This paper discusses the general problems of designing and calibrating sources of very weak light of known flux and spectral distribution, at levels necessary for measuring the performance of the various types of detector in current use in astronomy. After a brief introduction to ultra-weak-light photometry, the author describes a laboratory low-level light source built at the RGO for use as a primary standard for calibrating CCD's and PM-tubes etc. Alternative secondary standards, for use at observatories, are then discussed; these must be small, simple, portable and reliable, and should be suitable for permanent installation in telescope instruments, e.g. spectrographs and photometers etc. Finally, a few words are added on possible future developments.