In this entry, the illustration from the LCD definition is explained in more detail. It shows a seven-segment digit, which is commonly used to display 0 through 9, and the center segment is the focal point of the example. These segmented digits are used in all the passive technologies (see LCD types).|
Positive Image Twisted Nematic (TN) Display
(Image Courtesy of LXD, Inc.)
In this seven-segment digit, the unenergized center segment looks silver or light gray. In their natural state, liquid crystals twist by themselves and line up with the orientation "rubbed" into the front and rear glass panels. The rubbing is done by a cloth that is rolled over a transparent polyimide film that was applied to the glass, creating microscopic grooves.
In this "positive image" example, the rubbings and polarizers are oriented in the same direction, and the silver/light gray appearance (left) is the result of ambient light traveling from the viewer down the crystals and reflected back up the same spiral staircase.
When energized (right), the crystals reorient perpendicular to the rear polarizer, which is rotated 90 degrees from the front polarizer. The light is absorbed in the rear polarizer and causes the segment to appear dark.
Pulse Width and Amplitude Modulation
The seven-segment example above is simple compared to television. In an LCD HDTV, more than six million subpixel cells, each containing billions of liquid crystal molecules, are constantly and independently being pulsed on and off very rapidly at different rates of power to create the required intensity of red, green or blue for each pixel. The combination of pulse width modulation (PWM) and amplitude modulation (AM) causes the molecules to straighten from partially to fully and revert back to their spiral twist over and over hundreds of times per second. See PWM and amplitude modulation.
Rotate Polarizer for a Negative Image
To create a "negative image" readout, whereby unenergized segments look dark, the front and rear polarizers would point in the same direction. When unenergized (left), the light coming down the spiral staircase would be absorbed in the rear polarizer, which, unlike in the example above, would not line up with the crystals. When energized (right), the crystals would reorient perpendicular, but light would travel down and then back up, appearing silver/light gray, because front and rear polarizers would line up with each other.
In low-cost passive displays, ambient light is reflected back up the spiral. However, backlights are widely used to provide a source of light, and if used, the reflective layer is also transmissive. It allows ambient light from the viewer plus light from the backlight to move back up the crystal staircase (see transmissive LCD). See LCD and LCD types.