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Discussion Starter · #1 ·
There seems to be a push towards more and more lumen output. What effect does this have on black level. For example, would a Sanyo XP30 with 3000 ansi lumens in a relatively dark room have poorer blacks than a XP21N with 2500 ansi lumens in a similar setting.


Is there any way to reduce light spillage on higher lumen projectors?
 

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In general, for LCD's, blacks get brighter (in ft-lamberts) as the projector total light output (lumens) goes up...

UNLESS the measured contrast ratio scales with the lumens.


So, if a 1000 lumen LCD projector has a measured contrast ratio of 200:1, and a 2000 lumen LCD projector has a measured contrast ratio of 400:1, then the black level ft-lamberts *should* be the same (assuming the measured white level ft-lamberts at the projection screen scales with the lumens).


If the measured contrast ratio of the 2000 lumen projector in the example is 300:1, then the ft-lambert measured black level will be brighter than the 1000 lumen projector.


This is why you must be careful with 2000+ lumen light cannons that don't scale contrast ratio with their light capacity (lumens), if you are concerned with absolute measured black level. The *relative* perceived contrast may appear more pleasing to some eyes, but the instrument-measured black level will be higher if the contrast ratio doesn't scale with the lumens (ie double contrast ratio if you double lumens).


Now, to get to the point of the topic heading- SMPTE defines a standard brightness for theater projection. One SMPTE citation I just read in a JVC DiLA white paper states a reference brightness of 12 ft-lamberts. Others here state a brightness bogey of 15-20 ft-lamberts


A foot-lambert is a lumen/sq foot, adjusted by the gain of your screen. i.e.:


1000 lumen projector

4:3 LCD panel

4:3 8 foot wide, 6 foot tall screen, 1.1 gain


brightness= [1000/(8*6)]*1.1 = 22.9 ft-lamberts


This is almost double the SMPTE standard for film projection. Many HT enthusiasts prefer the brighter image, due to lack of light control, or the use of gray screens (which have gains < 1.0 usually). A screen with a gain lower than 1.0 will adjust your ft-lamberts down.


Soooo, to determine how many lumens is enough for your situation:


1) Do you have ambient light or good light control?

2) Do you want to use a gray screen?

3) What size screen do you want? (determines surface area used in ft-lambert computation)?

4) Do you prefer a TV-like "glowing" image (greater than 30 ft-lamberts) or do you want to emulate a typical commercial theater look and feel (without the sticky floors and funny smells http://www.avsforum.com/ubb/wink.gif )?




[This message has been edited by Rgb (edited 06-09-2001).]
 

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The SMPTE standard for film projection stems from the fact that you start to see flicker on a theater with more than 12 ft-lamberts.

But that is not true with LCDs, DMDs or D-ILAs, and having more light gives a very pleasant 3D look.

Also, the absolute black level does not matter: your eye only sees the difference between blacks and whites, and the low-level resolution depends on the contrast ratio only, not on the absolute black level.

So, the more lumens, the better. 40 ft-lamberts would be ideal if you can afford them.


------------------

Robert


[This message has been edited by robena (edited 06-09-2001).]
 

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Knuck,


Robert and RGB are both right in their explanations. One reason you will often see film ftLambert ratings higher than 12 is that film projectors were typically setup "open gate" meaning no film in the gate. By time you figured in the loss for the film, you would end up with about 12 FtL.


Robert is also correct about the flicker issue being the reason for film brightness not being set higher and I've been having just that same argument with many people when it comes to appropriate screen illumination...I think the level should be raised a bit for electronic projection but it really depends on the type of material you are viewing and the intention of the creator.


While your eye does see the contrast as the bright and dark difference, you really do have to concern yourself with absolute black level when it comes to dark scenes. I've seen many a projector that looks fine on bright, high contrast scenes look like garbage when it comes to trying to deal with a low contrast night scene and it comes out as all grey with little shadow detail. That's the toughest job for non-crt projectors and one of the reason's for D-ILA's popularity.




------------------

Tom Stites

Director, Business Development

Digital Systems Division

JVC Professional Products

"My opinions do not necessarily reflect..."
 

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Discussion Starter · #5 ·
Thanks everyone for the excellent feedback.


Rgb:


1. I have good light control, no ambient light. My kids,

however, use the room quite a bit and I prefer it to be

well lit when they play their games.

2. I do want to use the Stewart Grayhawk.

3. I will be using a 92" x 54" screen.

4. I prefer a more theatre like experience and would really

like a projector that can improve on shadow detail in

dark scenes, hence my concern for too much light.


 

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re:

"1. I have good light control, no ambient light. My kids,

however, use the room quite a bit and I prefer it to be

well lit when they play their games.

2. I do want to use the Stewart Grayhawk.

3. I will be using a 92" x 54" screen.

4. I prefer a more theatre like experience and would really

like a projector that can improve on shadow detail in

dark scenes, hence my concern for too much light."


1. It sounds like you need more ft-lamberts than a theater. let's use 20-30 ft-lamberts as a target bogey for the projector selection process.


2. ...which means a gain of .95


3. Area = 34.5 sq feet


4. Then you want to be sure the measured contrast ratio of the projector you choose is as high as possible, hoping that the measured contrast ratio is as close to the contrast ratio stated by the manufacturer as possible.


Now for the part that confuses so many HT hobbyists, new and old...


We want to compute the ft-lamberts at the screen from a 4:3-paneled digital projector.


Let's use the middle of our target range, 25 ft-lamberts over the image on the screen.


