Red pigment

The red in an LCD color filter usually comes from an organic pigment called diketopyrrolopyrrole, or DPP, especially the type known as Pigment Red 254. It's a powder of tiny crystals rather than a dye that dissolves, so it stays put and doesn't fade under years of bright backlight. In car paint it's famous as Ferrari red.

The molecule is flat and built mostly from carbon rings, with hydrogen atoms around the edges. At its center are two joined five-sided rings containing nitrogen and oxygen. A six-sided ring sticks out on each side, with a chlorine atom at the far end that deepens and brightens the red. Chemists make it by reacting a chlorinated benzene compound with an ester in a strong base. The raw pigment is then ground into particles well under 100 nanometers across, so it doesn't scatter light and make the filter cloudy.

In the color filter, the pigment is dispersed in color photoresist and patterned into the red windows. Makers often blend in a little yellow or orange pigment to fine-tune the shade, and some use other reds, such as anthraquinone pigments, depending on the manufacturer.