Blue Screen Process - circa 1980

The following submission is a verbatim copy of the script for Blue Screen 1980.

(Burn In) I have wanted to make this video for a long time.

Every so often I paw through my stuff and find things that remind me of topics I’d like to cover in a tutorial. This subject has been on my mind for years. I compare this era in cinema to our nations lunar program. An amazing cooperative of artists, imagination, optics, engineering and home brewed software.

The results of this effort changed moviemaking forever.

I learned the blue screen process at ILM on the Empire Strikes Back. At the time, this process was considered cutting edge, the culmination of decades of photochemical tinkering. 2k digital compositing is still years away.

But almost 100 years earlier…

George Melies was busy experimenting with in camera, multiple exposure photography. This amazing short film mesmerized the world and launched a new segment of filmmaking soon to be known as special photographic effects.

The Williams Process ushered in the next big leap forward with this stunning sequence from FW Murnau’s “Sunrise”. This is the first known popular use of bi-packed travelling mattes. Frank Williams was said to have shot his subjects against a black screen and then generated mattes through high contrast print backs, and hand painted matte clean up.

In the early thirties, the Dunning process was used extensively on the epic “King Kong” His subjects were lit with strong yellow light and photographed against a blue screen. With blue filters and the newly introduced panchromatic B&W film, the Dunning Process rendered excellent matte elements.  In addition, King Kong is credited as the first movie to use an optical printer for compositing. Linwood Dunn, ASC is known as the father of optical printing and was the man who convinced 20th Century Fox to give his printer a try on King Kong,

In 1941, Larry Butler won an Oscar for his blue screen process on “The Thief of Baghdad”. Maybe a bit crude by today’s standards, but given the inaugural task of compositing in color, this film represented a huge leap forward and was foundational for the modern blue screen process.

Ub Iwerks and Petro Vlahos developed the Sodium Screen Process at the Disney Studios. This ingenious technique used a two strip camera and a one of a kind prism that split the taking image simultaneously, to color negative film and B&W film. This highly praised process, was in use until 1990.

Here are the Bones of Blue Screen Photography.

The blue screen process relies on maximum separation of a
narrow slice of blue light from the rest of  natures visible
colors.  We filmed our subjects in front of rear lit blue screens that emitted light in the range of around 500 nanometers. A nanometer is a numerical value for energy waves on the electromagnetic spectrum. As seen here, visible light occupies a small slice of the electromagnetic spectrum.

If we were to print this chart to B&W film with blue light look what happens. It becomes clear and this is the key to the entire process, making the blue screen go completely clear.

This contact printed element is called a blue print. This is the first use of the original negative and the first step in the blue screen process.

The succeeding steps in the process require an optical printer so lets now introduce that to the mix.

An Optical Printer is a precision machine that re-photographs any number of printing elements onto one piece of film known as a composite.

Here’s a simplified look at an Aerial Image Optical printer:

There is a camera fitted with a really good lens. It has a front projector and a rear projector. The relay lens focuses the image from the rear projector – here - at the film plane of the front projector – here. The lamp house provides the illumination for the film in the camera. Notice that each projector has two sets of take up reels. This supports the ability to thread two pieces of film in the projector simultaneously. This technique is called bi packing.  

As you’ll soon see, to create blue screen printing elements, you’ll need to be able to bi pack.

OK – step one was making a blue print and the next use of the original negative is printing RED and GREEN color separations. The red separation is made with RED light.  Notice how the blue screen area has gone completely black. A green separation is made with green light. If we were to use a blue separation it would be made with blue light. We don’t use the blue separation for two main reasons: blue fringing and a matting issue will discuss shortly. Most colors in the visible spectrum are made up of equal amounts of green and blue. So, when we re-combine these separations, we use the green separation twice, once with green light and once with blue light.

3 – COVER MATTE – The next step is to make a cover matte for each separation. This is achieved by bi packing the original negative and the blue print together in the front projector of an optical printer. Cover Mattes are black and white elements and are made with Red light. During composite photography, cover mattes are bi packed with the separations to add density to the blue screen area so only the ship element gets exposure.  A normal cover matte isn’t dense enough for a blue separation and that’s the other reason we don’t use it.

4 – INTERMATTE –The intermatte is made just like a cover matte except it is a much higher contrast element. We use the intermatte to make the hold out. A hold out protects the area the ship will occupy in the composite.  Without a hold out the ship would be transparent on the background.

5 – BACKGROUND  -  We need just one more element and that’s an interpositive from the BG negative  Now all of the positive intermediates have been created and are ready for compositing on an optical printer to a duplicate negative.

First, bi-pack the separation and cover matte rolls in the front projector. Then thread the single hold out roll in the rear projector. Next, fit the separation to the hold out. An ideal matte fit is one with a little white line all the way around the ship, like that. After the fit remove the hold out, it will be used on the final pass in conjunction with the background.  Check for any hairs in the gate and we’re good to go. The first of 4 passes is the Red separation photographed with Red light. Once complete, cap the lens rewind the camera and shoot the green separation. Rewind the camera and shoot the green separation with blue light.

If we were to stop now – this is what we would have after development and printing.  So to finish, we need to shoot a background pass.. The background roll is threaded in the front projector and the hold out is threaded in the rear projector. Rewind the camera for the last pass and fire away! And that’s it - a simple two element composite is in the can!

If you think that’s a lot of work for a two element shot, try adding up to 62 more elements to the mix.  Thanks for watching!

Filming of the X Wing took place outside against a cheap blue felt backing that cost $9. The fabric was hung in the shade and the model was rigged and placed 7’ away, under a cloudy sky . A small LED light was used to give the ship an edge placed in roughly the same position as the sun in the BG plate. The blue screen exposure was 1 stop under key. Nothing says miniature like half your model out of focus. To fix that issue, I lit for an aperture set at F22 with a 3 second exposure. These settings gave me a depth of field of 11.5 inches more than enough ti cover the 9 inch long ship The 55mm lens was 3 ft  from the model and the ISO set at 1000.

Yes, this is kind of a nerdy topic, and by the way, completely obsolete. But I believe it’s a part of an inventive and historic chapter in film history and needed to be documented for all to understand and enjoy.

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