For a 4:3 image, centered on the screen at 54" high;


54" = 4.5'

width of 4:3 image = 4.5' * 1.333 = 6'

area of 4:3 image (assuming you zoomed down the lens to match the 4.5' height of the screen) = 4.5' * 6' = 27 sq ft


Solving for lumens in the original ft-lambert equation:


lumens = (ft-lamberts/screen gain)* (projected image area)

or

(25/.95)*27 = 710 lumens! for a 4:3 image zoomed to fit the screen you want. Re-compute if you want 40 ft lamberts:


(40/.95)*27 = 1137 lumens for a 4:3 image.


For a 1.78 image filling your screen (here's what confuses many):


If you use a 4:3 paneled projector:


Image area = 44.08 sq ft!


that is,


92" wide screen = 7.67'


height of image area projected (including black bars!) = 7.67' / 1.33 = 5.76'


area = 7.67 * 5.76 = 44.2 sq ft


The 16:9 screen image will have black bars above and below the screen, and the black bars count against the area over which the lumens of the projector are spread.


so,


(25/.95)*44.2 = 1163 lumens, assuming you want 25 ft-lamberts and use a 4:3 panel digital projector.


You can run the numbers for different desired ft-lamberts and a 1.78-paneled digital projector. Also, a Panamorph anamorphic lens will increase the ft-lamberts on a 1.78 screen from a 4:3 panel projector. Those computations are your homework for tonight, class.


This concludes Projector Luminosity 101 http://www.avsforum.com/ubb/wink.gif .



[This message has been edited by Rgb (edited 06-10-2001).]


[This message has been edited by Rgb (edited 06-10-2001).]
 

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Discussion Starter · #7 ·
Thanks Rgb. I'll play around with the figures. I take it the lumens result in minimum ansi lumens required and more lumens will not hurt and perhaps help the image.


Brian
 

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As discussed on another thread (, using the projector's rated lumens (even if accurate) is probably advisable. In video mode the effective lumens are less.


I'm not sure why. Perhaps it's the nature of the video gamma curve, or just the fact that compared to static presentation images, the average lumen level in video must be less than max to avoid white clipping in the brightest scenes.


------------------

Noah
 

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re:"I take it the lumens result in minimum ansi lumens required and more lumens will not hurt and perhaps help the image."


Yes. The lumens output from the computations in my last post represent the min lumens required to obtain the ft-lamberts specified in the equation (the left hand side of the equation).


Buying a projector with more lumens will raise the ft-lambert brightness you see on the screen (assuming the same gain and projected image area, including black bars for 4:3 digital projectors). Whether this is "better" is a matter of taste. It won't hurt if you have light control issues- just be sure that contrast ratio scales with lumens to maintain absolute measured black level in ft-lamberts, per the prior discussion.


The bottom line is, there are only a small hand-full of digital projectors in each type category (DiLA, LCD, DLP) that have best-in-class contrast ratios and lumen levels for your application.


DiLA: JVC G15/G20 or OEM clones


DLP: not my area of expertise- browse the forums for the latest hot models and those in the past year or so via the Search.


LCD: Sanyo PLV60/ XP21/ XP18, Sony 11HT (?), other up-and-coming models with 600:1+ contrast ratios in the 1200-2000 lumen range (models?)




[This message has been edited by Rgb (edited 06-10-2001).]
 

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I'll second just about everything said in this thread. Very few people that try to get 15 fl-lams know the origin of the spec. The reason film images are flashed twice is that at the 24 fps rate of the film itself, the flicker, even at 10 ft-lam, would drive you nuts. There are curves of flicker sensitivity versus brightness and frequency. You can run a digital projector very bright and not be bothered by flicker.

Regarding "video" lumens vs "presentation" lumens vs "advertised" lumens: even if the manufacturer meets the ad spec (and most I've seen don't), by the time you're finished tuning for video, count on being at least 25% below the ad spec. On DLP's that have a clear section but disable it for video, you'll lose even more.

What no one has said in this thread is that the bulb will lose up to 50% of it's output over its life. So, you should add another 20-30% to your lumen goal.

So, taking the above example, the 1137 lumens becomes more like 1900 lumens (advertised, when new) to get 25 useable ft-lams half way through the bulb's life.

And yes, at that output, the black levels are pretty bad unless you have a very good contrast ratio. For me, the best looking projo, spec-wise, is the Sanyo 21N, which I've been planning to see for a couple weeks now, but haven't been able to find the time. The PLV60 might make it for 16:9 viewing (because the 4:3 panel wastes a lot of light), assuming it loses less than most when tuned for video and really does have >600:1 CR.




------------------

Steve
 

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If I had 5000 lumens and 200 raw ft-L, I'd just use a darker screen.


I don't think there is too bright. However one needs a good contrast ratio to go with it.


The manufacturers commonly cheat on contrast to get more lumens.


------------------

Ken Elliott


[This message has been edited by kelliot (edited 06-10-2001).]
 

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re:"If I had 5000 lumens and 200 raw ft-L, I'd just use a darker screen.

I don't think there is too bright. However one needs a good contrast ratio to go with it."


Do you have fore-knowledge of a 5000 lumen LCD projector with 1600:1 true contrast ratio on the horizon?

C'mon, let us in on the secret...
http://www.avsforum.com/ubb/wink.gif

 

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Allow me to emphatically state, "The XP21N is not too bright!" I was concerned about this before buying one, but have since learned that every bit of its brightness is a good thing. The picture is still enjoyable with a room full of daylight. DIRECT sun on your screen is a different story however. Only another star (not the Holywood variety) can compete with that. At night, I've never wanted for a dimmer image.


As stated in another post, even low-ouput LCD have the same black-level issue. It's just the nature of the technology. If you want blacks that are totally black DO NOT buy *any* LCD projector--you will be disappointed!


Frank
 
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