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 (4) 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!
FRANK RAMAGE Posted Apr 22, 2022 6:04 PM PST
I assume the lumens drop as the bulb ages, correct? I'm wondering about how many hours into my 4000 bulb should I reasonably expect to see enough drop that I'd consider replacing it? For example, does it stay 90% through 4000 hours, or does it drop 50% at the half-way mark of 2000 hours?

Too bad I didn't have a luminometer (is that a thing?) to measure it when it was new!
Rob Sabin, Editor Posted Apr 23, 2022 1:53 PM PST
Frank, a budget luminance meter is actually a fairly inexpensive tool that can be had on Amazon for not terribly much money, and if you put up an all white test pattern you can measure how bright your image is at the screen and track it as your lamp ages.

To answer the question, lamp aging is not exactly linear. If you have a 4,000 hour bulb it will actually lose a good bit of initial brightness in the first hours (which is why some folks recommend you don't do a calibration on a projector till the bulb has settled in for about 100 hours). After that, you re in for some some steady deterioration until it reaches its half life at 4,000 hours. I'd think that by the time your reach 2,000 to 3,000 hours you are looking at an image that has lost brightness, though you may not notice it because of how slowly it happens. A recalibration may be in order at that point to see if any adjustments can restore some of that onscreen brightness -- keep in mind that if you've properly set up the projector for an accurate image, you are with most projectors not likely using the full lumens. So there may be tweaks that can be done to make the image brighter using the remaining reserves. I do know enthusiasts though who like to replace a lamp well before its rated half-life to restore lost brightness and combat potential color shift that would also otherwise have to be calibrated out.

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