How Flat Panels Work

Flat-panel monitors are digital video displays that use backlit liquid crystal technology to produce images on a transparent plastic screen. Because they don't need an electron gun to shoot electrons through a big picture tube, a liquid crystal display (LCD) can be just a few inches or less from front to back. Flat-panel monitors occupy less space than CRTs, consume about one-third as much power, and produce brighter, sharper images.

A liquid crystal is a semiliquid substance with long parallel molecules (crystals) that respond to an electric charge by changing the way that light passes through them. When a layer of liquid crystal material is placed between a pair of polarizing filters at right angles, the presence or absence of a charge determines whether light is visible on that portion of the screen. Different materials have different light-passing characteristics, so monitors made with those materials can produce different images.

Figure 11.2 shows the layers of an LCD monitor. From back to front, an LCD monitor has these major elements:

• A source of light behind the screen. This is most often one or more fluorescent lights and a diffuser that spreads the light evenly through the entire surface of the screen.

• The first of two polarizing filters.

• An active matrix of thin-film transistors, each with a unique horizontal and vertical address that defines its exact location, mounted on a glass substrate that provides the charge to the liquid crystal material.

• The liquid crystal material.

• Another glass substrate.

• Three adjacent color filters, each of which can produce 256 shades, depending on the amount of light it receives. Combining the three colors (256 red × 256 green × 256 blue) can produce more than 16 million different colors.

• The second polarizing filter, at right angles to the first one.

• The surface of the monitor screen.

Figure 11.2: Light passing through a liquid crystal is controlled by a thin-film transistor matrix.

The graphics controller refreshes the image on an LCD monitor by scanning across the matrix of horizontal rows and vertical columns of thin-film transistors. Each address in the matrix corresponds to a pixel in the bitmap image supplied to the monitor by the graphics controller. The controller refreshes the entire image by moving through every row and column. Every pixel in the screen is made up of three adjacent sub-pixels, one for each color. The matrix receives instructions from the controller for one pixel at a time that tells it whether that pixel should be light or dark, and if it's light, which colors to use.

The source of light behind the LCD matrix is constant (unlike the pixels on a CRT that fade between scans), and the transistors in the screen remain open or closed until they receive an instruction to change, so an LCD screen does not produce the same kind of flickering image as a CRT at a low refresh rate. Therefore, most digital flat-panel monitors have only one refresh rate, usually 60 Hz.

Alternatives to LCDs

In addition to LCD, flat panel monitors can also use other technologies, including plasma, EL, OLED, LCoS, DLP, and LEDs.

A plasma or gas-discharge display uses neon gas between sets of horizontal and vertical (row and column) electrodes. When one electrode in each set receives an electric charge, the gas lights up at the intersection between the two electrodes.

EL (electroluminescent) displays use a phosphor layer placed between the horizontal and vertical electrodes. Like the neon gas in a plasma display, the phosphor lights up a pixel when the electrodes that define its address receive an electric charge.

OLED (organic light-emitting diode) displays use a thin film of luminescent organic material in place of the phosphor layer. Like plasma and EL displays, the luminescent material lights at the intersection of the row and column that define each pixel's address.

LCoS (liquid crystal on silicon) is a form of LCD display that uses a liquid crystal layer over an extremely reflective substrate layer. Because the switching circuitry and wiring is located underneath the reflective layer, there is no black space between pixels.

DLP (Digital light processing) uses a type of integrated circuit called a Digital Micromirror Device (DMD) chip. DMD chips contain a light source and millions of microscopic mirrors (one for each pixel) that switch toward or away from the light source to reflect light onto a screen in response to instructions from the graphics controller.

LED (light-emitting diode) displays use a set of individual electronic components that light up in a specific color when they receive a charge. In an LED display, each LED has address that identifies the row and column where that LED is located.

Response time is related to the refresh rate, because they both affect the amount of time needed for an image to change, but it's not the same thing. The response time is the amount of time needed for a liquid crystal to twist from one condition to another to pass light to a pixel, block light, and pass it again. This is defined in greater detail later in this chapter. The important difference between response time and refresh rate is that refresh rate originates in the graphics controller, and response time is a characteristic of the monitor.

Because it's not possible to physically change the number of pixels in an LCD display, each flat-panel monitor has a native resolution that produces the best possible image on that particular screen. Some monitors allow you to use other resolution settings by scaling the image and combining pixels, but the effect never looks as good as the same image at the monitor's preferred resolution.