It uses a small spectrometer to bounce infrared light off particles in the sample.

ECONOMIST: Anti-IED technologies

Because the rims of these craters catch rays of light from the sun, this may provide sufficient illumination for the spectrometer to detect frozen water.

ECONOMIST: Astronomy

For more ambiguous types of waste, such as a piece of plywood with nails driven through it, the robot uses a spectrometer to recognise objects based on the unique patterns of light they reflect.

FORBES: ZenRobotics Develops a Garbage Sorting Robot