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Discussion Starter · #1 ·
I was just wondering how the brightness and contrast settings work on a DILA. I know that on tube units you can shorten the life of the tubes by overdriving them. How are brightness and contrast regulated when you adjust the settings? does it adjust the bias of the panels or does it in some way regulate the bulb intensity? can fiddling with it and running the contrast way up hurt the projector in the long run?


Thanks!


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ham
 

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Ham,


The brightness of any given pixel is determined by the

opacity of the D-ILA device. When that pixel is

opaque, or nearly so - the light output is low. If the

D-ILA pixel is transparent - the output is high.


Since the projector can control the brightness of each

pixel - the circuits can raise or lower the opacity of

the pixel for the proper brightness level.


Remember that's how the projector is mixing colors -

there are 3 D-ILA element in the projector and the PJ

is mixing the amounts of red, blue, and green to form

the various colors.


There is a basic limit on contrast due to the D-ILA

device itself - what is the ratio of light output from

full "on" to full "off". In addition to this - some

light is not fully dissipated by the optics - and hence

finds its way out the lens - thus lessening the true

contrast ratio below the theoretical limit of the basic

D-ILA device.


Dr. Gregory Greenman

Physicist
 

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Actually, D-ILAs are reflective devices, and their opacity does not vary.


What changes is the way they polarize the light they reflect. The light emitted by the bulb is first polarized, which means in layman terms that it is going to vibrate in only one direction, say for example vertically. Natural light is not polarized: it vibrates in all directions.


This light is reflected by the D-ILA towards the same polariser. If it stays polarized vertically, the polarizer will let it pass entirely, and you have white. If the D-ILA polarizes it at 45 degrees, only half (I schematize, this thing is not linear) will pass through and you'll have grey. Black is done when the D-ILA shifts the polarization at 90 degrees.


The whole effect is similar to the effect you get when you watch sunlight reflected on water with polarized Rayban sunglasses: a mirror, the water here, polarizes the light, which allows the Raybans to remove the glare.


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Robert


[This message has been edited by robena (edited 07-19-2001).]
 

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Discussion Starter · #5 ·
Thanks for the replies, guys.


However, I'm still a bit confused. How does the polarizer change the amount ot light which gets to the screen when I fiddle with the button on my remote? Or is it some other mechanism?


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ham
 

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D-ILA's work differently than CRT's in that the peak white level and absolute black level are always fixed. The brightness/contrast controls don't change the bulb brightness, so no matter what you do with these controls the brightest white and darkest black are always the same.


The controls only affect how the source signal is scaled within the range of the DACs. Turn the brightness to low and you compress the shadow detail into the black floor, turn the contrast too high and you'll crush the whites into the peak white level.


So on a D-ILA there is only one correct setting for the brightness/contrast controls - the one which matches the source signal to the dynamic range of the projector.


Dave.
 

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The real fun begins when you finally understand the true nature of what polarization and light actually are, not just textbook understandings...


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Ken Hotte

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OK. I get it. I never made that connection, as I read far too much sci-fi to find that movie even halfway interesting. It was fun, and OK, though. That was the Doc's name, wasn't it? The real source of the 'Krell'.


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Ken Hotte

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The same. I remember Forbidden Planet being the first movie to give me nightmares (from B&W TV viewing in the early 60's). Not great, certainly, but something of a classic in the genre.


- Marc
 

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Yes - Morbius is from Dr. Edward Morbius from the movie

"Forbidden Planet" played by Walter Pidgeon.


For an early '50s vintage movie - pretty good.


A good handle for a fan of Sci-Fi, and Shakespeare.

[ Forbidden Planet is loosely based on "The Tempest"]


In addition, to being a videophile, I'm also an audiophile;

and "Forbidden Planet" is the source of the name of

Dan D'Agostino's high-end electronics company, Krell;

whose fine electronics I wish I could afford.


Greg
 

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Robert is correct - I oversimplified the mechanism to

explain it easily.


If you want to see how polarization affects opacity -

you can do a simple experiment at home.


Get two pairs of "Polaroid" sunglasses - i.e. sunglasses

that have polarizing lenses - which cut glare.


Now hold them up so that you are looking through one lens

of each pair. Now rotate one of the sunglasses so that

you are still looking through one lens of each pair - but

the lenses are rotated with respect to each other.


What you will find is that you can alter the opacity of the

pair of lenses by this rotation.


At some rotation - the pair of lenses will be totally

opaque. This occurs when the polarization of the two lenses

are at 90 degrees to each other. For example, one lens will

polarize the light horizontally, and the other lens has its

polarization vertical - so it only lets vertically polarized

light through. Since the first lens kills off the vertical

component and only allows horizontally polarized light to

go through to the second lens - which kills off the

horizontally polarized light - but would let the vertically

polarized light through - if there were any.


The net result of the two lenses oriented at 90 degrees is

to kill off all polarizations of light - hence the pair is

opaque.


If you then rotate them another 90 degrees - you will line

up the polarizations - so that both lenses let through light

with the same polarization. You will get the most

transmission in this orientation.


Now rotate one lens by 45 degrees. The transmission will

not be as great as when the polarizations are aligned but

it will not be opaque either.


You'd have to get into a discussion of how the electric

field vectors are resolved - which I don't think you're

interested in - but suffice it to say that the electric

field vector is reduced to ~71% of its pre-polarized value

by this 45 degree orientation. Since apparent brightness

goes like the square of the field strength - you have

a 50% decrease in brightness.


You can get any value in between the two extremes depending

on how you rotate the lenses.


The D-ILAs do the same thing. The D-ILA panel itself works

like the lens that you are rotating in the above example -

it is changing its polarization direction. Somewhere in

the optical path is another polarizer - which serves the

same purpose as the lens you held still.


That's a bit of the details of how the opacity is varied in

the D-ILA.


Dr. Gregory Greenman

Physicist
 

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I'm having fun trying to figure this one out.. "Morbius". Hmm... Morbid-Mobius? Moirbund-Mobius? Just curious.. funky handle.


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Ken Hotte

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[This message has been edited by KBK (edited 07-19-2001).]
 
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