The photoresponse and reflectivity of new (∼ one month) and aged (18 years) silicon surface-barrier photodiodes have been measured over the photon wavelength range from 365–700 nm (3.4−1.8 eV). The photodiodes studied have either no gold film over the sensitive surface, or a thin (∼ 1−10 nm) gold film. The photodiodes are operated in the reverse-bias mode at room temperature. The local steady-state photoresponse of both types has been measured over the sensitive area (2.5 cm 2) and the pulse response of the photodiodes to incident α-particles and α-particle induced light pulses from a Csl crystal has measured as a function of gold film thickness. The results of these studies are reported and show that over the wavelength range studied, silicon photodiodes with no gold film need no initial photon calibration and serve as reliable steady-state photon flux calibration detectors for periods at least as long as 18 years, with no photon calibration being required during this time. All that is required is a measurement of the reflectivity of the photodiode (which changes with time) at the time of its use for a photon flux measurement. In addition, these studies show that photodiodes with gold film thickness ⩾ 7.5 nm have a photosensitivity for both steady-state photon fluxes and photon pulses which changes by less than ∼ 5% over a time period of 12 years, and serve as excellent charged particles detectors exhibiting stable charged-particle response over this same time period.