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Projectors: Relationship b/t Lumens and Contrast Ratio?


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#1 of 6 MichaelDDD

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Posted September 18 2004 - 03:09 PM

What's the relationship b/t Lumens and Contrast Ratio, when talking about projectors?

I know that:

Lumens = Candlepower; how bright the picture is
Contrast Ratio = diff b/t blackest blacks and brightest whites

But how are they inter-related? I've noticed PJs costing $XYZ that have a certain amount of lumens and a specific contrats ratio. Then I've seen PJs costing 2x what the first one cost w/the same lumens but a higher contrast ratio.

Can someone explain this to me, please?
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#2 of 6 MichaelDDD

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Posted September 20 2004 - 10:31 AM

Posted Image

Aye, I've been left to edumacate (sic) meself.

Anyway, I've been doing a lot of reading...and checking out reviews.

I've learned that in some cases, lumens are "sacrificed" or "designed out" in order to have a very high contrast ratio, which can compensate for the lack of raw lumens.
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#3 of 6 Mike Wladyka

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Posted September 20 2004 - 11:42 PM

Well here goes...

what you are saying about lumens and contrast is correct.

How are they inter related?

Logically, if two projectors have the same contrast ratio and different lumens, then the starting point for the contrast ratio will be higher on the brighter one making it have less dark blacks.


Quote:
I've noticed PJs costing $XYZ that have a certain amount of lumens and a specific contrats ratio. Then I've seen PJs costing 2x what the first one cost w/the same lumens but a higher contrast ratio.


They can arise from a difference in technology or design. Such as DLP's have better contrast than LCD's. Further, on the DLP side, some of the newer and more expensive DLP projectors have "dark chip technology" which is a black background on the chip itself.

Hope that helps somewhat.
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#4 of 6 Leo Kerr

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Posted September 21 2004 - 04:05 AM

Factors that affect brightness:
1. Lamp design / brightness (including wattage.) This is fairly simple: all else being equal, a 100 watt lamp is brighter than a 60watt lamp. Envelope shape, filiment / arc-tube design, electrode positioning, et cetera, all impact lamp output.

2. Gate efficiency. This is a little more complicated, as I've made up this term to cover a wide range of things. On a DLP unit, this includes the fill-factor of the DLP chip itself (how much is mirror versus inter-mirror spacing) and the actual reflectivity of the mirrors themselves. Also, the efficiency of the color wheel and/or color seperation optics. In a color wheel situation, also the 'exposure time' will have a relation on brightness. On a LCD panel, this also includes the fill-factor, the polarizers, and a few other similar things.

3. Optical system. A good lens won't loose too much light.

Now, factors that impact contrast ratio.
1. Gate design. This is very similar to #2 Gate Efficiency above. As mentioned by Mike W., a black-chip DLP has a higher inherint contrast because it has a dark 'backing' behind the mirrors. How good is the off-angle light-trap? How specular (as opposed to diffuse) are the mirrors?

2. Optical system. This is the deal-breaker in most inexpensive projectors. Often times for size and convenience, certain features are designed into the lens - off axis projection, small glass, et cetera. Good lenses can get obscenely expensive. What's a 'good' lens versus a 'not good' lens? The coatings on the glass may not be as good, so each surface (two per element) adds a touch more scatter than it should. The glass may be cheap glass, rather than, say, a good borosilicate or crown glass. They may be working too close to the limits of the system for throw distance and/or off-axis projection. They may not have blackened all the interior parts near the light path. They may not have isolated the lamp well.

It's all inter-related. One of the best projectors I've seen is the JVC HD-2 or something like that (don't remember off hand.) It has a beautiful picture, with stunning contrast. And 500ANSI lumen output.

With LCDs, you can generally improve the color purity and contrast by having 'deeper' cells. But when you do this, you seriously add to the 'screen door effect.'

The new Panasonic projectors are playing some games to get better 'marketing contrast.' (as opposed to ANSI contrast.) There's a circuit watching the signal and driving an iris in the light path. They can darken the blacks in dark scenes by throttling back the lamp intensity, and pump the whites in bright scenes by opening up the lamp to full output. Doesn't help the ANSI contrast (simultanious contrast,) but it does remarkable things to the 'total' contrast.

Leo Kerr

#5 of 6 ChrisWiggles

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Posted September 21 2004 - 05:41 AM

Leo has it pretty well covered, IMO.

#6 of 6 MichaelDDD

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Posted September 23 2004 - 02:26 AM

Thanks very much, guys.

Leo,

That was a well-written and educational read. Posted Image That smartened me up quite a bit.

The projector I'm looking at (Infocus 4805) uses the DarkChip technology...the PJ has been getting very good reviews.

Thanks again.
Yeah, I base all my HT purchases on the WAF, too. Wallet Acceptance Factor.