It is becoming more and more common for people to just think that any projector will work in any room for any type of viewing. This is just not the case.

First, understand that a projector's brightness rating isn't the bottom line for what gets to your screen. You can't usually get 3,000 usable lumens out of a projector rated for 3,000 lumens with the kind of image quality you want for a dark-room home theater. Most often, you only get about half of the rated brightness from almost any projector when it is adjusted for the best video quality. There are exceptions to this rule, but it is always a good rule to divide the rated brightness by 50% for a close estimate to how much real world brightness you should expect.

But, what does the resulting 1,500 or so lumens of that 3,000 rated-lumens projector actually provide to you in terms of screen size? Well, it all comes down to math. And not even hard math.

QA lumens calculations
The calculation for determining necessary lumens in a dark room actually isn't very complicated.

For home theater use in a dark room, 13-18 lumens per square foot is the recommendation. Some people like a bit more punch, and 20-25 lumens is more than enough to deliver that.

Looking at some common screen sizes, the square footage, and actual brightness needed for 18 lumens per square foot is as follows...

Screen Diagonal (in inches) Total Square Feet Lumens Needed
100 30 540
110 36 650
120 43 774
133 52 936
150 67 1,206

So, you can see, that even fairly dim projectors can deliver a 100-inch diagonal image that will be punchy and bright enough in a dark room.

The problem is, that once you start adding light to the room, things change dramatically. Projectors can't project black. This is the most important thing to understand. So, if a white wall is lit with light in the room, then that will be 'black', and the projector needs to get much brighter than that wall to make it appear as black.

This means that the 18 lumens you needed in a dark room which delivered a 1,000:1 contrast ratio, will now need to be 60 lumens in your lit room to give you a 50:1 contrast ratio (or less).

So, your 100-inch diagonal now needs more than 1,600 lumens to be usable, and it won't give you a great image, but a usable image. Your 120-inch diagonal will need almost 2,600 lumens. The 150-inch screen will need over 4,000 lumens.

Not advertised lumens... Real world, calibrated, color corrected, lumens.

QA lumens theater
Controlling light in your home theater will allow you to get the most from your projector's brightness, and introducing any kind of light will demand more brightness from the projector.

This is why controlling light in your room is the single most important thing anyone can do to make a projector look its best. You increase contrast and lower the black floor of the room so that the projector can look its absolute best. Plus, you can potentially buy a dimmer projector which can illuminate a larger screen.

What about screens? Maybe that's another topic for another day. But, my sub-1,500 lumen JVC has no issues filling my 161-inch diagonal screen in my basement, partly because of the 1.3 gain of the screen in use. There is a lot more to say about screens and what they impart on your final image.

 
Comments (2) Post a Comment
Mike Posted Dec 14, 2021 12:50 PM PST
I have been puzzled about this.

I know that most home theater designers try to hit 18-20fL or 40-60fL with ambient light. Most movies are mastered (With HDR) to 1000, 4000, or even 10,000 nits maximum brightness. With 1 fL about 3.5 nits, no projector will ever get close to those levels. Movies are made to be shown on commercial projectors that are in that 16-18 fL range. Why are they mastered so bright if they will never be seen at that level?

Should you try to get brighter than 18fL to get your high bright scenes closer to what the director wanted from the movie?
Deighton Posted Dec 14, 2021 1:52 PM PST
The lumens logic still has me confused? Need a guide to be sure!

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