An LED (light emitting diode) is a semiconductor device that emits light when activated by electricity. It consists of a p-n junction diode which emits light through electroluminescence when activated. The color of the light depends on the energy band gap of the semiconductor. LED displays use an array of LEDs as pixels and are used in applications such as televisions and public transport signage due to their long lifetime, ruggedness, and energy efficiency. While more expensive initially than other lighting technologies, LEDs have advantages including instant lighting, durability, and environmental friendliness.

2. WHAT IS AN LED?

3. LED
 Light Emitting Diode.
 An LED is a two-lead(a lead is an electrical
connection consisting of a length of wire that comes
from a device.) semiconductor light source.
 It is a p-n junction(it is a interface between two types
of semiconductor namely p-type and n-type inside a
single crystal of semiconductor) diode, which emits
light when activated.
 P-n Juntion is created by Doping.
 when a suitable voltage is applied to the leads, the
electrons are able to recombine with electron holes
within the device, releasing energy in the forms of
photons.

4.  This effect is called electroluminenescenes.
 The color of the light (corresponding to the
energy of the photon) is determined by the
energy band gap(energy gap) of the
semiconductor.

5. LEAD

6. LED DISPLAY
 An LED display is a flat panel
display,which uses an array of light-
emitting diodes as pixels for a video
display.
 the LED's brightness in also used for
outdoors in destination sign on public
transport vehicles.

8. HISTORY OF LED
DISPLAY
 The first LED flat panel television screen
was developed by James P . Mitchell in
1977.

9. BENIFITS OF AN LED
DISPLAY
 Energy efficient - LED’s are now capable of
outputting 135 lumens/watt
 Long Lifetime - 50,000 hours or more if properly
engineered
 Rugged - LED’s are also called “Solid State Lighting
(SSL) as they are made of solid material with no
filament or tube or bulb to break
 No warm-up period - LED’s light instantly – in
nanoseconds
 Not affected by cold temperatures - LED’s “like”
low temperatures and will startup even in subzero
weather
 Directional - With LED’s you can direct the light
where you want it, thus no light is wasted

10.  Excellent Color Rendering - LED’s do not
wash out colors like other light sources such
as fluorescents, making them perfect for
displays and retail applications
 Environmentally friendly - LED’s contain
no mercury or other hazardous substances
 Controllable - LED’s can be controlled for
brightness and color

11. DRAWBACKS OF AN LED
DISPLAY
 LEDs are more expensive than conventional
lighting technologies , They must be supplied
with the correct voltage and current at a
constant flow , And this requires some
electronics expertise to design the electronic
drivers .
 LEDs can shift color due to age and
temperature , And two different white LED will
have two different color characteristics, So ,
They affect how the light is perceived .

12. WHAT IS LCD?

13.  A liquid-crystal display (LCD) is a flat-panel display or other
electronic visual display that uses the light-modulating
properties of liquid crystals. Liquid crystals do not emit light
directly.
 LCD televisions are thinner and lighter than Cathode Ray
Tube (CRTs) of similar display size, and are available in
much larger sizes.
 The light was provided by a series of cold cathode
fluorescent lamps (CCFLs) at the back of the screen.
 use white or colored LEDs as backlighting instead.
 Millions of individual LCD shutters, arranged in a grid,
open and close to allow a metered amount of the white
light through. Each shutter is paired with a colored filter to
remove all but the red, green or blue (RGB) portion of the
light from the original white source.

14.  Liquid crystals encompass a wide range of
(typically) rod-shaped polymers that naturally
form into thin, ordered layers, as opposed to
the more random alignment of a
normal liquid.

15. LCD LAYERS
 Polarizing filter film with a vertical axis to
polarize light as it enters.
 Glass substrate with ITO electrodes. The
shapes of these electrodes will determine
the shapes that will appear when the LCD
is turned ON. Vertical ridges etched on the
surface are smooth.
 Twisted nematic liquid crystal.
 Glass substrate with common electrode
film (ITO) with horizontal ridges to line up
with the horizontal filter.

16.  Polarizing filter film with a horizontal axis to
block/pass light.
 Reflective surface to send light back to
viewer. (In a backlit LCD, this layer is
replaced with a light source.)

17. ADVANTAGES OF LCD
 Brightness: Produces very bright images
due to high peak intensity. Very suitable for
environments that are brightly lit .
 Emissions: Produce considerably lower
electric, magnetic and electromagnetic
fields than CRTs.
 Geometric Distortion: No geometric
distortion at the native resolution. Minor
distortion can occur for other resolutions.
 Screen Shape: Completely flat screen.

18.  Power Consumption: Energy efficient.
Consume less than 1/3 the power of a
comparable CRT. Consume less electricity
than a CRT and produce little heat.
 Physical Aspects: Take up about 40% less
desk space. LCDs are thin and compact.
 Sharpness: At the native resolution, the
image is perfectly sharp. Adjustments are
required at all other resolutions which can
result in measurable degradation to the
image.

19. DISADVANTAGES OF LCD
 Aspect Ratio: The aspect ratio and resolution are
fixed.
 Black-Level: Not proficient at producing black and
very dark grays. In a "standard" configuration, not
appropriate for use in dimly lit and dark conditions.
 Contrast: Lower contrast than CRTs due to a poor
black-level.
 Color and Gray-Scale: AccuracyColor saturation is
reduced at low intensity levels due to a poor black-
level. Images are satisfactory, but not accurate due
to problems with black-level, gray-scale and
Gamma.

20.  CostConsiderably: more expensive purchase price
than comparable CRTs . (Cheaper lifetime cost:
lasts about 13,000 - 15,000 more hours than a
typical CRT.)
 Gray-Scale: Have an irregular intensity scale and
typically produce fewer than 256 discrete intensity
levels. For some LCDs portions of the gray-scale
may be dithered.
 Motion Artifacts: Slow response times and scan rate
conversion result in severe motion artifacts and
image degradation for moving or rapidly changing
images.
 Resolution: Works best at the native resolution. The
native resolution can not be changed. All other
resolutions require adjusting procedures which can
cause considerable deterioration of the image.

21.  Viewing Angle: Restricted viewing angles.
Viewing angles affect the brightness,
contrast and colors shown. Wide angles can
lead to contrast and color reversal.
 White Saturation: Saturation and
compression can occur due to the bright-end
of the intensity scale becoming overloaded.
Contrast control must be carefully adjusted.

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