Technical Resources for LCD Touchscreen & STN LCD Displays

Twisted nematic display, (TN), is type of liquid crystal cell structure that aligns the LC molecules through a 90' rotation. This twisted nematics configuration can produce images in two modes: Positive or Negative. Positive Mode provides white background and white segments.

Twisted nematic LCD modules are typically the least expensive display option, But can only effectively display content up to 16 lines (or 2 rows of text) without losing performance.

Super twisted nematic is a higher information content liquid crystal cell structure that uses adjustable interference of two optical modes to achieve a large number of multiplexed lines, higher contrast and higher level of gray scale. This results in a birefringence effect. The STN LCD liquid crystal twist is generally between 240 and 270 degrees.

STN is capable of multiplex ratio up to 400:1, or 400 lines of text simultaneously, which makes STN suitable for higher content character displays and graphic applications. There are 3 common types of STN LCD displays.

Green Mode STN–Green mode is the most common STN type. It has a green background with a darker pixel.

Silver Mode STN–Silver mode uses a polarizer configuration that eliminates the green color of the background but at the expense of a lighter, more blue/purple pixel appearance.

Blue Mode STN–Blue mode is typically used in a negative (light pixel on a darker background) mode application with a white pixel on a blue background.

Black Masking

Through the use of back masking placed inside the liquid crystal cell, Phoenix Display International can create a unique custom application in either TN or STN technologies.

Color Masking

By utilizing different pigments and additional screen-printing processes we are able to create custom area color displays.

Color Filters

By utilizing tinted polarizers, we can create additional LCD background color options.

Film Compensation Super Twisted Nematic, FSTN, consists of an STN LCD with an additional retarder film added to the outside of the cell to compensate the color shift of blue on green to black on white, resulting in the compensation. Film compensation improves the viewing angle and provides the sometimes preferred black on white graphic images.

Color STN modules employ a FSTN cell structure combine with a internal color filter. Each square pixel of a color STN LCD is broken up into three rectangular sub pixels comprising of a red, blue, and green filtered sub pixel. The combinations of these three colors in different grey shades give the display a color pallet between 4K and 65K unique colors.

LCD

• Transmissive LCD
• Reflective LCD
• Transflective LCD
• Positive Image LCD
• Negative Image LCD

Active-matrix TFT LCDs utilize tiny switching transistors and capacitors, called thin film transistors (TFT), or thin film LCD. Each thin film transistor is arranged in a matrix directly on the glass substrate using amorphous silicon. In order to address a particular pixel on a thin film transistor display, the proper row and column is addressed. This charges a capacitor associated with an individual pixel. The capacitor is able to hold the charge until the next refresh cycle. Because the liquid crystal charge is held at the pixel location and does not dissipate like the passive matrix STN and TN structures, this liquid crystal cell is capable of achieving higher contrast ratios at much higher switching speeds. The result is full motion video at a much higher contrast, with a full color pallet of 262K or more unique colors.

Transmissive

The liquid crystal cell has no reflective properties. The image is created by utilizing a backlight behind the display. The liquid crystal cell acts as a light valve to either let the light though the display or block the light. This technology is used when there is little ambient light sources available.

Reflective

This liquid crystal device has a reflector located on the rear of the cell. Ambient light is passed though the cell and reflected back to the user. The application is best used in direct sunlight or a well-lit office environment. This technology has the lowest power consumption because there is no backlight.

Transflective

Utilizing a transflector, which is a reflector that will still allow some light to pass through, transflective displays can achieve both reflective and transmissive properties. This enables acceptable transmissive performance with a backlight in lowlight conditions, and allows the application to utilize a reflective lower power mode in adequate ambient light conditions, while maintaining good performance in direct sunlight. This is the most common monochrome LCD viewing mode.

Positive Image

Darker pixel on a lighter background, most common configuration for monochrome displays. In a non-powered state the display will retain the lighter background appearance.

Negative Image

Lighter pixel on a darker background, most common configuration for color displays. In a non-powered state the display will retain the darker background appearance.

Liquid crystal displays are typically non symmetrical with respect to their viewing direction. The viewing cones are focused either above or below the horizontal plane of the display. Bottom view or 6:00 will be used when the user is below the perpendicular plane of the display and top view or 12:00 is utilized when the user is predominately above the display.

