What is a Liquid Crystal Display?
Why are Liquid Crystal Displays important?
What type of technologies are used to display letters, numbers and graphics on a LCD?
What are the types of fluids most commonly used in LCDs?
What is Twisted Nematics?
What is Super Twisted Nematics?
What is Film Compensated Super Twisted Nematics?
When referring to LCD technology what is referred to a viewing angle?
What are the three primary Polarization Types in LCD technology?
Where is Reflective technology found?
Where is Transmissive technology found?
Where is Transflective technology found?
What are the differences between Reflective, Transflective, and Transmissive displays?
What is Backlighting?
What is EL (Electroluminiscent Lamp)?
What is LED (Light Emitting Diode)?
What is CCFL (Cold Cathode Fluorescent Lamp)?
How are the pixels turned on and off in order to create an image on the LCD?
What types of techniques are used in LCD technology to produce color displays?
Do computers use LCDs?
What is the definition of a Duty Rate?
What is the definition of Bias Ratio?
What types of interfaces are available on a single chip LCD Controller/Driver IC?
What are the differences between a Driver IC, a Controller IC and a Controller/Driver IC?
What is Contrast Ratio?
What is meant by Chip-On-Board?
What is meant by Chip-On-Glass?
What is meant by Chip-On-Flex?
What is the space containing liquid crystal fluid between two pieces of glass called?
What is used to protect the edges of the glass and to act as a pressure device, compressing the elastomer connector between the PCB and LCD glass?
What is meant by Direct/Static Drives?
What is the most common method of connection for LCD modules?
What happens when excess DC voltage is applied to an LCD?
What is meant by DC to AC Inverter?
What is a Zebra ( R) Connector?
What is meant by the "rainbow effect" in LCDs?
Q1. What is a Liquid Crystal Display?
A: A liquid crystal display consists of an array of tiny crystals which can be manipulated to present data, such as, letters, numbers and even graphics.
Q: Why are Liquid Crystal Displays important?
A: There are two factors why liquid crystals are important they are size and weight. A LCD consists primarily of two glass plates with liquid crystal material between them. There is no need for bulky picture tubes in order to display images.
- Segment system: The display units are arranged to form a figure "8" to display numbers and letters.
- Dot matrix system (Character Display): The display units are arranged in Rows and Columns to form characters.
- Dot matrix system (Graphic Display): The display units are arranged in rows and columns to depict graphics.
- TN (Twisted Nematics)
- STN (Super Twisted Nematics)
- FSTN (Film Compensated Super Twist Nematics)
Q: What is Twisted Nematics?
A: Twisted Nematics (TN) was one of the earlier fluid technology that was implemented in LCDs TN type displays are primarily used in digital calculators, digital wristwatches, digital clocks and in some Caller ID boxes.
Twisted Nematic uses a 90-degree twist to the molecules between one alignment layer and the other. A good example of this is fluid being used is in the meter that displays your cost and gallons pumped at the gas station. Gas stations screens are clear and legible from almost any position.
Q: What is Super Twisted Nematics?
A: Super twist nematics (STN) has the most options and features. It offers better viewing angles than the TN technology. The most common use for this type of technology can found on vending machines, portable bar code scanners, hospital equipment, PC storage devices, etc.
STN is ideal for dot matrix formatted displays, both character and graphics based. Super Twisted Nematic displays improve on the more common TN screens by increasing the rotation of the molecules in their off state to over 260 degrees. This brings a higher contrast ratio to larger screens.
Q: What is Film Compensated Super Twisted Nematics?
A: Film Compensated Super Twisted Neumatic (FSTN) is the fluid technology that provides the best contrast ratio, the best viewing angle, and the most expensive. More companies are now using this type of fluid for their products.
The major reason why this fluid has a better contrast ratio than the STN, is its ability to offer a much lighter color background and darker pixels than the STN fluid.
Q: When referring to LCD technology what is referred to a viewing angle?
A: A viewing angle is the direction in which the display will look its best. Viewing directions are specified in terms of a clock position, see below:
- East 3 O'clock
- South 6 O'clock
- West 9 O'clock
- North 12 O'clock
Q: Where is Reflective technology found?
A: Reflective technology is most commonly found on calculators and some inexpensive digital wristwatches. Since Liquid Crystal Displays are non-emissive type of displays this means that they do not generate their own light source, they rely on an external light source. The light source might be using the sun, a current ambient light in order to see the characters on the display.
So, when a display is "Reflective" it means that is needs to have a light source in front of the display in order for you to read the display.
Q: Where is Transmissive technology found?
A: Transmissive technology is the opposite of the Reflective technology. Transmissive technology can be found in Medical devices, test and measurement instruments, automotive audio, laptop computers etc. This technology requires a backlight in order to see the characters. In addition, most transmissive displays are negative mode, meaning that the text will be a light color and the background of the display is dark colored.
