Projection Design Brightness Megathead

[trashcan]

I'm checking myself for places in my projection design knowledge that need some improvement and this is something I'd like to work on. I also think this will be useful to others!

 

What is your method for determining appropriate projector brightness in a given situation?

 

In general, my rule is that projector brightness needs to be 50fL (foot-lamberts) or higher, but some situations call for higher brightness and in some situations lower fL is totally acceptable. I keep track of what fL worked for most of my projects so I have many frames of reference. Sometimes I even take photos of other folks' designs and projectors and calculate what their fL was and am sometimes surprised. Point being: I do it "to-taste" which seems to be outdated given this can be "calculated". 16fL for a well controlled room works pretty well, but I try to shoot for at least 30fL. 50fL for a convention center looks meh. 80fL in a retail situation where ambient light is fairly aggressive looks pretty good. 90fL in a broadcast studio can look good if lighting is well controlled. 

 

So for the experts out there, I wanted to start a thread that discusses your tricks of the trade and how you calculate what is acceptable and what isn't. 

 

How do you measure fL against ambient light to determine what level is acceptable? Do you have a rule that you work with? Do you measure ambient light and determine what works well against that ambient light? Do you use an ambient light sensor? Which one do you use? Is there a secret chart somewhere that says what fL range looks good against an ambient lx range? Is it something like: the projection brightness needs to be 5x the ambient light fL? Can you do this with a smartphone? 

 

(for reference foot lamberts is calculated by brightness divided by total square footage:  a 10,000 lumen projector hitting at 16x10' screen has a brightness of 62.5 fL).