The Optoma ML550 actually performed exceptionally well and I am happy to report it exceeded expectations in terms of brightness. There are a few issues to discuss, but overall this projector was a strong performer. As discussed in this review under the section "Image Quality" I mentioned that adjusting the white light output adjustment made for much better color reproduction. This adjustment however did lower brightness. The ML550 measured out to be 514 lumens in its bright mode with no adjustments in the menu system. When adjusted to produce what I felt was a more accurate yellow color, the light output measured 367 lumens.
PC Mode was also a powerful performer by producing 441 lumens. These numbers are of course very low compared to typical portable projector that are about 2.5 times the size, but 500 lumens is still extremely when consider just 12 years ago 500 lumens would have been considered unbelievably powerful.
Cinema Mode produced 346 lumens. It is common amongst almost all projectors for the Cinema Mode, or Movie Mode, with some manufacturers that lumens drop to create the sense of a blacker black.
Photo Mode had a strong light output of 400 lumens. This mode produced a decent image, and the image was viewable with light streaming in from a window, but it was a little washed out. Overall, running the projector in Bright Mode with a little adjustment down to 5 on the white output produced the best image.
The ML550 is a really quiet projector and only produces 30dB of noise output. It has two tiny fans inside its really small case. You will find that the Optoma ML550 is virtually silent and you will not be bothered by the noise. Considering that your audience may be closer to the unit because of its portability, noise would be a consideration, but not with the ML550.