Why are TVs Typically Brighter than Front Projectors?

Question Summary: Why are TVs typically brighter than front projectors?

Submitted by Jay

I had a technical question about projectors and their brightness levels.  Given identical pitch black darkness levels, why is the brightness level of a front projector system something like 13-15 foot-lamberts (fL), and the brightness level of an HDTV around 22-30 foot-lamberts?  Is there a reason why a smaller screen has to produce a brighter image than a bigger screen?

I kept reading reviews of projectors and TVs, and they always had that range (for non-3D images of course).  Any explanation would be helpful.

Answer (provided by Clint DeBoer)

Great question! One thing you would want to keep in mind is that a front projection system is very different, fundamentally, than a flat panel TV. A front projector is sending light to a passive screen from the front. This has two very important implications: 1) the only way you can get "black" is by the absence of light, and 2) any light hitting the screen also reflects back into the room and causes the potential for true 'black' to decrease. On top of that, a third factor would be that you are also measuring light output from a reflected surface, rather than from the emitted source itself.

A flat panel TV, on the other hand, is an emissive technology, and on a pixel-by-pixel basis it can (more or less) achieve full black or full white without a terrific amount of "crosstalk" or contamination of the black levels. This is even more so with plasma TVs, high-end LED backlights and the use of local dimming technology.

So, looking at this, you can imagine that a generally darker room and less light output is the key for getting a very dynamic, deep image on a projector screen. With a flat panel TV, you can actually achieve a much higher contrast ratio by increasing light output while not affecting the black levels too badly in the process. Tom (Andry of the AV Rant home theater podcast) and I have actually seen TVs that had too much light output (they were trying to demonstrate early local dimming technology) and the resulting image had an artificially high contrast ratio that was unrealistic.

Hope that helps!