Film-Tech Forum ARCHIVE: Lumens Question

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Posted by Robert E. Allen (Member # 1351) on 03-27-2014, 08:52 AM:

One of my auditoriums is very small (37 seats). I am considering getting a BenQ projector that puts out 2700 lumens for playing DVDs & BluRays. The screen is 20 feet wide. Is 2700 lumens bright enough?

Posted by Scott Norwood (Member # 30) on 03-27-2014, 09:06 AM:

No.

The math is essentially this:

8.5' x 20' = 170 square feet of screen, assuming a common-height 2.35:1
screen.

Footlamberts = lumens per square foot on a 1.0 gain screen, so

170 x 16 = 2720 lumens needed, assuming a 1.0 gain screen.

The video standard is 14fl, but you should probably shoot for 16.

The problem is that the video projector is probably using a 16x9 chip, not a 2.35:1 chip, so the actual size picture that it would need to project for a scope image is 11.2 x 20, which means that you really need 3584 lumens for scope and just over 2000 lumens for 16x9.

Additionally, most video projectors are pretty optimistically rated for light output, using the most favorable lens and new lamps. Especially with mercury lamps, the light output can drop by half over the life of the lamp, with the result that you really want a 7k lumen machine for that size screen or else you need to consider a gain screen surface.

Before buying something like this, get a rental or demo machine with similar specs and the same lens that you plan to buy and try it on your screen to make sure that you have the right lens and that the picture is sufficiently bright (recognizing that light output will decrease over the life of the lamp). Make sure that the lens that you have can zoom to accommodate 1.37, 1.66, 16x9, 1.85, and scope images on your screen with whatever masking system you have.

Posted by Edgar Prass (Member # 7558) on 03-27-2014, 01:06 PM:

I have a similar issue but the question is rather how much is too much?
We are about to move our main video projector to a smaller auditorium and based on Scott's math above it would give a max brightness of 28fl (just installed a new mercury bulb). Some days ago I tested the projector in the small auditorium and I would say that the image was "nice and bright", but would it become exhausting for eyes after some hours? Projector also has an eco-mode, perhaps should use that and save some lamp hours.

Posted by Mark Gulbrandsen (Member # 72) on 03-27-2014, 03:18 PM:

I am not aware of any projectors that use true 2.35 chips in them. Perhaps there is an LCD model that does but All the Dee L Pee projectors I am aware of have 16 X 9 chips.

Also, you should really measure the gain of your actual screen. If it's old it is probably less then 1.0 if it was a matte surface. Spectra FL meter and reflectance standard is all you need to measure it.

Mark

Posted by Robert E. Allen (Member # 1351) on 03-27-2014, 10:29 PM:

Scott: I was mistaken about the screen size. It's 8 x 12. The contrast ratio of the projector is 10,000 to 1. Does that make any difference?

Posted by Scott Norwood (Member # 30) on 03-28-2014, 10:58 AM:

It makes a big difference! Assuming a 1.0 gain screen, 2K would exceed the actual requirement, if you can get 2k lumens throughout the life of the lamp and with the lens that you are using.

Posted by Carsten Kurz (Member # 5396) on 03-29-2014, 10:12 AM:

If that reads 3,65m*2,43m, then it is possible to throw a nice and bright picture on it with a decent home video projector. Not any Benq is suited though. I prefer JVC or e.g. a Panasonic AT6000/AE8000. These also have motorized, presetable lenses with a wide zoom and shift range. These two are also very quiet, if you plan to mount them in the auditorium.

- Carsten

Posted by Mike Frese (Member # 4361) on 03-29-2014, 06:54 PM:

How many of you feel that this a wise investment for this theater to do? Would it not better to save the money for a switch to a real DCP cinema projector?

So few theaters are able to make a decent amount of money on playing movies on dvd. Those tend to be arthouse type theaters (who seem to get away with having a very small screen) in large populations. I have not heard of any rural theater in a small town of 3500 people do well playing movies on dvd.

Posted by Robert E. Allen (Member # 1351) on 03-29-2014, 07:21 PM:

As the folks put it around here Mike we are "...in the country". It is our plan to play on our 12 foot screen classics from the 30s, 40s, 50s & 60s.

Posted by Marco Giustini (Member # 4544) on 03-30-2014, 06:32 AM:

I can only stress out what Scott said: the lumens claimed by many manufacturers are seriously over-optimistic.

