LCD

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2 Νοε 2013 (πριν από 4 χρόνια και 2 μήνες)

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LCD

LCD

(liquid crystal display)
(from Whatis)
is the technology used for displays in notebook and other smaller
computers. Like light
-
emitting diode (
LED
) and gas
-
plasm
a technologies, LCDs allow displays to be much thinner
than cathode ray tube (
CRT
) technology. LCDs consume much less power than LED and gas
-
display displays
because the
y work on the principle of blocking light rather than emitting it.

An LCD is made with either a passive matrix or an
active matrix display

display grid. The active matr
ix LCD is
also known as a thin film transistor (
TFT
) display. The passive matrix LCD has a grid of conductors with pixels
located at each intersection in the grid. A cur
rent is sent across two conductors on the grid to control the light for
any
pixel
. An active matrix has a
transistor

located at each pixel intersection, requiring less current to control the
luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more
frequently, improving the screen ref
resh time (your mouse will appear to move more smoothly across the screen,
for example). Active matrix is the superior technology.

Color LCD

An LCD that can show colors must have
three subpixels

with red, green and blue color filters to create each color
pixel.

Through the careful control and variation of the voltage applied, the intensity of each subpixel can range
over
256 shades
. Combining the subpixels produces a possible palette of
16.8 million colors

(256 shades of red x
256 shades of green x 256 sh
ades of blue), as shown below. These color displays take an enormous number of
transistors. For example, a typical laptop computer supports
resolutions

up to 1,024x768. If we multiply 1,0
24
columns by 768 rows by 3 subpixels, we get 2,359,296 transistors etched onto the glass! If there is a problem with
any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels
scattered across the
screen.

LCD Features and Attributes

Fr o m ht t p://c o mp ut e r.ho ws t u f f wo r k s.c o m/mo n i t o r 6.ht m

To e v a l u a t e t h e s p e c i f i c a t i o n s o f LCD mo n i t o r s, h e r e a r e a f e w mo r e t h i n g s y o u n e e d t o k n o w.

Na t i v e Re s o l ut i o n

Un l i k e CRT mo n i t o r s, LCD mo n i t o r s d i s p l a y i n f o r ma t i o n
we l l a t o n l y t h e r e s o l u t i o n t h e y a r e d e s i g n e d f o r,
wh i c h i s k n o wn a s t h e n a t i v e r e s o l u t i o n. Di g i t a l d i s p l a y s a d d r e s s e a c h i n d i v i d u a l p i x e l u s i n g a f i x e d ma t r i x
o f h o r i z o n t a l a n d v e r t i c a l d o t s. I f y o u c h a n g e t h e r e s o l u t i o n s e t t i n g s, t h e LCD s c a l e s t h e i ma g e

a n d t h e
q u a l i t y s u f f e r s. Na t i v e r e s o l u t i o n s a r e t y p i c a l l y:

1 7 i n c h = 1 0 2 4 x 7 6 8

1 9 i n c h = 1 2 8 0 x 1 0 2 4

2 0 i n c h = 1 6 0 0 x 1 2 0 0

Vi e wi ng Ang l e

Wh e n y o u l o o k a t a n LCD mo n i t o r f r o m a n a n g l e, t h e i ma g e c a n l o o k d i mme r o r e v e n d i s a p p e a r. Co l o r s
c a n a l s o b e mi s r e p r e
s e n t e d. To c o mp e n s a t e f o r t h i s p r o b l e m, LCD mo n i t o r ma k e r s h a v e d e s i g n e d wi d e r
v i e wi n g a n g l e s. ( Do n o t c o n f u s e t h i s wi t h a wi d e s c r e e n d i s p l a y, wh i c h me a n s t h e d i s p l a y i s p h y s i c a l l y
wi d e r.) Ma n u f a c t u r e r s g i v e a me a s u r e o f v i e wi n g a n g l e i n d e g r e e s ( a g r e a t e r

n u mb e r o f d e g r e e s i s b e t t e r ).
I n g e n e r a l, l o o k f o r b e t we e n 1 2 0 a n d 1 7 0 d e g r e e s. Be c a u s e ma n u f a c t u r e r s me a s u r e v i e wi n g a n g l e s
d i f f e r e n t l y, t h e b e s t wa y t o e v a l u a t e i t i s t o t e s t t h e d i s p l a y y o u r s e l f. Ch e c k t h e a n g l e f r o m t h e t o p a n d
b o t t o m a s we l l a s t h e s
i d e s, b e a r i n g i n mi n d h o w y o u wi l l t y p i c a l l y u s e t h e d i s p l a y.

B r i g ht ne s s o r Lumi na nc e

Th i s i s a me a s u r e me n t o f t h e a mo u n t o f l i g h t t h e LCD mo n i t o r p r o d u c e s. I t i s g i v e n i n n i t s o r o n e c a n d e l a s
p e r s q u a r e me t e r ( c d/m2 ). On e n i t i s e q u a l t o o n c d/m2. Ty p i c a
l b r i g h t n e s s r a t i n g s r a n g e f r o m 2 5 0 t o 3 5 0
c d/m2 f o r mo n i t o r s t h a t p e r f o r m g e n e r a l
-
p u r p o s e t a s k s. Fo r d i s p l a y i n g mo v i e s, a b r i g h t e r l u mi n a n c e r a t i n g
s u c h a s 5 0 0 c d/m2 i s d e s i r a b l e.

