Important factors to consider when evaluating an LCD monitor:
- Resolution: The horizontal and vertical size expressed in pixels (e.g., 1024×768). Unlike CRT monitors, LCD monitors have a native-supported resolution for best display effect.
- Dot pitch: The distance between the centers of two adjacent pixels. The smaller the dot pitch size, the less granularity is present, resulting in a sharper image. Dot pitch may be the same both vertically and horizontally, or different (less common).
- Viewable size: The size of an LCD panel measured on the diagonal (more specifically known as active display area).
- Response time: The minimum time necessary to change a pixel’s color or brightness. Response time is also divided into rise and fall time. For LCD Monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult.
- Refresh rate: The number of times per second in which the monitor draws the data it is being given. A refresh rate that is too low can cause flickering and will be more noticeable on larger monitors. Many high-end LCD televisions now have a 120 Hz refresh rate (current and former NTSC countries only). This allows for less distortion when movies filmed at 24 frames per second (fps) are viewed due to the elimination of telecine (3:2 pulldown). The rate of 120 was chosen as the least common multiple of 24 fps (cinema) and 30 fps (TV).
- Matrix type: Active or Passive.
- Viewing angle: (coll., more specifically known as viewing direction).
- Color support: How many types of colors are supported (coll., more specifically known as color gamut).
- Brightness: The amount of light emitted from the display (coll., more specifically known as luminance).
- Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark.
- Aspect ratio: The ratio of the width to the height (for example, 4:3, 16:9 or 16:10).
- Input ports (e.g., DVI, VGA, LVDS, or even S-Video and HDMI).
[edit] Brief history
- 1888: Friedrich Reinitzer (1858-1927) discovers the liquid crystalline nature of cholesterol extracted from carrots (that is, two melting points and generation of colors) and published his findings at a meeting of the Vienna Chemical Society on May 3, 1888 (F. Reinitzer: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)).[1]
- 1904: Otto Lehmann publishes his work “Liquid Crystals”.
- 1911: Charles Mauguin describes the structure and properties of liquid crystals.
- 1936: The Marconi Wireless Telegraph company patents the first practical application of the technology, “The Liquid Crystal Light Valve”.
- 1962: The first major English language publication on the subject “Molecular Structure and Properties of Liquid Crystals”, by Dr. George W. Gray.[2]
- 1962: Richard Williams of RCA found that liquid crystals had some interesting electro-optic characteristics and he realized an electro-optical effect by generating stripe-patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electro-hydrodynamic instability forming what is now called “Williams domains” inside the liquid crystal.[3]
- 1964: In the fall of 1964 George H. Heilmeier, then working in the RCA laboratories on the effect discovered by Williams realized the switching of colors by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical problems with this new electro-optical effect made Heilmeier to continue work on scattering effects in liquid crystals and finally the realization of the first operational liquid crystal display based on what he called the dynamic scattering mode (DSM). Application of a voltage to a DSM display switches the initially clear transparent liquid crystal layer into a milky turbid state. DSM displays could be operated in transmissive and in reflective mode but they required a considerable current to flow for their operation.[4][5][6]
Pioneering work on liquid crystals was undertaken in the late 1960s by the UK’s Royal Radar Establishment at Malvern. The team at RRE supported ongoing work by George Gray and his team at the University of Hull who ultimately discovered the cyanobiphenyl liquid crystals (which had correct stability and temperature properties for application in LCDs).
- 1970: On December 4, 1970, the twisted nematic field effect in liquid crystals was filed for patent by Hoffmann-LaRoche in Switzerland, (Swiss patent No. 532 261) with Wolfgang Helfrich and Martin Schadt (then working for the Central Research Laboratories) listed as inventors.[4] Hoffmann-La Roche then licensed the invention to the Swiss manufacturer Brown, Boveri & Cie who produced displays for wrist watches during the 1970’s and also to Japanese electronics industry which soon produced the first digital quartz wrist watches with TN-LCDs and numerous other products. James Fergason at the Westinghouse Research Laboratories in Pittsburgh while working with Sardari Arora and Alfred Saupe at Kent State University Liquid Crystal Institute filed an identical patent in the USA on April 22, 1971.[7] In 1971 the company of Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on the TN-effect, which soon superseded the poor-quality DSM types due to improvements of lower operating voltages and lower power consumption.
- 1972: The first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.[8]
- 2008: LCD TVs are the main stream with 50% market share of the 200 million TVs forecasted to ship globally in 2008 according to Display Bank.[citation needed]
A detailed description of the origins and the complex history of liquid crystal displays from the perspective of an insider during the early days has been published by Joseph A. Castellano in “Liquid Gold, The Story of Liquid Crystal Displays and the Creation of an Industry” [9].
The same history seen from a different perspective has been described and published by Hiroshi Kawamoto, available at the IEEE History Center.[10]
[edit] Color displays
A subpixel of a color LCD
Simulation of an LCD monitor up close
Comparison of the OLPC XO-1 display (left) with a typical color LCD. The images show 1×1 mm of each screen. A typical LCD addresses groups of 3 locations as pixels. The XO-1 display addresses each location as a separate pixel.
In color LCDs each individual pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters (pigment filters, dye filters and metal oxide filters). Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. CRT monitors employ a similar ’subpixel’ structures via phosphors, although the analog electron beam employed in CRTs do not hit exact ’subpixels’.
Color components may be arrayed in various pixel geometries, depending on the monitor’s usage. If software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through subpixel rendering. This technique is especially useful for text anti-aliasing.
To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used.
Tags: history, lcd, pixel, Resolution