LED (Light-Emitting Diode)

LED backlights consist of surface mount LED’s, which are positioned in a matrix along the bottom of a shallow plastic tray and encapsulated with a clear epoxy (LED array type). For an edge lit display, LED’s are placed along the side or sides of a thin plastic light guide which then transmits the light evenly through the LCD panel.

Lifetime 50,000-100,000hrs
Power 5V DC typical

EL (Electro Luminescent)

EL panels are a thin membrane device consisting of two coated electrode plates sandwiching a thin film of phosphorescent substance between two plates. When AC voltage is applied to the electrode plates, the electrons collide with the light emission core. The energy given off is light. EL panels come in a variety of color and require an inverter to create the AC voltage.

Lifetime 5,000-10,000hrs
Power 70V - 150V AC typical

CCFL (Cold Cathode Florescent Lamp)

CCFL lamps are small high voltage field emission lamps made of lead–glass with mercury providing a fluorescent light source. The CCFL lamps are used in conjunction with the plastic light guide to evenly disperse light evenly through the front surface of the LCD panel.

Lifetime 15,000-30,000hrs
Power 300V AC typical

4-Wire Resistive

Resistive touch screens have a flexible top layer and a rigid bottom layer separated by insulating dots, with the inside surface of each layer coated with a transparent conductive coating. Voltage applied to the layers produces a gradient across each layer. Pressing the flexible top sheet creates electrical contact between the resistive layers, essentially closing a switch in the circuit.

Touch measurement in 4 Wire resistive touch screen is a 2-step process. The distance along the x-axis at the point of touch is measured by creating a horizontal voltage gradient on the top sheet, with the bottom acting as the return layer. Then a vertical voltage gradient is created on the bottom layer, to measure the y-axis.

The technology and electronics are simple, making 4-wire the cheapest touch screen technology. Any damage to either layer causes the touch screen to stop functioning. This lack of durability means that 4-wire resistive technology may not be ideal for applications like public access kiosks, or on displays larger than 12”.

5-Wire Resistive

In this technology, the main electronics are on the glass bottom layer. A uniform voltage is applied to the top layer. A touch causes an electrical contact between the top and bottom layers. Depending on the location of contact, the voltages at the 4 corners of the glass are different.

The 5-wire resistive touch screen is typically more expensive than the 4-wire technology. But the electronics make it possible to use 5-wire for applications up to 22”. In addition, the voltage measurements take place on the rear panel only, making the touch panel system more reliable and less susceptible to damage.

Capacitive

The capacitive touch screen measures the amount of capacitance in each of the touch screen electrodes. By sensing when the capacitance increases, the touch screen can tell when your finger is touching, and through measuring which electrodes have the most capacitance, the touch screen can locate to an accuracy of better than 1/1000th of an inch.

Digital Matrix

For digital matrix touch screens, the conductive material is patterned into rows and columns in the form of a grid. Each etched layer has a voltage connection. When the layers are pressed together, current will flow through the corresponding row and column where the touch occurs to calculate the position of the touch. The resolution, which is dependent on the number of rows and columns, is far lower than the analog touch screen versions. But the controller electronics are far simpler.

Contrast Ratio

The contrast ratio is the ratio of the luminance between a pixel in the “light” state compared to the same pixel in the “dark” sate. Typical contrast ratios for the different technologies are:

• TN 9:1
• STN 10:1
• FSTN 15:1
• CSTN 25:1
• TFT 300:1

Brightness

Brightness or luminance of a display is the measure of light output from the LCD panel. It is typically measured in “Candelas per square meter”, or “nit”.

Response Time

The response time of a liquid crystal cell is defined as the amount of time required for the LC fluid to switch from 90% of a given a pixel state to 90% of the opposite state, for a given temperature. This is also be considered, the amount of time required to change the entire image on the LCD panel. Therefore, the inverse of the response time will give the frames per second capability. Typical response times for the given technologies are:

• TN 200ms
• STN/FSTN 350ms
• High Speed FSTN/CSTN 80ms
• TFT 35ms

1/8VGA 160 x 120 QCIF 176 x 144
QCIF+ 176 x 220
QCIF ++ 176 x 240
QVGA 320 x 240
WQVGA 480X240
1/2VGA 640 x 240
VGA 640 x 480
WVGA 800X480
SVGA 800 x 600
XGA 1024 x 768