Feature Article

Understanding Contrast Ratios in Video Display Devices

Part III

May, 2006

Darin Perrigo


What is Black?

One definition of black is the total absence of light. However, if this were the only definition, then things like black cars and black cats couldn't be seen. Black can also be used to describe what we perceive as black. If that weren't the case, then we couldn't talk about blacks in any mixed images.

In the figure shown below, the circle appears dark gray when surrounded by black, but appears black when surrounded by white.

In video, there is also something known as video black. In 8 bit space (0-255) with the BT.1361 standard, this is level 16. Levels below 16 are known as below black. Things encoded as video black can end up looking like gray on a projection screen, or they can end up looking like black.

What is White?

While white is achromatic color that is bright, it also can be used to describe what we perceive as white. At the same level of brightness, we can perceive something as gray (or even very dark) under some conditions and white under others. For instance, 16 ftLs viewed on a screen can look like bright white in a dark theater, while that same level can come off the dark tires on a car viewed outside and look very dark gray or almost black under those conditions. This is one reason that side-by-sides of projectors viewed simultaneously can be very misleading (besides each one washing the other's images out for front projectors), and also one reason viewing two or more screen materials at the same time can give very misleading impressions. For example, the center spot in the two figures shown below are the same, but it should look white in the figure on the left, and gray in the figure on the right, because of the black or white surrounding material.

In video, there are also things known as reference white and peak white. In 8 bit space (0-255) with the BT.1361 standard reference, white is level 235 and peak white is level 254. Reference white (or 100%stim) is what should be used for the bright screen in the On/Off CR test and I refer to this as "white".

How Much On/Off CR is Enough?

To match film that most of us see in typical movie theaters, 2000:1 On/Off CR is generally considered enough in our home theaters. However, that is a limitation of the mechanism for playback, and I am more interested in what it takes to match what we can perceive in real life. Also, the specifications like BT.1361 call for video black to be at "no light" and not at the raised black level we generally get in commercial theaters.

I have seen some low numbers discussed for how much On/Off CR people can see, but as I will explain below, I have done some testing with a CRT front projector where it was determined that the On/Off contrast ratio was close to 700,000:1 after tweaking. This is very good, yet still left some room for improvement.

In real life, the ratio of luminance between white paper in sunlight and white paper in moonlight is approximately 1,000,000:1, while the ratio for white paper in sunlight to white paper in starlight is approximately 100,000,000:1. However, white paper in sunlight is too bright for comfortable reading levels, and the white levels that are used with projected images in commercial theaters are much closer to those comfortable reading levels. Even bright plasma screens are much closer to comfortable reading levels than white paper out in sunlight. In a dark theater, going from the white levels we see outside and back and forth with very dark images would bother a lot of people and result in eyestrain. We generally don't go from very bright to very dark conditions and vice versa as quickly and as often in real life as many movies do. For those who want to set up with white levels for movie watching that go much brighter than comfortable reading levels and who would get eye strain from transitions between very bright levels and very dark levels, backlighting can be used to keep things from going too dark in the dark scenes. This can be something as simple as a small table lamp behind the projection screen. However, under these conditions, the On/Off CR of the projector can become mostly irrelevant as the backlighting limits the absolute black level off the screen, and realistic blackouts become impossible regardless of the capabilities of a projector.

For those happy with white levels that are close to commercial theaters or at least in that ballpark, a question then becomes how black is black enough for blackout scenes. For those who want blackouts to hold so that they cannot see the screen (this means the screen also has to be invisible in the room when the projector is off, and the rest of the room is set up for viewing), we can look at some data and then examine some test results in more detail.

The lower cone threshold (for color vision) is about at the white paper in starlight level (3.2 x 10-4 cd/m2), but we can still see about 100 times lower than that with the rods (black & white vision) in our eyes. They are thought to go down to approximately 3.2 x 10-6 cd/m2. For reference, the upper limit where rods tend to get saturated is around 3.2 cd/m2, so by themselves they have a range of about 1,000,000:1.

If we use a fairly common white level for movies of about 40 cd/m2 (a little under 12 ft-lamberts), then matching that absolute lower limit for our vision would imply an On/Off CR of 12,500,000:1. But it takes time for our eyes to adjust (including dark adaptation after entering a dark theater), and when going from bright scenes to a blackout, it doesn't take nearly that much to give the perception of no light on the screen. In one test I ran with a projector that had about 9000:1 On/Off CR using the end of Eternal Sunshine of the Spotless Mind, I could not see the screen at all for the approximately one second of blackout that followed a full screen bright white image before the credits started to roll.

There are much more difficult scenes for absolute blacks than that one in Eternal Sunshine of the Spotless Mind though. For instance, Cast Away contains a sequence that goes from lightning flashes to very dark and ends with a 17 second blackout (near the end of chapter 8 from 29:49 to 30:06). Finding Nemo has an even longer blackout in the chapter called, "The Abyss", and that one has a slow fade into the blackout. So, in that case, there is a little less biasing of the eyes towards brighter things before the blackout.

