[gumshoe99]

At ProjectorCentral.com, they say that there is an optimal Foot-Lambert brightness measurement at the screen to watch movies. Apparently a brighter image would lead to eye strain/discomfort.

ProjectorCentral Info

The specific quote on that page says that in a dark room it's about 12 to 22 foot-lamberts.

Quote

a luminance level on the screen in the range of 12 to 22 fL is in the ideal comfort range

Here's an example of a inexpensive short throw projector available in my area I am using for illustration purposes. According to the numbers I get for a 96" diagonal screen (with Video/Movies selected) that projector could easily run in econo mode (assuming a 25% to 33% dropoff in brightness) and still provide plenty of brightness for watching movies. That's a pretty good feature of their calculator. Its better than manufacturer stated ansi lumen to help rate any given projector for home theatre use.

BenQ MP522

With the calculator it's easy to see if the fl (foot lamberts) for various screen sizes and distances meets the suggested brightness range for watching movies depending on ambient lighting.

If this is as important as it seems, as anyone done any similar calculations for any of the diy projector builds? ie... 15" or 17" lcd, 400 watt pro build or a 15" lcd, 400 watt, standard build.

I saw some scatttered threads discussing average lux measurements for some builds which might be helpful? Is there a formula to convert lux to foot-lamberts for comparison with the suggested 12 to 22 foot-lamberts numbers?

[dougr]

gumshoe99, on Dec 8 2008, 09:17 PM, said:

<snip>
I saw some scatttered threads discussing average lux measurements for some builds which might be helpful? Is there a formula to convert lux to foot-lamberts for comparison with the suggested 12 to 22 foot-lamberts numbers?

here you go... I've spent endless hours playing with this calculator to do just that (as well as play with screen types (gain) and size): http://www.carltonba...ter-calculator/

This post has been edited by dougr: 09 December 2008 - 05:49 AM

[dougr]

Just played around with that calculator... nice tool for commercial PJs.

I compared the numbers from that calculator for a 1.2 gain, 120" screen at 18 ft throw distance for the older mitsu HC4900 and JVC-DLA-RS1 and the newer epson 6500-UB against the PJ I'm working on (575w HMI wuxga long throw) in the C.Bale calculator. Strangely enough, every single projector is within 1 FL of the mimimum 12 for a dark room in the long-throw config... the numbers I get from Bale's spreadsheet for my soon-to-be PJ with an estimated 385 lumens? 11.2. All of the commercial PJs used in a calibrated mode lose alot from their ANSI spec... use them in a long-throw config, and they apparently become comparable (from a brightness perspective) to our brighter DIY PJs. So, if you need a long-throw setup (like I do), you really won't be losing much brightness from going DIY, but if you can do a ceiling mount (brighter at wide end of zoom), and run a commercial PJ in its non-optimal mode, you will get a much brighter projection than we can do with DIY (except for the 1200w DIYs that is

This post has been edited by dougr: 09 December 2008 - 06:29 PM

[SupraGuy]

Okay, since most of us are dealing with lumens and lux as our measurements, the first step is to convert from foot lamberts to lux.

1 lux= 1 lumen / 1 m^2
1 foot lambert = 1 lumen / 1 ft^2

Therefore 1 foot lambert = 10.76396 lux.

12 foot lamberts = 129.2lux
22 foot lamberts = 236.8 lux.

No, I don't think that most DIY projectors hit this range, though some come pretty close. Mine WAS in this range, and I fount it to be too bright, so I de-tuned it somewhat. At it's initial state, it was in that range at an average lux of 146. I found that to be too bright, and it hurt my eyes when a dark scene cut way to a bright scene. (Example, in Star Wars the cut from the escape pod in space to the desert scene with R2D2 and C3P0 on the sand. I thought my eyes were going to burn...)

I de-tuned it from about 440 lumens down to about 250 lumens, which brought the average lux down to about 84. That's a bit under 8 foot lamberts, and I find it to be much more comfortable, and I'm still able to watch very dark movies easily and comfortably. The caveat there being that I watch in a very dark room.

[gumshoe99]

Thanks for the responses.

I finally got some time to spend on this again after dealing with a few adventures including a little flood caused by a busted hot water tank.

The sheet is not too helpful with diy units but it seems within general specs for commercial models.

I tried it out and the numbers work out to be similar to the ProjectorCentral calculator but only in Presentation/Data/Text mode. In Video/Movie mode the foot-Lambert are far off.

I played with the numbers for 2 very different commercial projectors that I am curious about. The Samsung P400, 800x600 resolution LCD, 150 ANSI lumens LED projector and the BenQ MP522 1024x768 resolution DLP, 2000 ANSI lumens, zoom lens projector.