First, they do not consider any calibration. So a 3000lumen figure could mean "3000 lumen with severely yellowish colours".
Some projectors slide an extra filter in for the MOVIE preset, to make colours more accurate, reducing the light output.
Contrast is another catch: they will claim anything and usually those figures are far from being true. While JVC DILA are superb at ON/OFF contrast, they are not great at ANSI contrast.
A typical JVC contrast ratio can be 20.000:1 ON/OFF, but just 120:1 ANSI.

And lamps: those lamps will drop considerably over time. Meaning that the brightness figure already overenthusiastic I mentioned will only happen when the machine is brand new.

Bottom line: don't be fooled by the numbers on the box, check a professional review first. I also recommend JVC, they do really deliver a brilliant picture. It should be enough on a 3.5m screen, but you may want to replace the lamps a little more often to offer a good brightness on screen.

Posted by Carsten Kurz (Member # 5396) on 03-30-2014, 01:11 PM:

quote: Mike Frese
How many of you feel that this a wise investment for this theater to do? Would it not better to save the money for a switch to a real DCP cinema projector?

That would be true for an investment in the 10.000US$ range. A suitable home-video projector for a 3,60m screen can be bought below 3000US$ (and will usually show a better picture quality than a S2k DCI machine).

This is 3.000US$ vs. 30.000US$.

- Carsten

Posted by Mike Frese (Member # 4361) on 03-30-2014, 05:07 PM:

quote:
quote: Mike Frese
How many of you feel that this a wise investment for this theater to do? Would it not better to save the money for a switch to a real DCP cinema projector?
That would be true for an investment in the 10.000US$ range. A suitable home-video projector for a 3,60m screen can be bought below 3000US$ (and will usually show a better picture quality than a S2k DCI machine).

This is 3.000US$ vs. 30.000US$.

- Carsten

Carsten,
Yes the cost for a home projector is cheap vs a DCP projector. You can pick one up for less than $1000.

But........

My question is will enough people come in this small rural Oklahoma town of less than 4000 people to make this worthwhile? I have not seen a small town theater who normally plays mainstream movies do well with old movies.

Would it not be better to keep playing movies on film even if they are 10-12 weeks old?

Who knows of a theater that does well playing old movies on dvd that is in a small rural midwestern town? I know some theaters in cities can do well with these old movies on dvd.

Instead of spending $1000 on a home video projector that may not make any money for the theater, put that money to your digital fund. Andrew Thomas has shown examples of being able to go digitl for just $6,000 down.

Posted by Frank Angel (Member # 248) on 03-31-2014, 09:37 AM:

Scott, thanks for that formula -- very helpful.

Sounds like Robert has thought out the viability of an arthouse formula for his small auditorium, so let's treat that as a given. The question still remains, is a consumer projector going to work for that situation. And that will hinge on what you want to accept as "OK," given you have accepted that the less-than-professional-standandards of DVD are acceptable (which may very well work on such a small screen).

But let me say that with a similar size screen of 22ft, we are using a three chip LCD Eiki at 10,000 lumens on a high gain screen and I can tell you my eyes are still asking for a brighter looking picture, but for the purpose of this room, it did what was necessary when we need to show other than 35mm content (which now is most of the freaking time).

That said, let me add, the choice came after multiple trials with a variety of projectors from the cheapest to this Eiki - probably the most expensive one we tested - before eveyone agreed this would be "OK." It cost over $20,000 with proper long throw lenses; this was no $3,000 toy. So yes, you can get a workable image from a non-DCI compliant video projector, but don't think you will be happy with one of those home theatre jobs; expect to spend what comes close to buying a real digital unit like the digital-lite units by Christie and Barco, which you might want to consider going for if you really think this room will make a profit; if it is just a kind of throw-away thing you want to do to augment the main house and as a kind of public service marketing thing, then a consumer projector would work, but still, expect to spend some bucks if you want an image that won't scream, "Projectionist -- your carbons are forever going out."

Posted by Marco Giustini (Member # 4544) on 03-31-2014, 11:23 AM:

any chance your eyes are looking for more contrast ratio instead?

Posted by Robert E. Allen (Member # 1351) on 03-31-2014, 04:52 PM:

Thanks for your input Frank.

