In today's post we're going to talk about contrast and contrast ratio in the context of projected video. I'd like to take a moment to go over some video basics and then we'll dive right into Contrast.
Video – A Primer
Irrespective of shape, type, and application, all current display devices work in the same fundamental way. Taking advantage of the phenomenon, persistence of vision, that gives our wee blog its name, all display devices produce an image by very rapidly “drawing” a series of horizontal lines that are “stacked up” to produce what can appear to the eye to be an unbroken image. Appropriately enough given the recent holidays in North America, persistence of vision is the same effect that causes us to perceive lines drawn in the air with hand held sparklers as unbroken.
So, whether we're talking about phosphors triggered sequentially (CRTs and plasmas), polarized TFT panels (LCD, LCOS), sequentially flipped micro-mirrors (DLP) or individual LEDs , the basic mechanism of drawing and stacking horizontal lines to create the illusion of an image is the same. The same persistence of vision is what causes the illusion of motion when a series of images with slight differences between them is displayed sequentially. Think of a flipbook as an example:
So that's about it, in simplest terms: draw a bunch of horizontal lines really quickly to create the illusion of a picture and then change the picture quickly enough to create the illusion of motion.
Here's a little knowledge bonus- if you've ever had terms like 1080p, 720p and 480i being bandied about around you but were unclear on their meaning, here's the explanation:
Numbers like 1080, 720, 480 tell you how many horizontal lines on your display make up each of our pictures. This reflects how detailed the individual picture can be.
The letters, i or p tell us how frequently the pictures are being displayed, reflecting how convincing the illusion of motion can be. i or interlaced means the screen displays all odd-numbered lines (1...3...5...7...) in 1/60th of a second followed by the even numbered lines in 1/60th of a second, these two fields are interlaced into a full 1080 line 'frame' every 30th of a second.
p or progressive displays scan all 1080 lines are rendered sequentially, and in 1/60th of a second, making for a more convincing illusion of motion.
Essentially that means you have 60 frames/second in progressive vs 30 frames/sec interlaced. More frames per second equals more convincing motion!
Those are the basics! If you'd like more detailed info, this is a good place to start.
Back to Contrast
Contrast is the range of difference between the brightest area of a projected image and the darkest area of the same image. It's generally described by a ratio of brightest to darkest and typically expressed in the form of :
X : 1
where 1 is the lowest light level measured and X is the highest. Generally speaking, the higher the contrast ratio the better.
Good Contrast Poor Contrast Poor Contrast
(inadequate white level) (elevated black level)
Why is contrast important? At a coarse level, contrast is what makes text legible,
Good Contrast Poor Contrast
At a fine level, it's what makes it possible to see critical detail in projected medical imagery,
or allows you to see an enemy coming out the shadows sooner in your favourite video game,
or to see all of subtle gradations of dark grey in a night time scene from a classic movie,
We've defined contrast and established that a good contrast ratio is important for any display application. Next we'll look at the factors that compromise contrast, specifically in projection based systems. We'll follow that by looking at strategies for improving compromised contrast. Stay tuned for the next post!