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44 Glossary
Contrast Ratio— Range of light and dark values in a picture, or the ratio between
their maximum and minimum values. There are two methods used by the
projection industry to measure the ratio:
1
Full On/Off
— measures the ratio of the light output of an all white image (full
on) and the light output of an all black (full off) image.
2
ANSI
— measures a pattern of 16 alternating black and white rectangles. The
average light output from the white rectangles is divided by the average light
output of the black rectangles to determine the
ANSI
contrast ratio.
Full On/Off contrast is always a larger number than ANSI contrast for the same
projector.
dB— decibel—A unit used to express relative difference in power or intensity,
usually between two acoustic or electric signals, equal to ten times the common
logarithm of the ratio of the two levels.
Diagonal Screen— A method of measuring the size of a screen or a projected
image. It measures from one corner to the opposite corner. A 9FT high, 12FT
wide, screen has a diagonal of 15FT. This document assumes that the diagonal
dimensions are for the traditional 4:3 ratio of a computer image as per the example
above.
DLP— Digital Light Processing—Reflective display technology developed by
Texas Instruments, using small manipulated mirrors. Light passes through a color
filter is sent to the DLP mirrors which arrange the RGB colors into a picture
projected onto screen, also known as DMD.
DMD— digital Micro- Mirror Device— Each DMD consists of thousands of
tilting, microscopic aluminum alloy mirrors mounted on a hidden yoke.
DVI— Digital Visual Interface— Defines the digital interface between digital
devices such as projectors and personal computers. For devices that support DVI, a
digital to digital connection can be made that eliminates the conversion to analog
and thereby delivers an unblemished image.
Focal Length— The distance from the surface of a lens to its focal point.
Frequency— It is the rate of repetition in cycles per seconds of electrical signals.
Measured in Hz.
Hz — Frequency of an alternating signal. See Frequency.
Keystone Correction— Device that will correct an image of the distortion
(usually a wide-top narrow-bottom effect) of a projected image caused by
improper projector to screen angle.
Laser Pointer— A small pen or cigar sized pointer that contains a small battery
powered laser, which can project a small, red (typically), high intensity beam of
light that is immediately very visible on the screen.