LCD screens come in passive and active technologies, which are explained here. For more about LCDs in general, see LCD. For details about the way liquid crystals pass and inhibit light, see LCD example.|
Segmented characters (top) are widely used for small readouts, while a graphics matrix of rows and columns (bottom) is used for TVs and computer screens. The 5x7 character (middle) provides more flexibility than segments but not as much as a full graphics display (bottom). (Image courtesy of Ralph Sabroff of Varitronix, www.varitronix.com)
Passive displays are widely used with segmented digits and characters for small readouts in devices such as calculators, fax machines and remote controls, most of which are monochrome or have only a few colors. The passive technology is also used for graphics displays, comprising a matrix of rows and columns, typically not larger than 240 rows (320x240). Used in myriad applications today as well as in the first laptops years ago, these graphics-based "passive matrix" screens can be monochrome or full color.
Compared to active matrix, passive matrix is less costly because transistors are used only to activate rows and columns, not each subpixel, resulting in fewer manufacturing steps (see Active Matrix Displays below). However, passive matrix screens have a narrower viewing angle than active matrix and suffer from "submarining," which is the disappearance of the cursor when moved quickly.
TN - Twisted Nematic (90º Twist)
The first LCD type, TN is used in low-cost readouts for consumer products, and it is also the foundation for active matrix color (see below).
STN - Supertwisted Nematic (240-270º Twist)
Widely used in the past, STN LCDs use birefringence to absorb and pass selective light wavelengths.
FSTN - "Film Compensated" STN
Widely used for passive color matrix screens, an optical film layer turns the STN color into a neutral density light source (a "light valve"). Red, green and blue filters are added for full color.
DSTN - "Double Layer" STN
Used in high-temperature environments, a second, but inactive, LCD layer functions like the film in FSTN, except that the layer's optical properties change at the same rate as the working layer. DSTN used to mean "dual scan" STN, which enabled higher laptop resolution by addressing two modules simultaneously; for example, two 240-line passive matrix subsystems created 480 lines.
ESTN and ISTN
Proprietary STN displays from Varitronix. See ESTN and ISTN.
Passive displays are monochrome TN, STN and FSTN, and passive color displays are TN and FSTN. Active matrix displays are color TN. (Image courtesy of Ralph Sabroff of Varitronix, www.varitronix.com.)
Passive displays can be created with any element design. This ceiling fan readout uses segmented digits for temperature and custom elements for the lights.
Active Matrix Displays (TFTs)
Used in all LCD TVs and desktop computer monitors and 99.9% of all laptops, active displays are essentially "active matrix" displays and almost always color. The reason for the 99.9% is that OLED is emerging (see OLED).
Whether active or passive, a pixel matrix is addressed by rows and columns, one row (line) at a time for each electronic frame and then repeated for the next frame. Unlike passive matrix LCDs, which have no internal transistors, active matrix displays have a transistor at each red, green and blue subpixel that keeps the subpixel at the desired intensity until that row is addressed in the next frame.
More Contrast, Sharper, Faster and Brighter
By driving the subpixels independently, active matrix screens are sharper and have more contrast than passive matrix, and their faster response times eliminate submarining. In addition, active matrix screens are very bright indoors because they use a backlight (see LCD and LCD example); witness the extraordinary computer screens and HDTV sets on the market. However, when active matrix cellphones and laptops are taken into bright sunlight, they can be overwhelmed with reflected ambient light and difficult to read.
All High-End Displays Are Active Matrix
In the early days of laptops, active matrix screens cost a lot more than passive matrix, and both types were offered. Today, active matrix is the only type of LCD on laptops. Also called a "thin film transistor LCD" (TFT LCD) because a thin layer of transistors is deposited on the back of the screen (see amorphous silicon), active matrix displays use TN liquid crystals with a 90º twist. See bad pixel, LCD, LCD example and OLED.