This entry explains the illustration in the LCD definition in more detail.
The Seven-Segment Display
The Spiral Staircase
This example is a twisted nematic (TN) seven-segment digit, commonly used for numeric readouts (see LCD types
). For illustrations showing the changing LCD states, see LCD subpixels
. (Image courtesy of LXD, Inc.)
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 the "positive image" example above, the front and rear planes are oriented 90 degrees from each other. The silver/light gray appearance (top left) is the result of ambient light traveling from the viewer down the crystals and reflected back up the same spiral staircase.
When energized (top right), the crystals reorient perpendicular to the rear polarizer. The light is absorbed in the rear polarizer, which causes the segment to appear dark.
Pulse Width and Amplitude Modulation
In an LCD HDTV, there are six million individually energized subpixel segments (24 million in a 4K TV). Each subpixel contains billions of liquid crystal molecules that 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 (for details showing the changing states, see LCD subpixels
). 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 from 60 to 240 times per second. See PWM
and amplitude modulation
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