Transparency paper is a type of paper which is designed for use with overhead projectors. This paper is clear, so that things printed or written on the paper can be clearly seen when light is passed through the paper to make a projection. In addition to being used with projectors, transparency paper can also be useful for overlays and lightboxes. Art supply and office supply stores are good resources for transparency paper.
The earliest transparency papers were made from specially-treated wood pulp papers. Modern versions could more accurately be termed “transparency film,” since they are classically made from clear plastic films. The advantage to using transparency paper as opposed to to traditional paper is that the opacity of regular paper can interfere with the projection or clarity of an image. The use of overhead projectors is on the decline, but this specialized paper can be useful for other applications, and it continues to be produced by several paper companies.
Some companies make transparency paper which can safely be run through a printer, so that people can make prints and copies of prepared presentations. However, caution should be used before running this type of paper through a printer, because some transparency sheets will melt or warp when they are run through a printer. Paper which has been specifically marked as safe for use with a printer is the best choice, as a partially-melted piece of transparency film can gum up the works of a laser printer or copier considerably.
It is also possible to write directly onto transparency paper. In environments like offices and schools, a clear transparency sheet is sometimes laid onto a projector as a writing surface, with the presenter using erasable pens as a presentation aid. In a math class, for example, the instructor can work a problem on the projector, and students can see the projection at the front of the classroom. When the presentation is over, the sheet can be wiped off and re-used.
Because most transparency paper is plastic-based, users need to be careful about smears. Transparencies fresh out of the printer will tend to smear if they are not handled carefully, and brushing one's hand against a transparency while using it with a projector can result in a smudge which renders the transparency impossible to read. Reusable sheets will also tend to eventually become smeared with ghost impressions from previous uses, making projections hard to see.