Abstract Silicon pin photodiodes from different manufacturers were tested with synchrotron radiation and found to be excellently suited as detectors for the X-ray range between 5 and 25 keV. They have a high dynamical range (more than 6 decades) and a high degree of linearity. They can be used in the current mode down to 100 photons/s. This corresponds to a photocurrent of 50 fA. The performance of the signal processing (current-voltage amplifier, voltage-frequency converter, frequency counter) is of great importance. The Keithley current-voltage amplifier 18000-20, whose range can be switched electronically, is superior in performance to the Keithley amplifier 427, when the intensity range exceeds 4 decades.