We present results of the development of transition edge sensors (TES) based on Mo/AuPd thermistors aiming for photon number resolution in the visible up to NIR wavelength’s are deposited on an oxidized solid Silicon substrate and make use of electron-phonon decoupling at mK-temperatures generating a weak link for thermal relaxation. The Mo/AuPd bilayers have a tuned critical temperature of approximately 180mKand are being read out by making use of a series SQUID array, both implemented in a setup in an adiabatic demagnetization refrigerator. Optical pulses, generated via gated laser sources at room temperature, are fed via an optical single mode fiber to the TES. Their absorption is measured as a TES pulse response with a SQUID array. By means of pulse amplitude histograms (PAH), single photon resolution in the visible to NIR has been achieved. We have proven TES pulse response originating from single photon absorption in the TES via fitting the PAH with a Poissonian distribution. For 405 nm laser wavelength, we have achieved an energy resolution of about 0.7 eV, determined via analysis of the raw measurement data using PAH.