With this technology, the light source must be behind the LCD panel, so that the light shines through the display and the pixels that are activated, this will allow the light to pass.
Q: Where is Transflective technology found?
A: Transflective technology is probably the most commonly used polarization types in LCDs. Most cellular phones, PDA, GPS, etc. use this type of polarization.
Transflective is a type of backing, bonded to the rear polarizer, which enables light to pass through the back as well as reflecting light from the front.
Q: What are the differences between Reflective, Transflective, and Transmissive displays?
A: Reflective displays include a diffuser, this layer reflects the light that enters the front of the display. Reflective displays require ambient light for the light source since there is no backlight.
Transflective displays have a type of backlighting which is bonded to the rear polarizer. This enables light to pass through the back, as well as reflecting light from the front.
Transmissive displays do not have reflectors or transflectors laminated to the rear polarizer. A backlight must be used with this type of LCD configuration.
There are three common backlight technologies for the LCD:
- EL (Electroluminiscent Lamp)
- LED (Light Emitting Diode)
- CCFL (Cold Cathode Fluorescent Lamp)
Q: What is EL (Electroluminiscent Lamp)?
A: The EL (Electroluminiscent Lamp) backlight is very thin and offers a uniform light source. EL is popular due to its relative low cost, as compared to the other backlight sources
Although the EL has some great benefits, there are some drawbacks to its technology. The EL does not perform very well under high humidity conditions. When the display is subject to high humidity, the EL begins to delaminate and starts to malfunction. Another problems with the EL include the need to have a DC to AC converter also known as an inverter.
Q: What is LED (Light Emitting Diode)?
A: LED (Light Emitting Diode) is the most commonly used backlight for Cellular phones. The LED backlight offers some benefits over the EL backlight. To start, the LED backlight does not require an inverter, just a DC source of +3VDC~+5VDC. Secondly, depending on the backlight configuration, the brightness can very bright, and thirdly, the life of the LED exceeds 50K hours.
Q: What is CCFL (Cold Cathode Fluorescent Lamp)?
A: This type of light source is most common in graphics and color displays. It provides a uniform and bright white light. A common application that would use a CCFL backlight is a Laptop computer. Some other applications that use this type of backlight include, gas pumps, medical instruments, industrial PCs, etc.
The CCFL has a drawback that is similar to the EL backlight it also needs a DC-AC Inverter. Though not the same type as the EL, the CCFL inverter needs to generate more voltage than those used for the EL. A typical EL inverter outputs 120VAC @400Hz, whereas the CCFL Inverter needs to output 1000VAC @ 30kHz~40kHz.
Q: How are the pixels turned on and off in order to create an image on the LCD?
A: Addressing is the process by which pixels are turned on and off in order to create an image. There are two main types of display addressing:
- Direct Addressing
- Multiplex Addressing
Q: What types of techniques are used in LCD technology to produce color displays?
A: Color LCD use two basic techniques for producing color. The technologies are Passive Matrix and Thin Film Transistors (TFT) often referred as Active Matrix. Passive Matrix is the less expensive of the two technologies. TFT or Active Matrix produces color images that are as sharp as traditional CRT displays, but the technology is expensive.
Q: What is the definition of Bias Ratio?
A: Bias Ratio of an LCD is also known as the Voltage margin and is defined as the ratio of V on (voltage on pixels that are currently addressed to the ON-state) divided by V off (voltage on pixels that are not currently addressed).
Q: What are the differences between a Driver IC, a Controller IC and a Controller/Driver IC?
A: There are two types of Driver IC's, Common Drivers and Segment Drivers. Common Drivers output signals to create the rows or numbers of lines. The Segment Drivers output the necessary signals to create the characters or columns.
The Controller IC receives data written in ASCII or JIS code from the MPU and stores this data in RAM. This data is then converted into serial character patterns and transferred to the LCD driver IC.
Drive/Controller IC is probably the most commonly found in a graphics module. This IC receives data from the MPU and stores it in RAM. Also, it accepts commands directly from the MPU for both the common and segment drivers.
Q: What is the most common method of connection for LCD modules?
A: The most common method of connection for LCD is the Elastomer Connector, which is a silicone rubber strip made up of sequentially spaced conductive and non-conductive material.
Q: What happens when excess DC voltage is applied to an LCD?
A: A dead short is created. Conductive particles from one piece of glass are transferred through the liquid crystal fluid and deposited on the conductive surface of the opposite piece of glass.
Q: What is meant by the "rainbow effect" in LCDs?
A: The term "rainbow effect" refers to a red and green circle or rainbow on the LCD glass. The LCD panel under uneven pressure causes this problem from the bezel. This problem is very common in LCD modules and normally it will not affect the performance or the appearance of the display when operational.