A shutter, not a light source
An LCD does not create light. It controls light that comes from a lamp behind the screen. Each tiny region of the display acts like an adjustable shutter, and the material doing the adjusting is a liquid crystal. Understanding that one idea explains most of how these screens behave.
What a liquid crystal is
Liquid crystals are a state of matter between an ordinary liquid and a solid crystal. The molecules are usually rod-shaped. Like a liquid, they can flow and be moved. Like a crystal, they tend to line up in a common direction. The useful part is that their orientation can be changed with a small electric field, and their orientation affects light passing through them.
Light has a direction of vibration
Ordinary light vibrates in many directions at once. A polarizing filter lets through only the light vibrating in one direction, much like a picket fence passes a rope wiggled along the slats but blocks one wiggled across them. Polarized sunglasses use the same principle.
Put two polarizing filters in a row and turn the second one a quarter turn from the first (so their directions are perpendicular), and almost no light gets through. Align them, and most light passes.
The twist
An LCD sandwiches a thin layer of liquid crystal between two polarizers whose directions are perpendicular. Between them, the molecules are held in a twisted arrangement by tiny grooves on the inner surfaces. As polarized light travels through the layer, it follows the twist and rotates its direction of vibration. By the time it reaches the second polarizer, its direction matches that filter, so the light passes through and the pixel looks bright.
Now apply a voltage across the layer. The molecules untwist and line up with the electric field, and the light no longer rotates. It arrives at the second polarizer still perpendicular to it and gets blocked, so the pixel looks dark. That is the classic twisted nematic design, and it is the one behind TN panels.
Partial voltages give partial untwisting, which means in-between brightness. By varying the voltage across many small steps, the panel produces many levels of gray for each subpixel. The number of distinguishable steps relates to bit depth.
Some designs work the opposite way, with light passing when no voltage is applied and blocked when it is. The principle is the same either way: voltage sets the orientation, and the orientation sets how much light gets through the second polarizer.
From one cell to a picture
A screen contains millions of these cells, each tied to a thin-film transistor that acts as a switch with a small storage capacitor. A controller sends signals row by row, setting each cell’s voltage and then holding it until the next refresh. A color filter over each cell, red, green or blue, gives each one a color, and combinations of three neighbors form a full-color pixel.
Different arrangements of the molecules
Designers have found other ways to orient crystals, mostly to improve how the screen looks from the side:
- In-plane switching rotates the molecules in a plane parallel to the glass instead of tilting them out of it, which tends to keep color steadier at an angle. IPS panels are built on this idea.
- Vertical alignment holds molecules upright when no voltage is applied, which blocks light well and gives deeper blacks, but is more sensitive to the angle of view in the basic form. VA panels follow this approach.
These differences are a large part of why viewing angles vary from one panel type to another.
Limits that follow from the physics
- Black is never perfect. Polarizers and crystals leak a small amount of light, so a dark pixel in front of a bright lamp looks dark gray rather than truly black. That limit is why contrast in LCDs depends so much on the light source behind them.
- Speed depends on the liquid. Molecules must physically rotate, and thicker or colder liquid moves more slowly. Response time is the measure of this, and it can be slower when cold.
- Angle matters. Light passing at an angle travels a different path through the layer, which can reduce contrast or shift colors.
- The crystals only work in a narrow range of conditions. Extreme cold or heat can affect behavior, which is one reason environmental limits are listed for LCD products.
Why the lamp matters
Since the crystals only subtract, the light behind the panel sets the upper limit of brightness and much of the color quality. That is why LCD backlights are a topic of their own.