I don't have a display at my disposal that can hold either one of those blackouts to my eyes for the whole time without compromising shadow detail. CRT projectors generally have a problem where calibrating them to hold blackouts like those mentioned above compromises their shadow detail, since they then don't tend to come out of black fast enough for parts of images that are encoded a little above video black. However, some people have been working on improving this situation recently. One way to address the problem is to do some adjustment to the gamma down low to separate the levels. This is possible with a PC as well as with some external circuits some people are working on. I have a friend with a Sony G70 CRT who has done some gamma adjustment with his PC sources to improve this tradeoff between absolute black level and shadow detail in mixed scenes. Some of the best CRTs are rated at about 30,000:1 On/Off CR, but with his calibration, he is getting a higher CR than that. So, his display is a good one for me to do testing with to find out how much we can see and to try to get some idea about how much On/Off CR is needed before no improvement could be perceived.

In the Cast Away and Finding Nemo scenes mentioned above, that CRT goes dark enough that the screen is basically invisible for a few seconds into the blackout, with the length of time depending partially on how our eyes are adjusted before starting the test. Beyond a few seconds, we can see that the screen is being lit up by the projector, although it isn't a lot of light. I don't have any instruments that would allow me to measure the On/Off CR he is getting directly, so I came up with another way. I used a digital projector (a Sony VW100) from which I can measure the On/Off CR pretty accurately. I get close to 23,000:1 dynamic On/Off CR in the center of the screen with that projector at my favorite settings. We covered all but the center of the screen with dark material which blocked some lines that this G70 displays on blackouts and also blocked the outside of the image that has less CR than the center with this VW100. We each used a full screen video black test image, and I used neutral density filters (which were high enough quality to not skew the color balance too much) on the VW100 until the absolute black level from the CRT and the absolute black level from the VW100 were close to the same. With an 8x and a 4x filter on the VW100, the absolute black level was still higher from it, but with another 2x filter added, it was dimmer. We estimated how far the crossover point was by eye. With the filter in place, I then measured the resulting white levels from each projector and got 0.41 cd/m2 for the VW100 and 15.6 cd/m2 for the CRT. From our estimate of how much we had overshot with the last 2x filter, we modified the 0.41 to 0.5. We then removed the ND filters from the VW100, and I used a different meter to measure the white and black levels close to it as 14,200 lux and 0.62 lux. Then the calculation for On/Off CR from the CRT was basically the On/Off CR of the VW100 multiplied by how much brighter the whites on the CRT were with the filters on the VW100, or:

(14,200 / 0.62) * (15.6 / 0.5) = 700,000:1

So, still a ways to go for the digital projectors to achieve that and even further to be able to hold both the Cast Away blackout and the Finding Nemo blackout to our eyes in a setup like this.

Although it wasn't necessary to measure the white level on the screen from the VW100 without the filters, I know that it had the higher white level (and this can be seen from how much filtering we used), and readers may notice that the projector with the higher On/Off CR had the dimmer whites. I only mention this because it can be easy to make the mistake of thinking that higher On/Off CR means brighter whites. There is no way to know whether the projector with the higher or the lower On/Off CR will have the brighter whites without knowing more than just the On/Off CRs (like the absolute black levels).

One way to look at the room for improvement for digital projectors in On/Off CR is that a projector with 5000:1 On/Off can block 99.98% of the light (with just 0.02% getting through) and needs to get to the point of blocking 99.9999% of the light to get to 1,000,000:1 or just block that last 0.02% it is missing now to get to essentially infinite On/Off CR. However, that makes it sound easier than it is. Fortunately, there are some smart people working on these projectors and as long as they care enough about making these blackouts realistic, they will ultimately get there.

As far as the length of time that I say a projector maintains a blackout, I want to be clear that the projector is putting out a fixed amount of light, and it is our eyes that are adjusting with time and making it so that we can't see the screen one second and can the next, although the level of light reflected from the screen hasn't changed.

The above was of course for a full screen blackout. As I mentioned further up, there are simultaneous CRs in scenes that are not blackouts. For many very dark scenes, the difference between that 23,000:1 in the center (lower at the edges) and that 700,000:1 still makes a visible difference in the images. As scenes get brighter, ANSI CR comes more into play and then the On/Off CR generally matters much less toward the blacks that are on the screen than it does in darker scenes.

Here is an image from AVP: Alien vs Predator, where the extra On/Off CR of 700,000:1 helps:

Copyright 20th Century Fox

Now some people may say that cameras cannot capture 700,000:1 or higher, so there is no reason to want that much in a projector. However, contrast ratios with cameras are different than with projectors. A camera can get no light and register no light, so it wouldn't be very useful to rate cameras based on no light for black since they would all end up with the same CRs or dynamic ranges (basically infinite). It makes a lot more sense to rate a device that captures light with one step above the lowest. However, with devices that put out light like a projector, the absolute minimum amount of light they can put out is relevant and differentiates them from each other. And when standards for video call for no light, then any contrast ratio less than essentially infinite (a black level of no light) misses out on meeting that standard. People doing the telecine transfers from film to digital can choose to encode levels to call for no light or to encode them higher if that is desired, but if the display can't put out zero light, then even if encoded for that, it won't be met. If the display can put out no light, then encoding higher is a way to avoid that, in cases where more than no light is desired.

Go to Part IV.

Copyright 2006 Secrets of Home Theater & High Fidelity

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