Nowadays, there are two types of sensors to detect the low luminous flux, PMT (Photomultiplier Tube) and Geiger-APD (Geiger Avalanche Photodiode). The domain of Geiger-APD has reached an advanced development in the last years. The basic idea of this structure consists in polarizing an APD in Geiger-mode by applying a voltage beyond its breakdown voltage. In this case of polarization, the APD is working in a special mode and is able to detect the single photon. The theory of detection of single photon using this detector has been invented in the beginning of 90's and developed for detection of low light intensity. By using this kind of photodiode in the Geiger-mode, we designed a new type of detector and several applications have been explored. In astrophysics, we can use this detector for the detection of cosmic rays (through the detection of Cerenkov light generated by atmospheric showers). Another application is also possible and reveals an important method for detection of cancer cells.