Theory of LCD Operation

Liquid crystal displays are composed of thin, layered sheets. Each layer either modifies the light that passes through it or prevents the passage of light.

Layer 1: Reflective Layer

The reflective layer sits behind the backlight bulbs, reflecting their light toward the front of the screen. This shiny sheet is usually the rearmost layer in an LCD panel. Because the reflective layer maximizes the available light, LCD panels consume much less power than CRT displays.

Layer 2: CCFL Bulb Layer

The LCD's backlight source is the cold cathode fluorescent lamp (CCFL) bulb layer, sitting just in front of the reflective layer. The number of bulbs depends on the size of the screen. Small screens may have only one bulb, while larger screens, such as the Apple Cinema Display, have three or more.

LCDs typically use CCFL bulbs. These bulbs are about two millimeters in diameter and are constantly on while the LCD panel has power. The intensity of the light they provide changes only when you adjust the display's brightness (using Displays preferences or the brightness function keys). The voltage output from a separate module called the inverter powers the CCFL bulb or bulbs in the LCD panel. The inverter transforms the low DC voltages, used inside the computer, into the higher AC voltages that these bulbs need to illuminate.

Like other fluorescent bulbs, a CCFL bulb contains a gas that glows, or “fluoresces,” when energy passes through it. The degree of fluorescence varies according to the temperature of the gas. When the gas is colder, it fluoresces less. For this reason, it's important for an LCD panel to warm up, sometimes for as long as 30 minutes, before you determine whether it meets its brightness specifications. As CCFL bulbs age, they produce less light. This can affect the display's brightness and its color purity.

Layer 3: Light Pipe Layer

The light pipe layer sits in front of the CCFL bulbs and is the first tool in distributing the light evenly across the screen. This thin layer is a clear acrylic panel with a pattern of short lines, or “pipes,” etched on its surface.

Layer 4: Diffuser Layer

The diffuser layer sits in front of the light pipe layer, acting as a second filter to distribute light evenly across the LCD screen. This thin, “foggy” sheet of acrylic softens any harsh light that would otherwise reach your eye.

Layer 5: Rear Polarizer Layer

The rear polarizer, much like polarized lenses on sunglasses, blocks certain light waves. Thanks to the vertical orientation of the rear polarizer layer, only vertically oriented light waves from the CCFL bulbs pass through. Horizontally oriented light waves are blocked.

The rear polarizer layer can be oriented horizontally and the front polarizer layer (discussed in the section “Layer 9: Front Polarizer Layer”) vertically, as long as the two layers are oriented at 90-degree angles.

Layers 6 and 8: Transparent Electrode Layers

The transparent electrode layers are composed of etched electrodes, or contacts. One layer's contact points are oriented in rows, the other in columns, and they generate a charge at each point, where their rows and columns meet. The combined electrode layers map out a distinct intersection point for every subpixel on the LCD screen.

A circuit board holding the electronic drivers for the LCD is typically attached at the edge of the transparent electrode layers by Tape Automated Bonding (TAB). (TABs are easily rendered unusable if mishandled.) The drivers on the circuit board activate the electrodes at points of intersection to generate a charged field across the liquid crystal material. This is a “passive matrix” design, which can be found in Apple's older PowerBooks.

Layer 7: Liquid Crystal Layer

The liquid crystal layer is the most complex of all the layers because it is dynamic. It consists of a liquid crystal gel, several molecules thick, encased in glass. The liquid crystals form distinct molecular layers or planes that lie parallel to the glass enclosure.

Layer 9: Front Polarizer Layer

The front polarizer layer acts as the final traffic controller for light; only horizontally oriented light can pass through.

If the light that reaches the front polarizer layer has previously passed through charged, vertically oriented liquid crystals, it exits the liquid crystal layer in a vertical orientation; it is therefore blocked by the front polarizer layer. If the light has passed through uncharged (twisting) liquid crystals, it exits the liquid crystal layer in a horizontal orientation and passes through the front polarizer.

Black areas appear on the LCD screen where the transparent electrode layers delivered a charge to the liquid crystals, causing them to align vertically and allowing the vertically oriented light wave to pass through, only to be blocked by the horizontally oriented front polarizer layer. No light has succeeded at passing through at those points.

Illuminated areas appear on the LCD screen where the transparent electrode layers did not deliver a charge to the liquid crystals, which maintained their twisting orientation. Light passed through the liquid crystals, twisted into a horizontal orientation, and was able to pass through the front polarizer.

Layer 10: RGB Filter Layer

The RGB filter layer is the last internal layer, closest to your eye. It is typically made of a thin film with red, green, and blue stripes—the three primary colors that, when mixed in varying proportions, produce all the colors you see. These stripes are so fine that you can't see them with the naked eye, though you can observe them with a small magnifying glass or pocket microscope. Light that has made it through the front polarizer layer crosses the RGB layer and takes on the color of the RGB stripe from which it traveled.