Co nt r a s t Ra t i o

Th e c o n t r a s t r a t i o r a t e s t h e d e g r e e o f d i f f e r e n c e o f a n LCD

mo n i t o r's a b i l i t y t o p r o d u c e b r i g h t wh i t e s a n d

the dark blacks. The figure is usually expressed as a ratio, for example, 500:1. Typically, contrast ratios
range from 450:1 to 600:1, and they can be rated as high as 1000:1. Ratios more than 600:1, however,

provide little improvement over lower ratios.

Response Rate

The response rate indicates how fast the monitor's pixels can change colors. Faster is better because it
reduces the ghosting effect when an image moves, leaving a faint trial in such applicatio
ns as videos or
games.

Adjustability

Unlike CRT monitors, LCD monitors have much more flexibility for positioning the screen the way you
want it. LCD monitors can swivel, tilt up and down, and even rotate from landscape (with the horizontal
plane longer t
han the vertical plane) to portrait mode (with the vertical plane longer than the horizontal
plane). In addition, because they are lightweight and thin, most LCD monitors have built
-
in brackets for
wall or arm mounting.

Besides the basic features, some LC
D monitors have other conveniences such as integrated speakers, built
-
in
Universal Serial Bus

(USB) ports and anti
-
theft locks.


LCD Terms

* Bezel

-

This is the metal or plastic frame surrounding
the display screen. On LCD displays, the bezel is
typically very narrow.

* Contrast ratio

-

The difference in light intensity between white and black on an LCD display is called
contrast ratio. The higher the contrast ratio, the easier it is to see detai
ls.

* Ghosting

-

An effect of slower response times that cause blurring of images on an LCD monitor, it's
also known as latency. The effect is caused by voltage temporarily leaking from energized elements to
neighboring, non
-
energized elements on the dis
play.

* Luminance

-

Also known as brightness, it is the level of light emitted by an LCD display. Luminance
is measured in nits or candelas per square meter (cd/m2). One nit is equal to one cd/m2.

* Native resolution

-

This actual measurement of an LCD

display, in pixels, is given in horizontal by
vertical order.

* Response time

-

The speed at which the monitor's pixels can change colors is called response time. It
is measured in milliseconds (ms).

* Stuck pixels
-

A pixel that is stuck either 'on'
or 'off', meaning that it is always illuminated, unlit, or
stuck on one color regardless of the image the LCD monitor displays can also be called a dead pixel.

* VESA mount

-

With this, you can mount a monitor on a desk or wall. It meets recommendations
of
the Video Electronics Standards Association (VESA).

* Viewing angle

-

It's the degree of angle at which you can view the screen from the sides (horizontal
angle) and top/bottom (vertical angle) and continue to see clearly defined images and accurate c
olors.




Advantages:




Thin Profile

LCDs feature very thin profiles, making them the perfect choice for small work spaces.



Standard Screen Sizes


Unlike CRTs, there is no variance between the monitor size and the screen size. For example, a
17
-
inch m
onitor features a 17
-
inch viewable image.



Bright Screens

LCDs, thanks to the florescent backlight, produce bright, rich images



Low Energy Consumption

In general, LCD monitors consume about half of the energy of standard CRTs.



Flicker
-
Free Refresh Rates

Refresh rate is less of an issue with LCD monitors. LCDs are designed to have flicker
-
free refresh rates at
60Hz.



High Contrast Ratios

LCD monitors tend to feature excellent contrast ratios, which insure bright, clear images. Make sure the
monitor you pur
chase has a contrast ratio of at least 400:1 or more.

Disadvantages:




Single Resolution

LCDs are generally designed to work best in a single resolution. They tend not to have the flexibility that
CRTs have to display multiple resolutions well.



Narrow View
ing Angles

LCDs tend to have a very narrow viewing angle. As you vary from looking at a LCD straight
-
on the image
gradually degrades. Look for at least a 160° viewing angle.



Slow Pixel Response Time


LCDs tend to have a much slower pixel response time, wh
ich can mean image artifacts remaining onscreen
after movement. For example, when you move your mouse quickly across the screen you might see the
cursor create shadow images of itself briefly. These disappear instantaneously. In order to avoid this try to
find a monitor that has a response time of 25 milliseconds or faster.

DVI (Digital Visual Interface):


While most LCDs usually connect via an analog VGA interface, many newer ones feature DVI as an option. The
DVI interface provides a stronger, more relia
ble signal that can be transmitted over a farther distance.
Unfortunately, the DVI interface requires a video card that supports it, which can mean an upgrade in order to take
advantage of this feature.


The three types of DVI are:



DVI
-
D


Digital only



DVI
-
A

Analog only



DVI
-
I

Both Digital and Analog

1080p

is the shorthand name for a category of
HDTV

video modes. The number
1080

represents
1,080 lines of horizontal
resolution

(1,080 horizontal scan lines),
[1]

while the letter
p

stands for
progressive scan

(meaning the image is not
interlaced
). 1080p can be referred to as
full HD

or
full
high definition

although
1080i

(interlaced) is also "Full HD" (1920x1080 pixels). The term
usually assumes a
widescreen

aspect ratio

of 16:9, implying a vertical resolution of 1920 pixels.
This creates a frame
resolution

of 1920×1080.

HDMI

(
High
-
Definitio
n Multimedia Interface
) is a compact audio/video
interface for transmitting uncompressed digital data.
[1]

It represents a digital
alternative to cons
umer analog standards. Because HDMI is electrically
compatible with the signals used by
Digital Visual Interface

(DVI), no signal
conversion is necessary, n
or is there a loss of video quality when a DVI
-
to
-
HDMI adapter is used.