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I wanted to have a discussion about contrast and black level because it seems to be a serious issue with LCD/DLP/and LCoS projectors. I'm pretty new to projectors and have been reading this forum and studying up. My background is software engineering with most of my experience with video and graphics systems. I'm going to write what I see as definitions of and importance of contrast and black level in FPs.


Black level is the color of "black", the projector displays, where black is the absence of color/light.


Black level does not have much to do with contrast but more to do with viewing conditions. You can see this by displaying a picture, with black border (RGB 0,0,0), on your computer monitor. If your monitor is adjusted correctly for your normal viewing conditions this should look black or very close to black. You can verify you have black by turning down the monitor brightness until you don't see the black level change. Now the million dollar question; is that black? From a perspective of digital information it is because the pixel value is 0,0,0. From your perspective it is because it looks black. Now turn off the room lights and see if you still have black. Unless your room was already pretty dark you won't see black but grey. You need to re-adjust the monitor brightness to get black with the new ambient light level. To a user black level is seen as black relative to ambient light levels.


The black level experiment with a computer monitor is very pertinent to FPs. This shows why CRT projectors can display a very good black level in any lighting conditions. They have the alit adjust brightness to very low illumination levels. Brightness adjustments are basically a uniform shift in intensity level for all brightness levels; Io = f(Ii) = Ii + B where Ii is the initial intensity, B is the brightness shift and Io is the result. When B is negative, brightness is reduced. Lowering brightness adjustment can cause clipping. Clipping occurs when black does not get any blacker causing multiple intensity values to be displayed as the minimum light output level. In other words if RGB 0.1,0.1,0.1 looks just like RGB 0,0,0 clipping is occurring. This is bad because it effectively reduces your dynamic range and lowers contrast so you don't want to clip. It is easy to see that large negative values of B can cause clipping.


At the current technology level, digital projectors (LCD/DLP/LCoS) can't display a very low light intensity level. So they can't match a CRT for black level in very dark viewing conditions. This means that you need some ambient light to get a good black level with the current digital FPs.



Contrast is the difference in luminosity between bright areas and dark areas on the screen. In other words contrast describes the dynamic range of light intensity for the projection system. In a grey scale projection the maximum contrast would be seen between white and black. So whiter white or blacker black will mean higher contrast. Since CRT FPs can be adjusted to very low light intensity levels they have a very low black and therefore can easily achieve high contrast. Digital FPs don't have as good a black level but can output considerably more light intensity in white. The main problem with digital FPs achieving high contrast seems to be the transmissive or reflective (depending on the technology) component of the chip can not be turned completely off. This means some light is always there for black. Increasing bulb luminosity amplitude will not give necessarily give more contrast because although whites get brighter, blacks also get brighter.


Contrast can be adjusted by linear or non-linear functions. A linear function is Io = A*Ii. A is the contrast multiplier and when positive increased brightness for light areas more than dark areas. It does not take very large values of A to clip on the high end (anything past RGB 1.0, 1.0, 1.0 clips). The best contrast image can be achieved by adjusting contrast and brightness to use as much of the light intensity range as possible with as little clipping as possible. This means that different source materials may require projector adjustment to look their best. Some projectors may have automated or partially automated adjustments to do this. Different source material is not necessarily just for inputs such as DVD or HDTV. Among DVDs, each could have differing dynamic range and brightness levels.


There are some other approaches to contrast enhancement like contrast stretching which brightens lighter areas and darkens dark areas. Some projectors may implement this type of contrast control. It could be useful for some viewing material. Scaling chips used in many systems probably have the capability of doing contrast stretching. Has anyone played with this on their system? Some really trick image processors might have adaptive contrast enhancement that can adjust the projector to the best picture for a given scene.


So what's more important, black level or contrast? It seems to me that contrast is much more important. Black level can be improved by adjusting ambient light or using a screen like a GreyHawk. High contrast is harder to achieve.


So how will digital FPs get better contrast? Better optics seem to help but I think the limiting factor is still the chip itself. There are two ways of making the chip higher contrast; lower the reflection/transmission of light for black or raise the reflection/transmission of light for white. Obviously this assumes the other end of things is not changing or you don't get a contrast increase. This could be as simple as a higher reflective coating for a LCoS or DLP chip.


I hope this post does not bore everyone. I want to find out more FP performance and the contrast/black level seems to be major issue. I'm certainly not an expert on FPs and feel this forum can help me and others learn more about FP technologies and how to get the most out of what is available.


--sdc


 
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