Secrets Q & A
- Written by Darin Perrigo
- Published on 30 November 2007
Page 4 of 6
How Much Simultaneous CR Can We See at Once?
I don't have an absolute answer to this one, but I know the number isn't as low as I've seen some claim. I have seen several people say that projectors don't need to be able to do more than 100:1 CR at once since humans can't see more than that. I don't know where that number came from, and I'm somewhat baffled since I can easily prove that people can see transitions much higher than that to anybody with close to normal vision in less than 5 minutes in my theater, but for some reason it seems to persist. If we could only see transitions with a 100:1 CR or less range at once it would make projector design easier, but that isn't the case. When bright and dark objects are very close in proximity, our ability to discern levels can be low, but with images on projection screens, levels of very different intensities do not have to be right next to each other.
I have talked with one company who works on High Dynamic Range displays that can do way more than 100:1 at once and can been seen by people viewing their displays, and they don't know where that number came from either. I was told that vision specialists have said that the human eye can perceive somewhere in the range 100,000:1 without any adaptation of the iris. I haven't devised any test for measuring the most CR I can see this way since I have trouble measuring things close to that high with any measuring equipment I have, but I know that transitions that are beyond 2000:1 simultaneous CR from the brightest objects can be easy to see. This doesn't mean that I can see beyond 2000:1 in every image, but there are conditions where I can see beyond that easily. One test I've run is using an image I have of a skeleton on a "black" background. The skeleton takes up only a small part of the image, so the simultaneous CR is much higher than the ANSI CR from my projector and room, and lower than the On/Off CR. By zooming the image from my projector down to smaller than the screen area, I can get transitions from the projected "black" to an area that is lit from washout effects and then from that to the screen border. In this test, it is easy to see some detail in the whites on the skeleton, then the projected "black", then the area beyond the projected border, then the screen border. I cannot see the difference between the screen border and the black velvet behind it though. While I can do measurements to find that the CR from the white on the skeleton to the area inside the screen border is more than a couple thousand to one in my mostly black velvet room, I don't have anything that can measure the light level off the screen border accurately.
For a while I thought that the 100:1 simultaneous CR limit floating around came from confusion with the Contrast Sensitivity Function, but I'm not sure if that is the main source of this. In a book by Poynton, he says that humans can discern different luminances across about a 1000:1 range at a particular state of adaptation, but there are references on the Internet to him claiming a 100:1 limit. This may have ultimately come from his discussion of CSF and threshold of 1.01:1 though.
In any case, I will explain why the CSF does not tell us the upper limit for how much simultaneous CR we can see. The main reason is that CSF doesn't measure the upper limit, it is a measure of the lower limit, or how we perceive low contrast cycles, not high contrast cycles. The Contrast Sensitivity is the inverse or reciprocal of the Contrast Threshold. That is, as the Contrast Threshold gets smaller (like off-white on white), the Contrast Sensitivity goes up. A Contrast Sensitivity score of about 100:1 means that a person started to be able to differentiate levels when they got to around 1% or CR of 1.01:1 (although those aren't exact), and as the separations got larger than 1.01:1, they would continue to be able to see them. Because some people miss the inverse or reciprocal part here, they may falsely assume that higher Contrast Sensitivity is higher CR when the opposite is true. Or that Contrast Sensitivity going down as our eyes age means we would want less CR, when in fact lower Contrast Sensitivity scores mean that we need more differentiation (or more CR) between levels to be able to see them as our eyes age. However, we also might want brighter whites as we age and our eye's ability to pick up light decreases.
Readers are welcome to try their own experiments putting black posterboard over part of their screens with images of some white levels and a lot of video black or by using their hands or something else to create shadows in those kinds of images and see if they have any trouble seeing those dark transitions and white transitions at the same time. There is a spot in Sin City in chapter 20 at 1:28:45 on the regular DVD that can be useful for this, although it will probably require lowering the brightness setting a few notches since the background isn't encoded completely as video black. Here is a shot of that scene:
Copyright Dimension Studios
A projector with just 100:1 ANSI CR and 1000:1 On/Off CR could do much more than 100:1(and less than 1000:1) simultaneous CR in a scene like that, and most of the projectors discussed here can beat both of those. A white room could hurt the simultaneous CR off the screen, but the brightest part of this scene only takes up part of the image, and part of that could be blocked with dark material (leaving some white with detail) while testing to reduce the effect of reflections around the room, if desired. If the simultaneous CR off the screen is too high, a person might not see the difference between the screen and black poster-board or a shadow created in the darkest parts of the image, but I don't know of any projectors which would create that problem at the moment. Note that shadows with setups which have light coming from more than source (like dual projectors or projectors with three lenses) are not the same as shadows created from a single light source.
Why On/Off CR Matters More Than Absolute Black Level From a Projector
A common question I see is how projectors compare for absolute black level. I steer people toward On/Off CR even if what they ultimately care about is absolute black level, because front projectors cannot give you images on their own. They require a surface. Without a surface there are no images to see. And the user gets to determine what surface is used to a large degree, along with whether they will use any kind of filtering (like a neutral density filter) on the projector, although these mostly apply to digital projectors and not to CRTs. Until a surface (usually a screen) and filtering (or not) are chosen, there is no absolute black level in ft-lamberts or cd/m2 (values for light coming off the screen), but rather only in lumens (values for light going toward the screen). And if filtering and screens are chosen such that two setups produce the same ft-lamberts for white, then the one with the higher On/Off CR will have the lower absolute black level. The math just works out that way with the following equations which are all forms of the same thing:
(On/Off CR) = (white level) / (black level)
(white level) = (On/Off CR) * (black level)
(black level) = (white level) / (On/Off CR)
With any two of the above, we can determine the third, and the last one shows that for the same white level, a higher On/Off CR means a lower black level.
As an example, let's consider two projectors with the following properties:
Projector A: 1000 lumens and 2000:1 On/Off CR
Projector B: 500 lumens and 1500:1 On/Off CR
If we just look at the absolute black levels we get:
Projector A: 0.5 lumens
Projector B: 0.33 lumens
We can see that projector B has the better absolute black level if we just consider what is coming out of the projector. However, if we look at a complete solution which includes a screen (and possibly a filter), the results can change. Projector A is fairly bright, so let's put it on a screen with a gain of 0.5. And we'll pick a size which results in 15 ft-lamberts for white. Projector B isn't bright enough to use with that screen and have the whites be bright enough if somebody wants in the 15 ft-lamberts range, so for that one we will use a 1.0 gain screen. The results here are the following:
Projector A: 15 ft-lamberts for white and 0.0075 ft-lamberts for absolute black
Projector B: 15 ft-lamberts for white and 0.01 ft-lamberts for absolute black
As can be seen, the projector with the higher On/Off CR ended up with the lower absolute black level once things were set up to the same white levels, even though it had the brighter black level coming out of the projector lens. This is also not even counting that the darker screen that could be used with projector A will help kill reflections off the walls and help the ANSI CR. Even if a person was happy with 7.5 ft-lamberts for white and would have used the 0.5 gain screen for projector B, they could have put a 2x neutral density filter on projector A (if it was a single lens projector like a digital) for the same white level and once again that lower absolute black level off the screen.
People do not have infinite choices for screen gain among commercial screens out there, but even so, I would encourage giving the On/Off CR more weight than the absolute black level out of the projector lens (or lenses).