Color filter

The color filter is the front sheet of glass in an LCD panel. It carries a mosaic of tiny red, green and blue windows, one lined up over each subpixel, with a black grid between them. Each window passes only its own color from the white backlight.

It's built on a sheet of display glass by photolithography, patterning with light. First the black matrix is coated on, exposed to UV light through a mask and developed, leaving a fine black grid. Then a layer of color photoresist loaded with red pigment is coated over everything, exposed only where the red windows go, and the rest is washed away. The same is done with green pigment, then blue pigment. A clear overcoat smooths the surface, and in many panel types a film of indium tin oxide is added as a common electrode facing the liquid crystal. Tiny posts called spacers often go on last to hold the gap.

In the LCD panel, the filter turns a grayscale shutter into a color display. The cost is light: each window throws away about two-thirds of what hits it, one reason LCDs need bright backlights. Quantum-dot TVs widen the range of colors by making the backlight's own colors purer.