Non-verbal communication is an important and large part of our presentation. And clearly our facial expressions are a key channel of nonverbal communication. If this is so, then why do we insist on turning the lights off when we turn on the projector? How can people see our facial expressions (gestures, body movement, etc.) if all they can see is a screen? As Bob Smith (Director of Technical Services for the Stanford Center for Innovations in Learning) points out on his well-written blog, it probably is just an "odd learned behavior."
Ten years ago, projectors were not all that bright, so turning the lights off made more sense. Today, even the smallest business projectors are typically 1200-1500 lumens, which is usually bright enough for a smaller venue or teaching situation. There is no good reason for turning off the lights today in most situations. Yes, dimming, sometimes, but only then if you feel the detail of the image is just too important to miss. In this case you can dim the lights while you explain the image, and then turn up the lights again when you are done with your close examination of the slide.
Habit is clearly another reason presenters turn off the lights. As I have said before, it is common in Japan for someone — motivated by politeness — to turn off the light switch just before a presenter begins his PPT presentation. "Projector on/ lights off" is considered common sense. But this practice is not only unnecessary today, it goes against what we know about effective communication and the need for presenters and audiences to engage and connect. It is near impossible to connect, engage, and persuade if the audience and presenter can not see each other.