The photometer is a device that converts light to voltage which is read out by the digital multimeter (DMM). This is due to the photoelectric effect. (Einstein won his Nobel Prize for his explanation of the photoelectric effect.) In the photoelectric effect, light hits a piece of metal. Quantum mechanics tells us that light can be thought of as tiny packets of energy called photons. The photons are absorbed by electrons and increase the energy of the electrons. Some of the electrons become so energetic that they escape from the metal. In other words electrons are emitted from the metal, producing a current that is amplified by the photometer. The current goes through a resistor in the photometer. Ohm's law (V=IR) tells us that current I going through a resistor R is associated with a voltage drop V. The DMM measures the voltage drop across the resistor. Saturation occurs when the metal emits the maximum flux of electrons that it can produce. (Or it may be the maximum amplification of the photometer.)
What does a photometer do?
B. It converts light into electric current.

A. It emits light.
B. It converts light into electric current.
C. It measures the frequency of light.
D. It emits sound.