An indirect, photon-counting X-ray detector capable of detecting the low-energy X-rays includes a scintillator screen that is directly coupled to a two-dimensional optical sensor. A signal filter receives an electrical output signal from the optical sensor and removes high intensity signal contributions therefrom that are indicative of direct interaction between said X-ray signal and said optical sensor. The scintillator screen has a sufficient thickness to ensure a high absorption of incident X-ray photons, and uses phosphor grains with a relatively small grain size. A cooling apparatus in thermal communication with the optical sensor may be used to control its temperature. The signal filter maintains a running average of changes in measured pixel output values for consecutive measurements, and replaces a measured value caused by a direct interaction event with a value equal to a previous measured value plus said running average.