It is a photoelectric effect experiment, but the measurement should be current. The principle is that shining light on the metal will cause the electrons in the metal to overflow, and adding a forward voltage between the two metal plates will cause the current to reach the other side. A metal plate on one side, allowing current to flow in the circuit and measuring the current through an ammeter.

This is the famous photoelectric effect experiment. Einstein won the Nobel Prize by successfully explaining the photoelectric effect and proposing light quanta.

In fact, in the photoelectric effect experiment, as long as the light frequency is high enough and the distance between the metal plates is small enough, the current can be measured without applying voltage or even adding reverse voltage.