[scott_s]

I think that all of my math teachers in middle and high school used projectors with transparencies. It wasn't until college that most math and physics was on black boards. The major benefit of transparencies is that the teacher was always facing the class. So they could just look up to make eye contact with students.

I hadn't given it thought before, but I assume transparencies weren't as present in college because the room a given college class will be is essentially random. It's not an environment controlled by the person teaching. in high school and middle school, that was the teachers' classroom. The teacher stayed in their room, and we moved around. Projectors require more maintenance (bulbs going out!), and the college teachers would need to lug around their transparencies and pens. But all college classrooms have blackboards and some chalk.

[cr0sh]

It might also be due to lighting.

Overhead projectors require a fairly dark room, and only work well up to a certain size and distance. Much beyond that, their light gets too dim, and if it is a large lecture hall, you need a large projection with very bright light for everyone to see.

Then there's off-axis people who might find a projector difficult to read. In a small classroom, both of these issues don't really come into play.

A blackboard, however, can be seen in a brightly lit room, almost anywhere in the room, even if the room is very large. For the really massive size lecture halls, you can have multiple blackboards that span the width of the room, plus sliding and portable ones.

Each system has its benefits and detriments I guess...

[rtkwe]

> large lecture hall, you need a large projection with very bright light for everyone to see.

And the hard limit on how bright you can make that is that the lecturer is staring into that light while writing so you can only make it so bright before it becomes painful or damaging. Anecdotally when I was in elementary/middle school we'd sometimes have to work things out writing on a projector and it was tough sometimes and looking up you couldn't really see because your eyes had adjusted to the brightness of the projector. Maybe they could improve the lenses to spill less light out but it wasn't back then.

[scott_s]

Absolutely true, and I didn't think about this. In retrospect, my intro physics courses (which were 90% engineers) were in such classrooms, and could not have used a projector. But the rest of my physics classes, and all of my math classes, were in rooms the size of my high school classrooms.