February 2005 - page 2
 The movie theater is a controlled environment based on a black background and very specific lighting values. There is a maximum light level (10-foot lamberts reflected from the screen on a pure white frame) and a reference color temperature. All of the color values are based on these. What about the reference black level? There is none. You just want to get as close to absolute black as possible. Film black level is excellent, but it is not the absolute black that can be had on a CRT-based system. A CRT-based system can achieve black levels that make images pop out in a way film cannot, but if film is the reference, and directors work with it in mind, then the system should at least be able to achieve film-level black.
Film is a “pass-through” medium, meaning that light is filtered through it. Some light passes through even the darkest black on film. The mechanics of a light filtering system make achieving what we would perceive as absolute black impractical. Film is also an analog medium, meaning that it can show all values of color and gray from the projector’s darkest black to its maximum output.
Video systems differ in many ways from film. While film shines light through an image on a sheet of celluloid, video systems chop up the picture into little pieces, then put them back together to create an image. The most basic element of a video image is the pixel. It shows us a light value at a particular point on the screen at a particular time. The number of pixels used dictates the detail level, and the light they put out, dictates how well the video system can achieve the reference light values. The values of gray displayed are discrete, but if there are enough levels of gray, we don’t notice them; if there are enough pixels, these should be too small too be seen at the normal viewing distance and meld in our eyes into a contiguous image.
Rather than our eyes, I should really say our minds, for it is how our minds use our eyes to see that is the entire basis of the magnificent deception that is the cinematic experience. We watch an image on the screen that has a maximum light output that is not even as bright as a well-lit room at night, and we can deceive ourselves into thinking we are looking at ancient Rome in a brightly lit outdoor scene. What is it that allows this deception? In order for us to see in the vastly different lighting that is indoors or at night as compared to outdoors on a bright day, our eyes and minds compress the lighting dynamics. Our irises open and close to accommodate the available light, and our minds process differences in light level logarithmically. This means, as an image gets brighter, our eyes become less sensitive to light variations. The effect is that it is the relative light level that we see, not the absolute light level. Therefore, if our eyes settle into the darkness of the movie theater, a mere 10-foot lamberts of light can look like an outdoor scene that originally measured over 1,000. If a little light is allowed to corrupt the black level, our eyes readjust and the lighting dynamics break down.
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