effective key source, but will only make it more difficult to key. It is not how bright the background is, but instead how evenly the light washes over the green or blue background.


Figure 5

 


o Soft Lights
Whether your setup is lit with quartz, halogen or fluorescent fixtures, the background lighting should be separate from the lighting used to illuminate the talent. Many lights are designed to create an adjustable hot spot in the center of the beam, which must be minimized. This is usually accomplished with a translucent diffuser in front of the light. The diffuser can be made of glass, spun fiber or heat-resistant gels. This causes the light to scatter, eliminating the bright spot on the background.

Fluorescent lighting is preferred to light a blue/green screen, the easiest lighting setup is to use all fluorescent fixtures but this tends to light the talent too soft with little or no shadowing. A combination of fluorescent and tungsten is a good combination to achieve a nice flat blue/green screen light and still be able to control shadows on talent. It is important that the color temperature of all lighting is the same.

Figure 6
 


o Cool Lights
Fluorescent lights offer several advantages: lower energy costs to create the same amount of light as quartz or halogen lighting; a fluorescent tube presents larger source of light than a small, intense halogen bulb, so the light is already somewhat diffused; and most importantly, lower electrical consumption which means a cooler surface that will not burn someone if they accidentally get too close to the light nor require gloves to adjust the light. Fluorescent lighting can be purchased at a hardware store for as little as $30, and can provide a very soft wash of illumination. There are also special fluorescent fixtures for video production (pictured above), with an emphasis on long-life, accurate color reproduction and dimmer compatibility. A single fixture unit costs about $650, and a double fixture about $900. Satisfactory lighting of both talent and background can be achieved with just three units, so lighting setups can range in price from $150 for shop lights to $2,300 for pro fixtures.

o Separate Lights
It is important to light the background with one set of lights and the person with another set. This allows flat lighting on the background, with freedom to light the talent as gently or dramatically as you wish. It helps to use lighting above and behind to separate the talent from whatever image will replace the background. Don’t overdo this back lighting, however.
The limited dynamic range of the camera means there will be little useful color data in overexposed highlights, which makes it next to impossible to separate fringe zones (such as hair detail) from the background.