Electronic Projection and Overhead Projector Systems

Electronic Projection System Terms

Active Matrix
Each pixel is actively controlled by a diode or a transistor. Advantage: allows each pixel to be independently controlled. 

ANSI Lumens
ANSI lumens is a scale to measure the overall brightness value for projectors. The measurement represents the average value of 9 points on the projected screen image. 

Aspect Ratio
The ratio between the width and height of the output (whether it is a monitor, LCD projection panel, overhead or slide). 

ANSI
American National Standards Institute. 

CGA
Color Graphics Adapter. This is the card added to an IBM® PC & XT that gives the computer the ability to handle graphics and color. Resolution of this card is 640 x 200 pixels. 

Composite Video
A video signal that combines all the color and timing components of the image into a single input line. 

Compression Mode
A method of displaying images in a reduced size format. A compressed image usually has part of the image information discarded. The result is a projected image that has light and dark lines and text characters with thick and thin line widths. 

Contrast Ratio
The ratio of the brightest and darkest images a display can reproduce. 

Convergence 
The alignment of the red, green, and blue video image signal on a projected display system. 

Display Size
Display size is the diagonal length of the LCD plate. Typical sizes are 8.4" to 12.1". 

DSTN
Double Super Twist Nematic. Where two separate LCD plates are combined to form a single panel. 

Distribution Amplifier 
A device that amplifies and transmits a video signal over a distance using shielded coaxial cable. 

EGA
Enhanced Graphics Array. This card is the second generation of the CGA card in that it gives IBM PCs, XTs and ATs greater resolution (640 x 350 in all models). 

Hz, (KHz, MHz)
Hertz or (Kilohertz or Megahertz). Cycles per second. (Kilo = 1,000, Mega = 1 million). These terms are used to express the frequency of an electrical signal or event. 

Image Compression (Compressed)
Compresses higher resolution images into 640 x 480. 

Interlaced
Every other line is scanned during each total vertical (full) screen refresh. 

IR (Infra-Red)
A type of wireless transmission using infrared light waves. 

LCD
Liquid Crystal Display.

Non-Interlaced
Each line is scanned during each total vertical (full) screen refresh. 

NTSC
National Television Standards Committee. The standard for broadcast color television and other video equipment signal in North America, established in 1953. 525 lines/60 Hz. 

PAL
Phase Alternate Line. The phase of the color carrier alternates from line to line. PAL is used extensively in Western Europe. 625 lines/50Hz. 

Palette
The number of colors available for use in creating an image. The use of a standardized palette in a presentation allows the user to create a consistent look. 

Panel
The same as liquid crystal display (LCD). 

Passive Matrix 
The use of simple driver electronics in an LCD projection panel where the pixels are turned on and off using a row-and-column format. The amount of control on each pixel is limited, which results in lower contrast ratios and a slower response time than active-matrix LCD projection panels. 

Pixel
A unique position on a display that consists of a single dot or group of three dots (red, green and blue). Total pixels are usually expressed in horizontal x vertical dimensions (e.g., 640 x 480). 

Polysilicon TFT 
Polysilicon TFT is a type of LCD technology that allows more light at high temperatures through the LCD. 

Refresh Rate
The number of times the screen image is "painted" or refreshed per second, expressed in Hz. 

Resolution
Resolution is the ability of an imaging system to faithfully reproduce fine detail information and transitions between dark and light parts of an image. The more pixels the display systems can address (e.g., 800 x 600 ) the higher-quality image with more detail. 

Response Time 
The time it takes for a pixel to turn on and off. Typically measured in milliseconds, an active-matrix LCD projection panel's response time is fast enough to display full-motion video and rapid mouse cursor movements. 

RGB 
Red, Green, Blue. The basic signal components of the color video system. 

SECAM 
Sequential Couleur Avec Memorie. The color television standard developed in France. SECAM is used mostly in France and Eastern European countries. 625 lines/50Hz. 

Serial port 
A data I/O port on the computer enabling other devices or computers to link with the computer. Also referred to as RS-232C or COM port. 

STN
Super Twist Nematic. 

Super VGA (SVGA)
A graphic of 800 x 600. This standard has versions with different vertical frequencies. 

S-Video (S-VHS) 
A video signal that separates luminance (Y) and chrominance (C) signals. 

TFT
Thin Film Transistor. This is a developing technology that attempts to place the controller of the panel directly on the surface of the glass. 

Transmitivity
The percent of the light that is transmitted off the stage of the overhead projector that reaches the screen at a given distance. Typically, LCD projection panels are able to use less than 10% of the total light available. 

TSTN
Triple Super Twist Nematic. Where three separate LCD plates are combined to form a single panel. 

VESA
Video Electronics Standards Association. A non-profit group of companies organized to define and improve computer graphics standards. VESA standards usually achieve a higher display quality by increasing the resolution (e.g., 1024 x 768) while maintaining a high vertical refresh rate (e.g., 72 Hz) to reduce flicker. 

VESA Standard
A set of display specifications agreed upon by the VESA organization, usually referred to by resolution and vertical refresh rate. 

VGA
Video Graphics Array. This is the standard interface for the IBM PS/2®. It is the only analog graphics card IBM has used (other cards handle digital information) 720 x 400 in the text mode, graphics mode 640 x 480 resolution. 

Video
The capability to project images from a VCR, laser disc, or PC with CD-ROM drive. 

VirtualMouse Remote Control
3M's lightweight remote control offers all of the functionality of a computer-compatible mouse. 

XGA
Extended Graphics Adapter. IBM's graphics standard that includes VGA and extended resolution up to 1024 x 768. 

Y-Cable
With many computers, there is only one monitor output. Subsequently, a cable is necessary that will split the monitor signal so it will work simultaneously with both a monitor and an LCD projection panel. 

Overhead Projector 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. 

Closed Head
A projection head where the mirror and lens is enclosed. 

Convection Cooled
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 installed into the head assembly. 

Doublet Lens
A projection lens that has two elements contained in a single assembly. 

Elevation Angle
The elevation that the projected image can be tilted to, and still project a full image onto the screen. 

Flip-In Magnifier
A 1.3X enlargement lens used to enlarge LCD projection panel images. 

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. This 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. 

High/Low Intensity Switch
A control switch that decreases (LOW) the lamp output by 10% and doubles the lamp life. The HIGH setting should be used with LCD projection panels. 

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. 

Lamp Changer
A control mechanism that allows easy exchange of replacement lamps by rotating a replacement lamp into operation position after the primary lamp has failed. 

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 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 lightweight, portable projectors. 

Safety Thermostat
See Overheat Thermostat. 

Singlet Lens
A projection lens that has only one element. 

Stage
The flat area of the projector where the transparency film or LCD projection panel is placed. 

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 by varying the focal length. (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" (293 mm) or smaller. 

IBM and PS/2 are registered trademarks of IBM Corporation.