There’s no shortage of confusion in the television market as new technologies emerge. A prime example of this is the presence of two related yet different technologies in LCD and LED-based LCD high-definition televisions. Most older and currently less expensive displays use cold cathode fluorescent lighting, or CCFL. Newer and higher-end displays use light-emitting diodes, or LEDs. Added to this confusion is the fact that LED technology has two sub-genres: locally-dimmed and edge-lit. Although the core liquid crystal-based technology remains the same, the difference in the way the on-screen images are illuminated often results in considerable performance and price differences.
Liquid crystal displays operate using a sandwich of technologies, all of which work together to form crisp images. A constrained layer of liquid crystal solution is applied between two layers of polarized glass. The liquid crystals inside the solution possess traits of both a solid and a liquid, and respond to magnetic fields. These fields are applied by the electrical current running through the solution, causing the crystals to twist and untwist to varying degrees. The amount of twisting is proportional to the amount of light emitted. This light passes through three primary color filters and on to the video processor, resulting in a beautiful HD image. Both traditional LCD and LED-based LCD panels perform these essential functions.
Traditional CCFL Backlighting
Cold cathode fluorescent lighting or CCFL bulbs comprise the majority of backlighting on computer and laptop monitors as well as televisions. This is a bright and relatively energy-efficient lighting type, bettered only by LED LCD displays. However, it does often create uneven black levels and relies upon the degree of twist in the LCD layer of the display to create a true black. This commonly results in black areas depicted as a dark gray, which limits color accuracy. CCFL-enabled LCDs are normally less expensive than LED-based LCDs as one would expect given the maturity of the format and inherent performance limitations.
Locally-Dimmed LED Panels
Black on a television display ideally represents the total lack of color or light. Locally-dimmed LCD LED displays create black by turning off the relevant diodes on the display, creating a total absence of light in the areas directed by the set’s video processor. The extreme difference between light and dark areas of an LED display create wide contrast ratios, a key metric used to differentiate modern displays. LED-based LCD sets often have contrast ratios in the millions-to-one range, meaning black images of space and black suits are truly black. Since color accuracy is an offshoot of contrast ratio, high-contrast displays are more likely to offer rich natural hues. The only drawback to this display type is the visibility of light “blooming” around bright objects, but this is typically offset through proper calibration.
The most common LED panel offers edge lighting. This type of display arranges its LEDs along the perimeter of the screen, using various optical tricks to evenly spread light throughout the screen. However, this technique often results in the edge of the display appearing brighter, becoming progressively dimmer as you look toward the center of the screen. As manufacturers move away from CCFL lighting on their smaller, budget-oriented displays, edge-lit LEDs without local dimming will increase in prevalence.