Creating the illusion: Green screen lighting

The illusion
If good cinematography is about creating the illusion of three dimensional reality, on a two dimensional screen. Working to create reality using background plates offers a whole new variety of technical issues that need to be addressed with skill if it is going to be successful. In this article I will touch on some of the basic issues that need to be taken into account in order to pull off the illusion of "Reality", and done well it is a creative process and can be very rewarding.
First Step
The first step in the creative process is to evaluate the background plate or overlay image. You may or may not have shot the background plate yourself, either way you will need to make some decisions, for example:
Camera: Position of the camera, set up of camera including lens and lens height, iris and white balance.
Optical quality of Key and Back light: What type and time of day is it? Exterior, interior, sunny, cloudy, evening or night? What quality key light will you need to use to replicate the background lighting. Obviously a sunny day would be represented by a hard key with crisp shadows. A cloudy day would use a softer diffuse light and a nigh exterior could have some wonderful moody colours motivated by streetlights and shop windows
Direction of light: Most importantly, where would the key come from - which direction are the shadows falling? Get this wrong and there is no true illusion. The human brain subliminally picks up these subtle and fine details.
Colour quality: You will need to match the foreground to the background
Subject position in frame: Where can you naturally place the subject?
Lighting the Screen
Essentially there are a few rules for trouble free green screens. Here are some of the most essential:
Even Flat Lighting - to within 1/3rd of a stop. Try using a spot meter. This works best for me. The most common method for this and an international standard are 4' 4Banks from Kino Flo. These have several advantages. Firstly they are large soft sources with wide reflectors that help multiple units blend the light together seamlessly. This system will dramatically reduce the set up time , the amount of units required and offer excellent colour that keeps contamination of the subject to a minimum.
Lamp Colour Choice - It is essential to avoid the contamination of green light onto the subject. There are several ways to do this, however with digital cameras and green screens the most successful combination I find is the use of the KF3200 lamp from Kino Flo. In features and commercials on large stages the use of narrow wavelength (special effects) blue and green tubes are used, however in broadcast studios it is more common to see the screen lit with KF3200 bulbs - this provides good colour for the green screen (better that naked tungsten) but further to that, any direct contamination from the lights will not affect the foreground. In some cases where space is an issue the Kino's can be used in a vertical position and light both the screen and rim light the subject at the same time. Other colour choices for lighting the screen are appropriate depending on circumstance for example blue screen - or large screens however the above system is the most popular professional system currently in use – and has a proven track record.
Avoid contamination - keep your subjects as far from the screen as possible. In my book, a minimum of 2m is required If this is not possible, careful use of backlights can help alleviate some of these issues.
Avoid light with corrupt or a non-continuous colour spectrum- It can be tempting to use lights with a green spike for example (Architectural or non-Kino Flo fluorescents or cheap LED sources) and this may work well if controlled correctly (it can also reduce the amount of light needed) however most shooting environments are in constant flux and unless you are renting or using the fixtures for a single purpose or set up, contamination (especially on skin) can be a major issue in pulling a clean key.
Green is your enemy - If producing high quality images is our aim, we need to take real care of skin tones, but we need to ensure we can easily pull off detail like hair convincingly. In order to do this we need to work to keep all green light including spill and bounce light confined to the screen area itself. We need to be aware of the psychology of colour on the human - Green denotes the colour of illness and or decay and it is a colour we should not really use on skin. It creates a sickly looking person that is not pleasing to the eye. High quality skin tones make images look glossy.
Fighting Green - There are times when contamination cannot be avoided generally due to lack of space. In this situation some minus green gel (magenta) on the back lights can counter the contamination and do a great job of helping to provide that clean problem free key. But an issue may arise when you put the background in as you will see a magenta rim around your subject in the foreground, so beware.
Summary:
The art of pulling a good matte is the careful and full analysis of the foreground subject and the background to be inserted. Recording as much resolution and digital information that the edit system may need, including recording at a 4:2:2 sub-sampling compression ration or better and lighting the foreground sympathetically with the light in the background. Lighting the Green or Blue screen evenly and at an exposure level of 1 F-stop below the foreground, and placing the camera at a correct orientation as to create a foreground image that sits seamlessly into the back ground.
If you can achieve all this, you will find the postproduction processes a pleasure and the illusion will be seamless and complete.


David Morphy is the technical director and one of the founding partners at Cirro Lite a company renowned for its technical innovation and market leading solutions. David has been involved in high end lighting technology for over 30 years, he started his career in the music industry working with visually creative acts like Pink Floyd, Genesis and the Rolling Stones. In 1992 he started Cirro Lite with John Coppen to promote new technology with the aim of launching Kino Flo in Europe, his first major screen job was for ILM (George Lucas) who were contracted to do the special effects for the “Helicopter in the tunnel” sequence on the first Mission Impossible film on the 007 stage in Pinewood. For this Sequence ILM insisted that a plane full of Kino Flo units loaded with Spiked Blue 420nm bulbs to light the screen in order to hit the release date for the feature. This was a first in Europe and the start of a whole new wave of digital effects work now common place in the industry. David brings this knowledge and more to the creative use of light in image creation.