Overhead projectors are stand-alone electrical/optical devices that allow you to present images onto a screen through transparencies. Light shines through the images and the optics of the projector focus it up on screen. However, they can not display opaque or solid images, such as pages from a book.
Key terms and definitions for overhead projectors:
1. Fresnel lens – A specially constructed lens that produces a soft-edged concentration of light. The Fresnel lens is about 1″ below the surface of the stage glass.
2. Stage Glass – The glass surface where the transparencies are placed, between the light source and the optics.
3. Lens – Part of the optics that focuses the image for projection. There are three lens types:
a. Singlet – one lens
b. Doublet – two lenses
c. Triplet – three lenses
4. Head Assembly – The combination of the optics and mirrors. There are two head types:
a. Open Head – uncovered open optics, brighter image
b. Closed Head – covered, enclosed optics, stays cleaner
5. Thermal Switch – A temperature sensitive switch that senses heat build-up and removes power from the lamp if cooling fan fails.
6. Safety Interlock Switch – A switch that removes power to the overhead projector if the unit is opened. Prevents danger of being electrocuted.
7. Focal Length – The minimum distance from the stage glass to the lens. The focal length will determine the size of the projection at a given distance. A short focal length (about 11″) will be a wide-angle projection requiring the projector to be fairly close to the screen. A standard focal length (about 14″) will have a narrower projection, so the unit may be further away.
8. Throw Distance – The distance from the head to the projection screen.
Some additional features found on most overhead projectors today include a quick lamp changing system and a hi/low switch for lamp brightness. The lamp change feature allows you to keep a spare lamp inside. Should one go out during the middle of your presentation, you can simply flip a switch and change lamps without interruption. A hi/low switch not only reduces or increases lamp brightness; continued use of the low setting can extend lamp life!
Visual Presenters and Document Cameras
Unlike those big and bulky opaque projectors from years past, visual presenters are extremely flexible and highly powerful and presentation tools. They offer much value and affordability and they’re much smaller and compact too.
You can display any solid object; printed/opaque documents or book pages, small 3D objects, photos, slides, negative and positive film, and even X-Rays! Examine the tiniest of objects in minute detail as if through a microscope, except everyone else in the room will be able to see it at the same time. A presenter with a zoom feature allows even closer inspection.
Visual presenters offer a color or black and white camera mounted on a moveable arm that you position over your document or object. Most include flexible lights for illumination. The base, or stage, usually has side or bottom lights.
Types of Visual Presenters/Document Cameras
There are two types of visual presenters: analog and digital. Depending on the type of input connections provided, you can display objects anywhere from a computer screen, TV monitor, plasma panel,or through a data/video projector.
Analog visual presenters produce images in NTSC video format (same as a standard VCR). They will connect to a TV or VCR where images can even be displayed and even recorded onto a VHS tape for later viewing.
Digital visual presenters use the same technology found in most digital cameras. They will typically have a color CCD (Charged Coupling Device) image sensor for better quality. Most plug directly into a data/video projector. Some can be connected through a computer. A digital-based image shown on a computer can be saved as a file (much like from high resolution scanner). Some have a built-in memory to store and capture images internally for later viewing.
Digital presenters can often produce a rapid series of images to form a short movie. Unlike real-time video recording through a camcorder, presenters are capable of producing a series of images in fps (frames per second-a.k.a. frame rate). This tells you how smoothly each frame will transition from one to another. Therefore, the larger this number is, the smoother movements will be.