All modern light meters make use of photodiodes. So, they convert the light that falls on them into electrical energy. The magnitude of the electrical energy that the photodiode produces will be directly proportional to the intensity of the light. Rather than using one large photodiode, light meters use an array of smaller photodiodes for better accuracy and reliability. That means the meter will not stop working if one photodiode inside the meter fails. By measuring the amount of electrical energy generated by the diode, the light meter will be able to tell you the intensity of light with reliable accuracy.