LCD panels have not changed fundamentally over the last 10 years. LCD is short for “Liquid Crystal Display” and describes the active element of the display that is made from liquid crystals. In an LCD display the light source is located behind the panel and emits light from a rack of LEDs (Light Emitting Diodes), whereas earlier, manufacturers would use more bulky fluorescent lamps (CCFL). The LEDs can either be placed either along the edges of the LCD panel (Edge or edge-lit LED) or behind the LCD panel (direct or backlit LED).

With edge-lit LED it is difficult to control the light intensity locally across the screen, as the LEDs emit light from the edges only. This light has to be reflected to cover the entire screen area with the help of a light guide. Therefore light intensity can only be controlled in bands. With backlit LED the diodes are placed behind the panel, which allows for better local control as the LEDs can be arranged in a grid and potentially be addressed individually or in zones. The highest number of diodes in an LCD display to date is 5200, which is only possible on a large 100” panel. Each diode will therefore illuminate approximately 6400 pixels (Ultra HD resolution). 

Light is typically reflected through several plastic layers that serve the purpose of making light distribution homogeneous. These light diffuser layers will sit between the LEDs and the LCD panel, meaning that the complete backlight unit takes up a considerable amount of space, making the TV thicker.

To control light intensity for each of the basic colors, LCDs use liquid crystals inside each pixel. These crystals can be rotated in the cell by manipulating an electrical field, making it possible to adjust the light throughput. Once the light intensity of a basic color has been adjusted to its desired level, it passes through a color filter that filters out all wavelengths except red, green or blue. If you move close to the screen you can see these red, green and blue sub pixels that in pairs of three make up one pixel.

How fast an LCD panel can change color depends on how fast the liquid crystals can move into a new position. This determines the amount of blur/trailing you see in the picture. Ideally, you want the liquid crystals to assume the new position (color) instantaneously. However, that is not possible and it usually takes the liquid crystals a little time (measured in milliseconds) to do so, meaning that blurring/trailing can occur.

When light passes through the liquid crystals light intensity can no longer be sustained in all directions. If you think about an incandescent bulb it can emit equal amounts of light in all directions (it is called a Lambertian emitter) but in an LCD display the use of liquid crystals means that the panel will emit most of the light directly forward. Or in other words; light and color intensity will look strongest, and purest, only right in front of the screen. This phenomenon is important in order to understand the concept of “viewing angles”.