[Andy238]

Am I missing something?

I'm trying to digest the concept of lumens and how many are needed for nice looking HT. Now, I see ads (Tweeter) for a $7K Sharp projector for HT and they're all warm and fuzzy about having 400 lumens. Now I'm thinking, after seeing all the fuss about 1000 lumens and higher being the norm, that I'm missing something.


Also adding to my indigestion is the SVGA vs. XGA resolution thing. That same $7K prj is SVGA. I thought XGA or even higher is needed. No?


Can anyone "shed some light" on what is acceptable? (sorry, couldn't help it)


Thanks.


Andy

 

[Phil Olson]

Andy,


It depends on your screen type and width. I have a 12' wide GreyHawk screen and 1500 lumens is just adequate. What you want in an ideal world is 16 foot-lamberts, (fl). Divide lumens by the area of the screen then multiply by the screen gain.


Assuming a 4x3 screen and projector, a 8'x6' GreyHawk would provide (400/48)*.9 or 7.5 fl. Not ready for prime time.


On a 4'x3' screen it would be 400/12 *.9 or 30fl. Get out your sun glasses.


Probably a 6' wide screen would be about right for that projector. It being an LCD, you would definitely want to go with a GreyHawk, ( or at least grey paint! ). Note that you could solve the above shortcoming by going with a 2.0 gain, but you don't want to do that with an LCD.


There are other things that can effect this number but that should get you in the ball park.

 

[Movie Mike]

Thanks, Phil, for that formula. I'm constantly amazed at the tremendous knowledge of AVS forum members like you.


This forum is a very good resource.


Mike

 

[Andy238]

Thanks Phil,


That does help a lot. At least now I know what I'm aiming for.


Andy

 

[Dave T]

A couple more comments:


Another critical factor is the light control of your room. If you can have a pitch dark room, you can get by with much, much less lumens than if you have light in the room - either because you can't block light coming in from elsewhere (windows, other rooms), or because you just don't like watching in the dark.


Another thing (and it's not relevant to this case), but I think I disagree with Phil on one point - how many lumens is too much. I have a theoretical 60 ft lamberts with my setup, and no way is it too bright. It just means that I can watch with some light in the room, but even in a dark room, nobody is reaching for sunglasses.


Finally, Phil - why do you say Andy needs a Greyhawk? Because it will hide screen door? That may be so (I've personally never seen an LCD on a Greyhawk - you may be right about the screen door), but having a little more screen door may be well worth it, if the projector is luminosity-challenged, not that 400 lumens is that bad.


- dave

 

[Augie]

My Screen Dimension Calculator will do the lumen math for you.

 

[jamoka]

The formula is correct. You can not predict screen brightness using it this way though. The problem is that digital projector lumen ratings are often inflated and therefore unreliable when used to predict real world screen brightness.


In this thread

http://www.avsforum.com/avs-vb/showt...threadid=23857


the Seleco was measured to have only about half the manufacturers' rated lumens when measured in an actual theater setup.


CRT ANSI lumens are generally rated lower, but are more accurate.

 

[lennyr]

I'll second DaveT's post - I am set up at a theoretical 54 or 43 ft lamberts (depending on what mode I'm running my PJ in, if its brightness specs are to be believed). I don't find that too bright at all. I generally like to have a little ambient light in the room, and that really ups the brightness requirement.

 

[kelliot]

IMO, the 16 ft-L benchmark is way to low. If you have any ambient light, it should be up around 50 ft-L or higher. I myself have a hard time salting popcorn in the dark.

 

[Jimmer2]

If this is correct, my calculations are:


48"x27" screen, or 4'x2.25' = 9sqft


So assuming a 1000 lumen pj:


1000/9*1.2= 133 foot lamberts... !!!


...am I in any danger of getting a severe 'TV tan'?

 

[Phil Olson]

I guess I shouldn't use shorthand explanations. The 16fl is for a darkened theater. For more ambient light, you need even more lumens, as has been suggested.


Also, if you don't have the right numbers from the manufacturer, then it's garbage in-garbage out, right? Note that even for a D-ILA, the lumens may be honest but 20% or so will disappear when it has been calibrated. As I said, there are other things that effect light output.


