Overhead Projector - Glossary of Terms

Overhead Projector - Glossary of Terms

APERTURE - The area of the stage that is available for projecting an image. Usually comes in the
following sizes: 10“ X 10”, 10.5” X 10.5” and 11.25” X 11.25” (A4).
ARTICULATED HEAD - A closed projector head where the mirror will move one-half the distance when
the image is tilted up on the screen.
COATED STAGE - Hard coating on a reflective optical stage (fresnel lens) that resists scratching, so that
the projected visuals stay sharp and clear.
COLOR TUNING - Adjusts the edge-to-edge uniformity of the projected light to eliminate yellow or blue
corners. This gives an optimized image, no matter what the projector to screen distance, within the
prescribed area.
CONVECTED COOLED - Natural cooling of the projector lamp by the upward flow of heated air, without
the use of a fan. Usually used in reflective type projectors that have the lamp in the head assembly.
DOUBLET HEAD - A projector head that has a two element lens.
ELEVATION ANGLE - The elevation that the projected image can be tilted up to, and still project a full
image onto the screen.
FOCAL LENGTH - The value given to a lens, stated in inches or millimeters. The smaller the focal length,
the wider the angle of the image. Focal length is the distance between the lens and its focal point.
FOCUS CORRECTION - A feature that is used when it is necessary to project an image onto a vertical
surface (such as a wall) with a high tilt angle of the head, making it possible to obtain uniform focus from
top to bottom of the image. The feature does not eliminate keystoning.
FRESNEL LENS - A flat lens that is composed of a series of closely spaced grooves that control the
refraction of light. It is usually part of the stage.
ILLUMINATION - The density of light projected onto a screen or other surface. Stated in lumens.
KEYSTONING – Keystoning is caused when the projected image is not perpendicular to the screen.
Correct keystoning by tilting the screen until it is perpendicular to the light beam axis.
LET RUN THERMOSTAT - See Post Cool Thermostat.
LIQUID CRYSTAL DISPLAY (LCD) - A display panel that sits on top of the projector stage which creates
an image that is generated by a computer.
LUMEN - The unit of illumination on a screen or other surface. One lumen is the light of one candle power
on each square foot of a surface of a sphere at a radius of one foot from the light source.
OPEN HEAD - A projection head where the mirror and the lens is not enclosed. The image is raised on the
screen by tilting the mirror up.
OPTICAL TUNING - See Color Tuning.
OVERHEAT THERMOSTAT - Turns the lamp off if the projector reaches an unsafe temperature.
POST COOL THERMOSTAT - A thermostat that allows the projector fan to continue to run after the lamp
has been turned off, which reduces the temperature of the unit.
REFLECTIVE TYPE PROJECTOR - An overhead projector where the light source is located in the head
assembly and shines down onto the stage. The light is then reflected from the fresnel lens, back through
the head and onto the screen. Usually used in light weight, portable projectors.
SINGLET HEAD - A projector head with a lens that has only one element.
STAGE - The flat area of the projector where the transparency film is placed.
SAFETY THERMOSTAT - See overheat thermostat.
TRANSMISSIVE TYPE PROJECTOR - An overhead projector where the light source is under the stage
and light is transmitted through the transparency film to the head and onto the screen.
TRIPLET LENS - A projection lens that has three elements contained in a single assembly.
VARIFOCAL LENS - A projection lens containing movable elements to permit focusing of the image. (Not
a zoom)
WIDE ANGLE LENS - A lens that will project a larger image on a screen at a closer distance than a
standard lens will project. Usually has a focal length of 11.5” (293mm) or smaller.

Quick Reference Guide 3M™ Overhead Projectors
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