The Optoma PK301 has two brightness modes. One is considered the Standard Mode and that is the mode you will be forced into if you choose to run the projector without AC power. If you want to produce the 50 lumens stated by the manufacturer you need to be plugged into AC power. Then you will want to enter the menu system and change the projector to Bright Mode."
It has been my experience that one can expect to get at least half the lumens stated by the manufacturer when it comes to Pico projectors. Standard projectors tend to produce 20 percent less lumens than the manufacturer specifies. Of course some projectors are the exception. Pico projectors are short on brightness, but this just means you really need to use them in environments where lighting can be tightly controlled. Depending on the Pico you purchase, some ambient light can be present in the room.
The Optoma PK301 during evaluation was being used in a room with some ambient light that came in threw the slits in my window blinds. And, even with this small amount of light the image was acceptable. Totally dark would have best, but was surprised to see the projector perform like it did.
Most impressive, is that the PK301 did hit its claims of 50 lumens on AC power and 20 on internal battery. With 50 lumens, the PK301 really can do some serious work.
What I really liked though, was getting the XP8000 external battery pack. The PK301 still managed to do it's 50+ lumens as claimed, and so far, that makes the PK301 the brightest pico we've seen, when not plugged in. The price to pay - $99 for the external battery pack - the XP8000, which weighs (per Optoma) one entire gram more than the projector - 228, vs 227. Together - that's 455 grams - one pound is 454 grams. Bottom line - with the battery pack you have a 1 pound portable 50 lumen PK301 pico projector!