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what determines DLP contrast? (hardware tinkering)


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I knew how DLP and LCD optical engines work for a long time but never thought about this. With LCD contrast can't be perfect because it is never 100% opaque and some light can leak through the black pixels.

But with DLP you have micromirrors. And the same micromirrors sold by the same company (Texas Instruments) to the projector manufacturers. Its not like one DMD chip with the same specifications inside one projector is better than the other.

If a micromirror faces a different angle than the one going to to the lens assembly, black pixel should be perfect black, but it isnt, with most projectors.


Even if this wasn't how DLP worked and the imperfect color filtering of the color wheel was to blame, then contrast isn't perfect even with LED powered DLP projectors, where the red, green and blue channels come from a monochrome source and they blink on and off when needed, so the color wheel with single chip DLP cant be to blame as well.


So what is the deal? How come most projectors have something like 2000 - 10,000:1 contrast ratio and you can clearly see a dim grey frame projected in total darkness when displaying a black frame, and only a small portion of projectors have something like a 250,000:1 contrast? What makes them special? Or rather, what makes the others not have perfect contrast?


The only thing comes to mind is reflections inside the final zoom and focus lens barrel. But then again, if that were the case, with completely black frames with all pixels being black you shouldn't have seen anything anyway.


So what is going on?

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  • 3 weeks later...

Cost and age!


Older version of the DMD chips had a reflective mask behind the mirrors. Light can sneak past the gaps between the mirrors and be reflected back out, reducing the contrast ratio.


In the later versions the gap between the mirrors is considerably smaller and the mask behind is black, these were referred to as dark metal chips.


The amount of angle the mirror can move has also increased in later versions this again improved the contrast ratio.


The glass the prism is made out of is critical as is it's condition. Any contaminants , dirt, dust condensate residue will significantly reduce the contrast ratio.


Within a short period of time a rental projector used for concerts and live events will not look as good as one used in an all but sealed air conditioned projection room.


Don't forget the lens, contaminants in here again reduce the contrast ratio.


Some earlier lenses for 3 chip DLP projectors had an iris ring fitted to the lens, this increased the contrast ratio but reduced the total output.


Some of the many things that can alter the contrast ratio.



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DMD - you would need to know if two different projectors (with the same resolution) had the same Ti DMD array, http://www.ti.com/dlp-chip/overview.html


Black Level vs. Contrast - pretty interactive measurements and as ever the marketing department tend to get hold of what goes in the blurb as 'bigger' numbers always sells.


Huge (Full On/Full Off) Contrast ratio figures are often pretty meaningless as they don't tell you if there was any semblance of accuracy in the video image being displayed when you take the measurement.



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