In an age of digital projectors that visualize files from computer to screen, opaque and overhead projectors see far less use in classrooms and offices than they did in decades past. You may find their lower-tech approach to the job of projection useful, however, especially by comparison to the data hookups and expensive lamps that their digital cousins require. Which of the two device types suits your business needs depends on the kinds of materials you want to project.
The opaque projector takes its name from the types of materials it projects: bound or unbound printed materials or other reflective objects. As its cost and construction quality rise, the opaque projector offers greater fidelity in its ability to reproduce fine details accurately and clearly. The technology uses a very bright light source to illuminate projected material, and a system of lenses and mirrors to display it on a wall or screen, usually at a much-enlarged size. These projectors can include adjustments to change the scale at which your source material projects.
The overhead projector works with transmissive, or transparent, objects, such as sheets of plastic film, on which you print or write messages, artwork or other source material. A bright light shines through the transparent substrate and onto a mirror, which reflects the image through a lens and onto a flat surface. Overhead projectors enable you to write in real time on a transparency that's placed on the machine, sharing what you write with an audience. Although the technology requires electricity and a projection surface, it offers the advantage of enabling you to write or draw comfortably on a flat surface, compared to the awkward writing position required for traditional whiteboards or lecture-hall chalkboards.
Opaque projectors form a mainstay of the muralist's toolset. An artist who creates large-scale drawings at a smaller size on a traditional drawing medium uses an opaque projector to enlarge the work on a surface for retracing. In meetings and lectures, a presenter can share a page from a book without copying or reproducing it. By contrast, the transparent media required for overhead projection lend themselves to handwriting with wax pencils or liquid-ink markers, or, more recently, to computer-printed transparencies made from digital files.
Both opaque and overhead projectors use light sources bright enough to generate considerable byproduct heat. If you're projecting materials that suffer from heat exposure, or you must leave transparent materials on a running overhead projector for long periods of time, exercise care to avoid damaging your equipment or your sources. Because these projectors only work with one type of material -- reflective or transmissive -- you'll need one of each if your workflow requires projection of opaque as well as transparent sources. Although opaque projectors can display on a wide range of surfaces, overhead projectors work best when their output displays on a traditional screen, which means acquiring and caring for an additional piece of equipment, along with its stand or wall-mounting hardware.