"Reversed" photodiodes as quanta counter. Diode on the left is green, on the right is red.
"Norma" photodiodes connected to voltage source emits the light (red, green). "Reversed" photodiodes are connected to voltmeter: when they are in light they become voltage saource (it is photovoltaic phenomena).
The light have to be of course with appropiate wavelength - ligth of the Sun is sufficient. Magnitude of recorded voltage is a result of change light of the color (red, green) first into wave frequency n and then into quanta energy defined in electron charge units (eV) diminished by work function of the electrons at p-n junction of silicone (about 0.8 eV).