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# How to determine what Projector brightness (lumens) is required for a specific site?

• I have an event coming up that requires developing the specifications for an architectural projection. I was looking for some formulas that might assist with estimating Projector brightness for the site. I came accross the following in AV Magazine:

“The desired brightness output is most commonly the screen area in square metres multiplied by the room light falling on the screen surface in lux, multiplied by the desired contrast. For example, a meeting room screen of 6m² with a room light situation of 100 lux typically requires a contrast of 10:1, resulting in a required brightness level of 6,000 lumens (6 x 100 x 10).”

Does this sound right? Does anyone use other methods for determing projector requirements for non-theatrical settings?

I was also under the impression that Projector staking is not a simple equation of 1 + 1 = 2, in terms of brightness and contrast. Has anyone come across a good rule of thumb when getting an accurate output of stacked projectors?

Best Wishes

Bonemap

• Hi ,

Yes it sound right for me.

If we consider  a videoprojecteur:

Videoprojecteur 6000 lumen

if projection distance is 10m

Ratio 1.2 :1

Format 16/9

10m/1.2=8.33m width

8.33/1.77=4.70m height

8.33*4.7=39.15m2

6000/39.15=153 lumen/sq meter

=153 lux as 1 lux is one lm/sq meter

Contrast 153/100=1.53 :1

Videoprojecteur 6000 lumen

Distance projection 10m

Ratio 3.33:1

Format 4/3

10m/1.2=8.33m width

3/1.33=2.25m height

3*2=6.75m2

6000/6.75=888 lumen/m2

=888 lux

if the room is 100 lux average

Contrast 888/100=8.88 :1 ( in real some more assuming that average illuminace measure  in lux are usually token on floor  or horizontal task usual plan. on vertical plan its less if room is enlighted by downlights.)

You have to consider lumens , distance throw and projection ratio of the lens to get the surface screen .

Best wishes.

• @dbo

thank you, that gives an idea.

best wishes

bonemap

• @bonemap

It really depend of the angle of the beam. The solid angle is determinant in the calculation.

Brigtness is in lux assuming that rule lux variying whit distance square. lux = candela/distance square. ( candela is intensity ).

here is a well done topic on that but sorry it'in french.

If google can translate it ... it can help.

http://ressources.univ-lemans....

Best whishes

dbo

• @dbo said:

if the room is 100 lux average

I have used lux meters to work with lighting and am starting to think that it would be a useful tool for specifying projectors.

Best wishes

Bonemap

• Rules of thumb:

You want your final image to be an absolute  minimum of 18fL and ideally around 24fL+ in a space that has designer controlled ambient light such as in a theatre design situation.

For spaces and performances that have constant ambient light you want to aim for around 50fL+.

fL = foot Lambert

Check out these books if you want to start digging into this further: https://www.routledge.com/Digi...   and https://www.routledge.com/Medi...

(Fair warning - I am biased about the first link as I am one of the authors.)

For stacking projectors check this out: http://forums.prosoundweb.com/...  read the bit from Brad Webber:

He quotes from Da-lite's "Angles Of View", Vol. IV, 11 titled "Light Reading":

"A misconception prevalent in our industry and erroneously promulgated by the author of these articles has been that double stacking projectors produces less than twice as much light. This assumption, we are now pleased to understand, is not true. 1 + 1 does = 2; every time.

In an important sense, however, this lineally additive quality of lumens is true only if the device measuring them is other than the human eye. Our eyes, you see, do not respond to changes in brightness in a straight, linear fashion. Instead, they react logarithmically to changes in their input and thus, for instance, we are able to find our way out of a darkened movie theater, yet continue to see when we emerge onto the brightly lit street outside it.

If we array a human audience before some screen and first illuminate it with a 1,000 lumen projector and then turn on a second 1,000 lumen projector, will the audience see twice as bright an image? No, it will not. The perceived brightness will increase only by about 50%. It will not double."

Best,

Alex

• There is a useful tool at projectorcentral.com you can use to calculate every needed aspect of your projection.
https://www.projectorcentral.c...
It even gives you recommendations for the limits of the ambient light.

As  you see, there are some more values you can change. There are some you should think about, especially as you saying you want to project onto buildings! Buildings are most times not plane white. Therefore they doesn’t have a gain of 1, reflecting less of the projectors light.

The other big factor is the content you are working with. High contrast content like text, presentations or geometric objects without fine colour gradients, doesn’t need a high contrast/brightness to be perceived well. The same is for b/w movies. If there is a lot of small steps of colours/brightness, especially in darker parts of the content, like in many movies, you need more basic power.

You should always calculate with the native aspect of the projector. This means, if the projectors native aspect is 16:10 you should calculate with this and not 16:9, even if your content is FullHD 16:9. The difference between these aspects results in more than 11% bigger projection surface to spread the sources light on.

If you project onto surfaces with parts in different distance (like two walls in different distances) with the same projector, you should calculate a virtual screen size for the most distant surface. For example, if you project on two parts of a building, each the half of the picture, but the one half some meters behind, the ‘virtual’ screen size of the remote surface is bigger then the one in the foreground. This is especially true, if you use short throw lenses! Based on a 1:1 throw ratio lens, the ‘virtual’ projection its 56% bigger, if the remote surface is only 1 meter behind!

The same counts for keystoned pictures. The farthest part of the projection will be the darkest. But not the real projection is the one to calculate with but the ‘virtual’ that would be the size, if you would put a plane screen at this distance.

One could think the only reason not to use the most brightest projector to get, would be the budget. But there is another one. The brighter the projection, the brighter are the dark parts of the projection. Projections of black pictures are never true black. The darkness of the black depends on the projectors capabilities, but is still dependent on the basic brightness. If you have allot of darkness in your content you could profit from less power of the projector!
Another factor comes up on high gain screens and rear projections, as higher brightness tends to have hot spots, or even see the light source through the back projection screen.

One more hint; To really get the most of your content, you should always check the color profile settings of your computer, found in the monitors setting of you operating system! Depending on the settings you’ll get a much more detailed and sometimes brighter picture! There are sometimes icc profile files to download for the specific projector, but you might find good results within the default list.

• @vanderzee said:

@bonemap
Here is the tool I use:
https://itunes.apple.com/us/ap...

Seems to be a Webapp, using the projectorcentral.com webpage as base. To bad it's iOS only again...

For Android, I use Projectionist: https://play.google.com/store/... Its handy for calculating edge blended, multi projector setups too.

edit:

there is a version for iOS, too.

http://nfc.neodarque.com/Proje...