The "photometric" technique consists of measuring the slight brightness variations of the stars.
The presence of the terrestrial atmosphere is a problem for this technique, in particular the scintillation due to the atmospheric turbulence.
These perturbations limit the accuracy of the photometric measure on the ground to a few hundredths of a percent, and prevent the detection of solar-type oscillations or terrestrial planets.
It is for this reason that we must go into space to avoid these perturbations of the terrestrial atmosphere.
The atmospheric turbulence perturbs the beams of light arising from astronomical sources, and is responsible for the scintillation phenomenon which limits the accuracy of the photometric observations from the ground.
Counting the photons
A radiation source is a beam of numerous particles called "photons".
The intensity of the source is directly related to the number of photons it emits. Therefore, to measure the intensity of this source, we can count the photons that we receive.