Bob

Posted by Jarod Reddig (Member # 6363) on 04-01-2014, 04:08 PM:

You guys are also forgetting about throw distance. Longer the throw distance the dimmer the image and vice versa. Most home theater projectors only have one choice of lens as well so it may not zoom out enought to fill the screen. JVC and Sony home theater projectors have some of the best contrast ratios. True 10,000:1. You can not tell if they are on or off the blacks are so dark. Not just on/off wither. The JVC doesn't need a dynamic iris to get very high contrast ratios. The Sonys also have excellent black levels but they implement a dynamic iris to get as deep of blacks. This is the one area I believe home projecters hands down beat DCI pjs and even film. If playing mostly black and white movies black level is very important. Both aren't know to be the brightest either tho. Always a trade off. Make sure you do all the math before committing. For my home cinema I use a BenQ W7000 and its rated at 2,000 lumens. I use it on a 1.0 gain 1.75:1 10ft acousticaly transperent woven screen with a throw distance of 20ft. In high lamp mode im getting around 19 fl for 2D and only about 4fl for 3D when bulb had about 100hrs on. There is 1,300 hours on the lamp and theres a slight drop in brightness. Just some info to let you know what to excpect with such a projector.

You might look into this company for a nice 3chip DLP projector thats cheaper than a DCI pj but plenty of choices.

http://www.digitalprojection.com/

I believe some of the pricier models have 2.35:1 chips but not positive.

Posted by Steve Guttag (Member # 268) on 04-01-2014, 04:35 PM:

Throw has zero effect, in and of itself, on brightness. Light travels through air just fine. Light travels through no air too just fine. Anyone that told you otherwise is wrong.

What MAY affect light as a function of throw (total lumen output) is the lens and its speed and its speed at a particular zoom position. However that is dependent on the particular lens one is using.

As for contrast ratio. While it is true that a projector with a dynamic iris system can achieve a greater range if one were to measure only black or only white, a DCinema projector will have a greater range (ratio) if there is a mix of anything else. Put up a 50% checkerboard pattern and see which projector does better. The DCinema will win every time.

The problem with dynamic iris for cinema use is if a movie is graded and one knows the detail in the image based on standard light levels, if one has a dynamic iris and the scene is on the dark side, then it will likely also kill any shadow detail as the entire image is lowered in output to get those blacks extra black.

I've yet to see any digitally projected image to compare favorably to film with respect to contrast ratio (presuming both are set up properly and exhibiting what they are capable of). Of course, I still like the image off of film better too but that is an entirely different discussion of little point anymore.

Posted by Jarod Reddig (Member # 6363) on 04-01-2014, 10:53 PM:

Thanks for the info. Ive must of always been misinformed of distance in relation to light loss. Home Theater magazine reviewer Tom Norton would speak of the effect of light loss at further throws. And referring to the lens specs like you said. But anyways Like if you had a flashlight shinning on a wall and you walk backwards its intensity weakons. I could be way off base but not sure.

Im definitely against dynamic iris's and there effects on the image which is why I like the JVC D-ILA HT projectors so much and ther high contrast. They truly have the deepest blacks that ive ever seen in an image digital or film. Reminded me of film whenever I demoed them for sure.

Edit; I Think where my confusion was is keeping the distance in spec with the lens like you say above. I think that's what I had in mind. Thanks again for info

Posted by Steve Guttag (Member # 268) on 04-01-2014, 11:14 PM:

The flashlight example is the flaw. Think of the flashlight reflector as the lens. It will create a "spot" of a certain size at a certain distance. When one walks backward, the spot grows in size so the brightness appears to be darker. The lumen amount (what is coming out of the flashlight) does not diminish...it is the area that the flashlight is being asked to cover is growing...at the exact same rate as the reflected light is diminishing.

Now look at a projector. if our screen is 20-feet tall. Regardless of where our projector is, we will select a lens such that the image remains 20-feet tall. It doesn't matter if our projector is 50-feet away from the screen or a 100-feet away. So the area that the same quantity of light has to cover is the same...hence the throw has zero effect on light levels.

I have found that for most manufacturers, the fastest lenses tend to be in the middle of the range. So that the longest lenses and the shortest tend to be less efficient on light. For zoom lenses, often, for any particular zoom lens, the light tends to be more efficient with the lens zoomed for the shortest focal length (largest picture). However, unless you have a Throw Ratio such that you have a choice of 2 or more lenses, this is moot since the projector and screen are going to be at a distance dictated by architectural considerations so you will be stuck zooming the lens to the size needed to fill that same 20-foot tall screen.

Posted by Jarod Reddig (Member # 6363) on 04-04-2014, 12:45 PM:

I see your point.

