These pretty circles of light are plastic diffused backlights, probably originally used in LCD gauge displays. Normally they're tucked behind the LCD and shine white light from below. Here, we've got the LEDs available individually, and they're really cool! Each LED is encased in a 2.2mm thick semi-circle of acrylic. The acrylic is a special type that is very good at diffusing light, so even though there are only two white LEDs, it provides near-uniform lighting. The other flat side of the diffuse has a reflector on it so you only get light from the top (otherwise the backlight would lose 1/2 the brightness unnecessarily)