Black matrix

The black matrix is the thin black grid that frames every subpixel on an LCD color filter. Its lines are only a few micrometers wide, but without them light would leak around the edges of each colored window, colors would bleed into each other and blacks would look gray.

It's made from color photoresist loaded heavily with carbon black, the same sooty pigment used in printer toner. The black resist is coated over the bare glass, exposed to UV light through a mask in the shape of the grid, and developed so only the grid remains. It's the first pattern laid on the color filter, and the red, green and blue windows are all aligned to it. Because so much carbon black is packed in, getting UV light deep enough to harden the bottom of the layer is a real challenge, and the recipe is carefully balanced.

The grid also hides the transistors and wiring on the back glass and stops room light reflecting off them. Older panels used a thin film of chromium metal instead. Black resin replaced it because it's cheaper, avoids toxic chromium chemicals and reflects less light.