Photometry is a method based on light radiation. It is mainly used to determine the cell density in a medium. For example, if you want to know how many bacteria are in a water sample, you irradiate them with light in a photometer. A detector measures the extinction, which is a parameter for the reduction of the light rays. If there are many bacteria in the water, they absorb, reflect and refract the light.
Explanation for children: A photometer is a machine that can, for example, calculate how many bacteria are in dirty water. To do this, the water sample is irradiated with light in the machine. When the water is pure, the light simply shines through without being blocked. But if there are many bacteria inside, they absorb or reflect the light. The photometer then measures how much light goes through the sample and tells us whether there are many or few bacteria in it.