So, what are the causes of poor contrast? Contrast in a projected image can be compromised in two ways: inadequate, or elevated black levels and inadequate white levels. Let's start with elevated black levels as this is a more common concern with projection systems. We'll begin by offering a definition of black:
black is the absence of light
A surface that we perceive as black absorbs all visible light and reflects none of it. Given that (front) projected imagery is based on reflecting projected light back towards an audience we can begin to see a problem: how do we create the impression of an absence of light (black) using a device that produces light (projector?). There are a couple of strategies available, but first we have to digress for a moment and talk about how display devices work.
Getting Back to Black
When the content that we're watching calls for black (the absence of light), all display devices are essentially “turned off” during the time black is meant to be seen. And what does the average direct view display device look like when it's turned off? Black!
But what about projection? What do the reflective surfaces look like when the input signal calls for black, i.e., when the projector is off?
Oh, oh! If there's enough ambient light in the room to see that the screen surface is white, we've got a problem: we are looking at the black level of the projected image in those lighting conditions! This is exactly how elevated black level compromises contrast. Ambient or unwanted light is reflected off the screen along with the content that we're looking to see. Let's go back to the illustrations from the previous post see how elevated black level compromises contrast.
Good Contrast
Poor Contrast due to elevated black level
Good Contrast
Poor Contrast due to elevated black level
Good Contrast
Poor Contrast due to elevated black level
Inadequate white level is the other factor that can compromise contrast. This is a matter of there being insufficient projector light available to properly illuminate a screen of a given size and reflectance.
Good Contrast
Poor Contrast due to insufficient light levels
Good Contrast
Poor Contrast due to insufficient white level
Good Contrast
Poor Contrast due to insufficient light level
In upcoming posts, we'll be taking a closer look at contrast ratios: how they're arrived at, how most of the published ratios for display devices are, at best, fictional and how to determine “real” contrast ratios. We'll then propose guidelines for the minimum “real” contrast ratios required for acceptable results with different types of content. Lastly, we'll provide you with the tools and techniques to predict the “real” contrast ratio of a proposed installation.