I think that LCD's have the worst blacks of the three digital projector types. They also have the largest fill factor, (screen door). For both those reasons, the consensus is that GreyHawk gives the best picture. I think that GreyHawk is a good bet for all digital projectors but LCD needs it the most.


You can always compromise. If you have great low light vision, you could get away with less. But for most people, one needs a minimum of light to excite the cones in the eye properly, (you can't see color in really dim light).


Also, one could use a 2.0 gain screen. If you don't mind blacks looking like greys, you'll save some money. In addition, it will accentuate the screen door effect.


At any rate, as you can see there is no one right answer. You should first set down what you want.


Light controlled room or not?

Screen size?

Screen material?

Picture quality?


Of the above, light control and screen size are the biggies. Screen gain only makes a difference if you are going to use a high gain screen.


If you can, visit some HT's in your area and see what they are doing. Do the calcs on their equipment. Most HT people are quite hospitable.


Hope this helps,

Phil

 

[steve5097]

I agree with Phil about the Greyhawk.

Among others, I disagee about brightness, for their reasons and a couple others. The 16 ft-l number was derived based on flashing light through film at 48 Hz (24 fps x 2 flashes per frame). The way digital projectors work is completely different. Flicker isn't an issue under reasonable circumstances. I regard 30 ft-l as a BARE MINIMUM, based on the projector's specs, even in a dark room. As others have pointed out, specs are inflated and tuning for video reduces the number further. In addition, bulbs age, with life typically quoted as half output. I usually watch with some light on, and find 50 ft-l (actual, measured) okay, but I'll admit that might be too bright for a lot of folks.

 

[KFung]

FWIW, the threshold for image brightness in a dark room is somewhere down in the 7-8 ft-lamberts range (in real ft-lamberts). As you go up from that you get a definite impression of a brighter picture, although the whites will look white in any case. I would question how necessary it is to go to 50 ft-lamberts, measured, unless one has ambient light because you get a proportional increase in black level as well. The perception of brightness shouldn't be too bad, though. I've stared at much brighter images before! I would be concerned about the performance in the black level though, if that is something one is sensitive to.


I definitely agree with Steve, though, 30 ft-lamberts is a pretty good minimum when calculating off of manufacturers specs. It should give you a number well over 12-13 ft-lamberts (which is what SMPTE specs turn out to be once you add light lost going through the film) with a new bulb and it should also keep you out of the danger range at the end of your bulb life.


A grey screen is always a good idea on digital projectors if you have the light for it.


Regards,


Kam Fung

 

[steve5097]

Kam made a good point, that I didn't say clearly. At 50 ft-l, the black level isn't very good. In a dark room, it's noticeably bad. Since I usually have light in the room, it's a good tradeoff for me. A better answer is to have a "black" mode, ala Sony 10HT, so the light level can be easily reduced. The problem with the Sony is that even its regular mode isn't bright enough and its blacks are pretty poor.

 

[noah katz]

Kam,


"the threshold for image brightness in a dark room is somewhere down in the 7-8 ft-lamberts range"


What is on either side of the threshold?


Thanks

 

[Cameron]

Back to the original post. There is no way on earth that I would buy a 400 lumen SVGA LCD projector for $7,000. That sounds like a total rip to me. For $7,000 you can get a much better projector than that!!!!


Cameron

 

[KFung]

Noah,


A dim looking picture. Past that point your brain has more trouble convincing itself that white is perceptually "white" because the brightness is so low. Over that value the brightest neutral coloured object is generally taken by the brain to be "white." So you don't usually want to get below that kind of brightness level, of course that's just a rough number, nothing scientific.


Regards,


Kam Fung

 

[noah katz]

Kam,


OK, I get you. It will look white even though, for my tastes anyhow, dimmer than acceptable.


Thanks

 

[Dave T]

Quote:

Originally posted by KFung I would question how necessary it is to go to 50 ft-lamberts, measured, unless one has ambient light because you get a proportional increase in black level as well.

Is that necessarily true? Can't a projector still have good blacks, but have very bright whites?

 

[noah katz]

Dave,


Same blacks + brighter whites = higher contrast; not so easy