Introduction to LCD:
Liquid Crystal Display (LCD) consists of rod-shaped tiny molecules sandwiched between a flat piece of glass and an opaque substrate. These rod-shaped molecules in between the plates align into two different physical positions based on the electric charge applied to them. When electric charge is applied they align to block the light entering through them, where as when no-charge is applied they become transparent.
Light passing through makes the desired images appear. This is the basic concept behind LCD displays.
LCDs are most commonly used because of their advantages over other display technologies. They are thin and flat and consume very small amount of power compared to LED displays and cathode ray tubes (CRTs).
LCD Technologies and types:
Some of the LCD technologies are,
Blue Mode STN - This is the basic LCD, which needs lot of improvement on contrast ratio and viewing angle.
FSTN (Film STN) - Comes with an additional linearization film to offer better contrast.
CSTN (colour STN) - Layers of color filters are added to create up to 65,000 colors.
DSTN (Double STN) - Improves contrast and eliminates any other colors appearing on the screen.
Types based on displayed data:
1. Segment LCD: Displays numbers, letters and fixed symbols and were used in old style industrial panel display and such standard where we need to display fixed number of characters.
2. Graphical LCD: Instead of segments it has pixels in rows and columns. By energizing set of pixels any character can be displayed.
3. Color LCD displays: Are of type passive matrix and Thin film transistor/ active matrix.
Two types of color displays:
Passive Matrix
· Row & Column approach
· Apply small bias to perpendicular lines of electrodes
· Bias strong enough to darken bit at line intersection
· Multiplexed addressing scheme
Advantages: Simple to implement
Disadvantages: Can cause distortion
Active Matrix
· Each cell has its own thin-film transistor (TFT)
· Addressed independently from behind LCD
· Direct addressing scheme
Advantages: Sharp display, better viewing angle, 40:1 contrast.
Disadvantages: Need better backlight, complex hardware.
Recent passive-matrix displays using new CSTN and DSTN technologies produce sharp colors rivaling active-matrix displays.
LCD Module Backlighting:
LCDs unlike LEDS do not produce light, they need some external light source to view. So most the latest LCD screens used in notebook computers use a light source. They use CFT or EL Panel or in recent times white LED as source of light fixed in the background of LCD.
LED lamp is a better choice over CFT and EL due to its ability to offer variety of colors, intensity, long life (>100K Hrs), wide temperature range, and low voltage operation.
Electroluminescent/EL Panel: The EL backlight consumes less power but require high voltage (120VAC @ 400Hz Typically). EL panel is also not long lasting and is sensitive to wider operating temperature range.
Cold Cathode Florescent Lamp: This backlight is most popular and is suitable for large sized LCD displays. CFT has same drawbacks of EL panel i.e. short life span and limited temperature range, and need high voltage to operate (>300VAC @ 30-80KHz) and is also prone to vibrations.
Different viewing modes:
Reflective: In the reflective mode LCDs use ambient light to illuminate the display making them more suitable for outdoor use.
Transmissive: Transmissive mode will depend on internal backlight and is only viewable in indoors.
Transflective: Transflective mode LCDs employ both Reflective and Transmissive types and switch based on the availability of ambient light.
Special characteristics of LCDs:
Liquid Crystals are very sensitive to constant electric fields. Only AC-voltages should be applied as DC voltages can cause an electrochemical reaction, which destroys the liquid crystals irreversibly.
Temperature dependent and in a very cold or hot environment LCD may not work correctly. This is a reversible effect. Some displays need temperature compensation circuits to automatically adjust the applied LC voltage.
Key specifications of LCD display:
Important factors to consider when evaluating an LCD monitor:
1. Resolution: The horizontal and vertical size measured by number of pixels (e.g., 1024x768).
2. Dot pitch: The distance between the centers of two adjacent pixels. Smaller dot pitch indicate sharper image.
3. Viewable size: The diagonal size of the LCD display panel
4. Response time: The minimum time LCD takes to change a pixel's color or brightness.
5. Refresh rate: The rate at which the data is loaded in the monitor measured with the unit of number of times per second. Higher the refresh rate lesser is the flickering.
6. Matrix type: Active TFT or Passive.
7. Viewing angle: The angle from which the user can view the image on the display clearly.
8. Color support: The types and number of colors supported by the LCD (known as color gamut).
9. Brightness: The amount of light emitted from the display (known as luminance).
10. Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark.
11. Aspect ratio: The ratio of the width to the height (for example, 4:3, 5:4, 16:9 or 16:10).
12. Gamma correction: It is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems
Advantages of LCDs:
· Consumes less power and generates less heat.
· Saves lot of space compared picture tubes due to LCD's flatness.
· Due to less weight and flatness LCDs are highly portable.
· No flicker and less screen glare in LCDs to reduce eyestrain.
Drawbacks:
The disadvantages of LCD displays are,
· LCDs cannot form multiple resolution images.
· The contrast ratio for LCD images is lesser than CRT and plasma displays.
· Due to their longer response time, LCDs show ghost images and mixing when images change rapidly.
· The narrow viewing angle of an LCD weakens the image quality in wider viewing angles.
How LCD Display Interface Circuit works?
An LCD -liquid crystal display is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome.
They can be optimized for static text, detailed still images, or dynamic, fast-changing, video content.
Old type of LCD are monochrome types which only display one certain color while the modern types are colored ones that can display rich text and images.
LCD's resolution of display depends on the amount of pixels into it, the highest amount designed looks and displays best.
Now, LCd wont work without a light source and a reflector to drive it pixels to form an image information.
This typical block diagram below will help us a brief explanation oh how the LCD can produce an text and images on mobile phones handsets.
The block diagram shows the LCD gets a data source from the application processor, so therefore LCD is being controlled by the application processor to produce a detail images,
LED is a light emitting diode that can produce light, this light source of an LED is the one that reflect at the back of an LCD, without this LED light reflection on the back of an LCD it will result a black or dark screen displays.
LCD also needs a power supply voltage to activate its liquid crystal arrays inside of it, so that is why a voltage supply is also very important for that matter.
How the whole circuitry of an LCD being connected and designed. A circuit start from an application processor that controls and sends data to LCD connector which where the LCD is being connected. Before the data reach to the LCd connector it is being filtered for Electromagnet Interference protection. The LED light circuit and a power supply voltage is also provided for it is also work an important part on LCD circuit.
Always keep in mind that LCD needs the following sources to make it work completely,
1. Data control signal from the application processor
2. LED light that reflects on it back so that display will reveal completely.
3. A power supply voltage to turn the LCD activated.
A failure of these three sources will result on display problem issues.
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