For the P400 led projector 4:3 format, 72" diagonal, screen gain 1.0 :

Using the spreadsheet it states that at that size it produces 9.8 foot-Lambert. The ProjectorCentral result is 9 foot-Lambert in presentation mode but only 4 foot-Lambert in Video/Movie mode. According to their calculator, in movie mode, this LED projector can only hit 12 FL or more if the image is 44" diagonal or less and a median 16 FL with a 36" diagonal image in a 4:3 format. This is not unexpected for this low light LED projector but it would be disappointing if our diy units don't do much better than this.

For the MP522 projector 4:3 format, 72" diagonal, zoom off, screen gain 1.0 :

The Excel sheet gives 130 FL and the ProjectorCentral site gives 109 Foot-Lambert in presentation mode for the same diagonal size. In movie mode the ProjectorCentral calculator shows 55 Foot-Lambert and even at 108" diagonal it's still 24 foot-Lambert. So although this projector has a short throw as I said before, in movie mode, it could easily operate in econo mode and produce a pretty bright viewable image well within expected standards.

I'm not comparing these 2 units to each other because they use very different light engines but I wanted to see how practical each unit is and how they compare to diy models.

This is where SupraGuy's comments come into play.

As I've stated elsewhere I'm in the early stages of redesign and want to see what if anything is worthwhile. I have the parts to go 400 watt (Pro) long throw, or standard short throw or even sell what I have and start all over again but I need to know what is a realistic expectation for various style units. The issue of Foot-Lambert had never sunk in until I saw ( and paid attention to) the Movie mode results on the ProjectorCentral site.

Using SupraGuy's numbers, can a LL 400 watt light engine and a 15" lcd LL standard or LL Pro lens (or 17" and Pro lens) , hit a median 16 Foot-Lambert or roughly 172 lux. If so, since smaller screen projections are brighter, at what size screen would I expect the sweet spot of 16 FL to happen?

Is there a better combination to hit the optimum (12 to 22 fl) brightness.

For argument sake, which would be brighter at 96" diagonal, a 400 watt, 15" lcd shorter throw standard LL lens pj or a longer throw 400 watt, 15" lcd Pro lens pj? Any estimate of Lux or Foot-Lambert? Does the square law come into play here and favor the short throw unit?

This post has been edited by gumshoe99: 17 December 2008 - 11:35 PM

[SupraGuy]

Total lumens depend primarily on the light engine and the LCD. From there, lux (or foot-lamberts) depends on screen size. Since the total number of lumens is predetermined, the larger the screen, the dimmer the image.

Say you can get 200 total lumens with your lamp, reflector and LCD. If you r screen size (4:3) is 1.6m by 1.2m your total screen area is 1.92 square meters, so you'll get an average lux of just over 104. This is pretty good (And about the target that I personally shoot for) but it's still shy of your goal. This is an image about 78" diagonal. If you want to raise that to 129 lux (12 foot lamberts) then you'd need to reduce the image size to about 1.45m by 1.1m (About 71" diagonal)

71" diagonal is still a respectable size, so that shoud be fine.

Say though that for whatever reason you can only get about 150 lumens, that means that you can only cover 3/4 of the area and get the same lux.

Note that once the total number of lumens is determined (Which is light engine, LCD size and distance to collimator fresnel) it does not matter if you get the 71" diagonal by use of a long throw lens or a short throw lens. It's the total projected image size that matters.

[dougr]

SupraGuy, on Dec 18 2008, 06:50 PM, said:

Note that once the total number of lumens is determined (Which is light engine, LCD size and distance to collimator fresnel) it does not matter if you get the 71" diagonal by use of a long throw lens or a short throw lens. It's the total projected image size that matters.

true... the primary reason to consider the short/long throw of the lens is that if you decide to compare your stats to a commercial PJ, its nice to compare apples-to-apples, and full tele is where commercial units are their dimmest (and often comparable in best mode to 400 or 575w DIY PJs). So, if you want a long throw, while it may not matter for the output of your DIY projector (given a proper choice of optics), it will matter for that $2000 commercial unit you are comparing it to at the same throw distance. The advantage of designing your own is that you get to choose the optics for your primary use, while a commercial PJ is necessarily a compromise to meet the diverse needs of a wide audience. That is not to say that a commercial PJ will not outperform a DIY PJ across the entire range of conditions, but the extent to which it outperforms it (especially in brightness) will vary depending on your intended use. phew... lots of words for something that should have taken a single sentence!