I think whats going on is in HT projectors many if not most don't come with different options of lenses; just the lens that comes with it. It has a required throw distance to be most effecient like you said. So your stuck. Most are only effecient in a very short range, hince my argument and have very little vertical and horizontal zoom so you have way less throw distance to deal with as opposed to with cinema projectors where you can just pick the required lens for the distance. Thats what im saying. The OP better make sure the BenQ projector he selects has the ability to change lenses. I did and experiment last night and moved my projector closer to the screen by roughly 10 feet and adjusted focus and it became much brighter. Much less deeper blacks too at the expense of brightness. After moving the projector back to its original mount you could clearly see how less bright it was. It really is about the lens like you said.

Thanks again.

Posted by Steve Guttag (Member # 268) on 04-04-2014, 03:13 PM:

Is your projector of a fixed focal length? Most are not for home use...when you moved it...did you zoom the lens so the image was the same size?

Posted by Jarod Reddig (Member # 6363) on 04-04-2014, 03:29 PM:

No it is not fixed. I should have added that i adjusted zoom as well so image fit the screen. Theres not much play though. If I were to go a few feet further back from my normal 20ft throw it would not be able to zoom out enough to fill the screen.

Posted by Carsten Kurz (Member # 5396) on 04-05-2014, 07:43 AM:

As Steve said, depending on lens characteristic, the difference in brightness with the same image size adjusted with a zoom lens is small. Typical behaviour is slightly brighter, with slightly less contrast, in the wide position, somewhat less bright, but higher contrast, in the tele position (larger distance).

Even some cheap home cinema projectors may have a zoom ratio of more than 2.1, so you can double the distance while keeping the same image size. The brightness though will not vary much. Usually you can measure it, but hardly see it with the bare eye. If you are free to choose the position, you can of course still optimize for either brightness or contrast a little bit.

But it is NOT distance in itself. A piece of window glass will usually eat more light than 100m of free air.

- Carsten

Posted by Jarod Reddig (Member # 6363) on 04-05-2014, 11:21 PM:

Thanks for the info. Yes the difference wasn't huge but was noticeable.

Posted by Frank Angel (Member # 248) on 04-06-2014, 06:11 AM:

That long throw/long focal lenghts negatively affect image brightness vis-à-vis the optics is a misconception that seems to be prevalent and probably originated in Drive-In circles because there is some truth there with those exceeding long throws out in open fields. But the conclusion that the lens optics cause any bright degredation is incorrect -- yes, distances there can show some negative effect on brightness, but it has nothing to do with the optics of the lens vs distance -- it has to do with other factors: 1) the amount of moisture and the dust/dirt particles in the air over that long distance that light has to punch thru and 2) very long focal length lenses that are needed for those distances tend to be more inefficient and with more elements needed to achieve those focal lengths. Even zoom lens change efficiency as they travel thru the zoom range.

Posted by Jarod Reddig (Member # 6363) on 04-06-2014, 04:52 PM:

Thanks Frank. Good points. Appreciate all who responded.

On a differenct note I still hold true that the best HT projectors have better contrast than any DCinema projector that I have personally experienced. Blacks that make a star field appear to float inside a black void is what im talking about. Not grey, or dark grey blacks, or even really dark grey but true black while retaining brightness. My single biggest thing I hate about D-Cinema is its contrast performance. If someone could reccomend a theater or projector that has the best contrast they have seen Id love to expereince what is considered the industries best. So far the best Ive seen has been Sonys DCinema pjs.

Thanks again.

Posted by Steve Guttag (Member # 268) on 04-07-2014, 06:43 AM:

As I said before...put up a 50% checkerboard pattern...the DCinema projector will have superior contrast performance. HT projectors don't properly render contrast as they use a dynamic iris...they make a presumption that if most of the image is dark to lower overall brightness. So, if you have a lowly lit scene indoors (say by candle light)...then shadow detail will be lost in people's faces...a problem a DCinema projector will not have.

Posted by Jarod Reddig (Member # 6363) on 04-07-2014, 10:43 PM:

I agree the ones that must rely on dynamic iris's for there best contrast are infirior. The best JVCs do NOT implement a dynamic iris to achieve it's best contrast. I'm referring to native contrast performance like a check board pattern and such. I'd love to see DCI content thru one.

http://procision.jvc.com/product.jsp?modelId=MODL029155&pathId=140&page=10