Dark or light? Until recently, it looked like a no brainer and I was advising everyone to use dark backgrounds. Now experience makes me modify that advice. It's not so simple after all and you'll have to make a judgement call.
Why the original advice to use a dark background?
You only have to compare a slide with light words on dark background, with the same slide the other way around, and you'll see that the first is considerably easier on the eye. It was so obvious that I missed something subtle but important.
It turns out that bright white words on black background are harder to comprehend than the other way around... so while you gain some in eye comfort, you lose some in comprehension. (If you try this comparison yourself, make sure that font, font size, and font characteristics are identical in both cases - then ask someone who doesn't know the content which of the two is easier to understand. Also, do your test on the big screen, not on the computer screen.)
So, here's my amended advice.
Increasing comprehension but keeping it easy on the eye
For screened words, use light words (not bright white) on dark background, but make sure that the font is bigger and bolder than you might normally allow. Which means that you should have even fewer words on the screen - for PowerPoint, that's a good idea anyway. But if you have any doubts about comprehension, return to black words on a light background.
- If your PowerPoint uses photographs of objects, you will almost certainly need a white background.
- Many people make two versions of a PowerPoint show - one (the detailed one) for the handout and the other (highly simplified) for the screen.
- A dark colour doesn't necessarily mean black. Some find that too sombre.