The basic function of an overhead projector is similar to that of a slide projector. It can show on a screen what has been written beforehand on a transparent sheet. The teacher can directly write on the transparent sheet instead of on a blackboard.

• 投影仪的基本功能与幻灯机相似。它可以将事前写在透明薄片上的文字等投影在银幕上。教师可以直接书写在透明薄片上而不用写黑板。