Contrast ratio measures the difference in brightness between the light and dark areas of a television screen. If you have backlighting on your TV, the technology may make it easier to create a strong contrast and an improved picture. Some forms of backlighting, however, make it impossible to darken the blackest parts of the image completely.
Static Contrast Ratio
If a white area in a television image has 500 times the brightness of a black area, the static contrast ratio — also called native contrast ratio — is 500 to one. Static contrast ratio makes a big difference in how sharp and visible television pictures are, but there is a limit to how much contrast our eyes can discern. A ratio of 100,000 to one doesn't look any different to the naked eye than 1,000 to one.
Dynamic Contrast
Dynamic contrast ratio achieves higher figures than native contrast, because it compares separate scenes. In a dark scene, the rear-projector or adjustable backlight turns down the light level, and then increases it in brightly lit scenes, so there is greater contrast. Adjusting the backlighting improves the look of individual scenes, but it isn't as effective as the static ratio: Dimming backlighting means that the bright parts of dark scenes become dimmer, too. If you own an LCD television, the black areas never become completely dark as long as the television is on.
LEDs
An LCD television that uses LEDs — light-emitting diodes -- for backlighting is able to black out the dark areas of the image completely. LED televisions also use local dimming, turning off the light for the dark areas of the screen but leaving the brighter parts unaffected. If the television uses LED "edge lighting" to illuminate the screen from the side, local dimming doesn't work as well. Some manufacturers, however, have developed edge lighting that provides some degree of dimming.
