Doug Blackburn – Contrast Ratios

From a thread in AVS Forum.  By Doug Blackburn

The OP is confused in the extreme.

16-235 is digital values for video… there are no units.

When you MEASURE how black (or white) a TV is with a meter, you are going to measure some UNITS OF LIGHT like cd/m2 (metric) or fL (foot-Lamberts),

The thing is technology is not perfect. If the TV was perfect, 16 would ALWAYS produce total blackness from every video display (assuming you were viewing consumer video sources where black is represented by 16 and 100% white I represented by 235. But TVs always have some residual amount of light they reproduce when the screen is creating what passes for black. A few TVs (some Pioneer Kuro plasma models, not made for several years now or some brand new Panasonic plasma models) have gotten very very very close to digital 16 being truly black, but many TVs still have a visible dark gray displayed instead of black.

Let’s say you measure digital level 16 (and the TV is correctly adjusted so 16 really is as black as the TV ever gets) and you measure 0.001 fL (foot-Lamberts) and you measure 100% white at 30 fL. The contrast ratio is 30 divided by 0.001, your contrast ratio would be 30,000:1

Images just plain look better and more lifelike with higher contrast ratios (as long as 100% white isn’t so bright that it causes you to squint which leads to eye fatigue over the long haul.

Let’s say somebody makes a PERFECT video display and digital level 16 produces total black from the screen… no measurable or visible light output from the TV when it is displaying black. In that case, the contrast ratio would be infinite because whatever level you use for 100% white will be divided by zero and anything divided by zero is infinity. A calculator will tell you that you cannot divide by 0, but any number divided by zero really results in infinity being the answer. Infinity is a valid mathematical concept and dividing by zero is an equally valid concept. Divide 30 by .001 to get 30,000. If you divide 30 by .00001, you get 300,000… make the divisor smaller and smaller and the result gets larger and larger. Zero is the “smallest” number and produces the larges result… infinity. So a “perfect” TV with truly black blacks will have an infinite contrast ratio no matter how bright or dim you set 100% white.

When it comes to projectors, manufacturers do all kinds of trickery to result in higher-than-real-world numbers. I’ve heard of some cases where the projector is placed so far from the screen, that a single pixel becomes large enough to measure with a meter (generally, your meter will measure some significant number of pixels, at least several thousand of the more than 2 million pixel triplets (each pixel has individual red, green, and blue pixels producing the light representing a single pixel, in projectors, those Red, green, and blie pixels are stacked on top of each other, in flat panel displays, the 3 colors are side-by-side for crt or plasma and on top of each other for LCD) that make an HD image frame. At any rate, the trick used by some projector manufacturers is to measure just ONE pixel so they get all the light and none of the dark area between pixels. That method over-states the real-world contrast ratio and often leads to the 6-digit or higher contrast ratio claims you see today.

Black level is more important to contrast ratio than the 100% white level. Example:

100% white is 100 fL and black measures 1 fL. Contrast ratio is 100:1
If 100% white s 200 fL and black is 1 fL the contrast ratio is 200:1
But if 100% white is 100 fL and black is 0.5 fL, contrast ratio is 500:1
So obviously, it is much better for contrast ratio to cut the black level by 1/2 than it is to double the white level.

Depending on where you read it, human vision is capable of only about 400:1 to 800:1 contrast ratio in any one scene. But the world around us has a contrast ratio that is infinity though there probably aren’t very many people who have experienced an infinite contrast ratio… even in the middle of nowhere on a moonless night, the light from stars will create a contrast ratio lower than infinity. To experience an infinite contrast ratio, you have to be in a laboratory that is capable of total blackness (much like an anechoic chamber for acoustic research attempts to produce total silence, it’s really difficult), though total blackness is easier to make than total silence.

Anyway, images where black is actually dark gray instead of black look milky and lack dimension. The blacker you make the blacks, the more dimensional and realistic the images will look. That’s why we like displays that produce very dark blacks rather than being obviously gray.

You can setup a video display to produce its darkest black at digital level 16 as required for consumer video sources, or you can also setup a video display to produce its blackest black for digital level 0… which is what computers do… computers operate in the 0-255 realm. If you view consumer video on a computer setup for 0-255, the consumer video source will never have a digital level below 16 (there can be occasional mistakes with numbers lower than 16 but those are mistakes not actual information). So consumer video played on a computer setup for 0-255 will never create images with true black (or as close to black as the computer display can get). Sometimes the computer video system is smart enough to know the incoming video is consumer video in the 16-235 format and the computer will switch to that mode automatically for you, but some computer video systems don’t change automatically, you have to find a setting somewhere and change it for the consumer video you want to view… if you want the images to look pretty good.

Let’s say the computer monitor produces 0.0001 fL for digital level 0 (zero). When you change the computer video system to work with consumer video and your lowest digital level will be 16 and you want 16 to produce the blackest black, when you have setup the computer video system to produce its best black for digital level 16, you’ll still measure 0.0001 for digital level 16… just as black as digital level 0. It’s all in the settings… either in the computer or in the TV (and if you can’t change the computer to show consumer video correctly, you may be able to change a setting in the TV so that the TV goes into 0-255 mode).

 

The iris in our eyes can only have one opening size at any one time. If the scene is mostly bright with only a little darkness in it, brightness will win and our iris will close to keep the perceived light level comfortable… but that will darken the shadow areas and we won’t “see into the shadows”… but if we look away from the bright area of the scene and focus only on the darker area, our iris will open farther to allow us to see shadow detail easily, though the bright area will be uncomfortably bright if it attracts our attention… at least it will be uncomfortably bright until our iris closes enough to make it comfortable again. It’s the action of our iris that allows us to tell that 100,000:1 contrast ratio makes better-looking images than 1000:1 contrast ratio.

But when you look at one scene with an “average” luminance level that keeps our iris at a fixed opening (say half way between fully open and fully closed), we can only perceive somewhere between around 400:1 and 800:1 (depending on your reference source) contrast.

Average Picture Level is a way to define how bright images are on a projector or flat panel TV. An APL of 100% could only be achieved if you displayed a 100% white pattern that filled the entire screen. A 0% APL could only be achieved on a perfect video display that produced ZERO light when it displayed 0% white (AKA black)… all the video displays we have available today are below 1% APL when displaying a full-screen black pattern, but they are also somewhere above 0% APL because their blacks aren’t perfectly black. If you display an image with an APL of 10%, we can see 400:1 to 800:1 contrast within that image. If you display an image with an 80% APL we will still see THAT image with contrast of 400:1 – 800:1 because our iris attempts to keep dark scenes bright enough to see detail and bright scenes dim enough to not cause discomfort (i.e. squinting, shading eyes, blinking uncontrollably, etc.).

Bottom line… we can see over a very wide range of luminance levels, but not all at the same time. Our eyes have to adapt (via the iris opening changing sizes). This is most easily observed when you turn off bright lighting in a room and it becomes very very dark all at once. As you stand there waiting, the room changes from being so dark you can’t see anything to being able to make out at least shapes and sizes of objects as you are in the dark environment longer and longer. And when a flash goes off in a dark room, we’re essentially blinded momentarily until our iris opens again. So we WANT video displays with high contrast ratios and the higher the better… infinity would be great! We just can’t see black and near black (i.e. shadows) and white and near-white (i.e. highlights) clearly all at the same time in the same scene.

 

Written by Doug Blackburn