Theoretical considerations indicate that by applying the Magnephot principle to photomultipliers, a simple and rugged industrial microphotometer may be built in the range of extremely low light intensities. The requirements for a low input impedance and a conservative input signal level can be successfully combined with a satisfactory signal-to-noise ratio without the use of mechanical light chopping. The external-shield method of dark current suppression may be usefully employed to extend the sensitivity limit to lowest light values, limited only by the discontinuous nature of the light flux.Experimental investigation confirmed these assumptions and led to the construction of a sturdy and simple ultra-photometer of inherent electrical stability. The limit of useful sensitivity is around 10-12 Lumen, corresponding to the light collected by a 24" telescope from a star of the 14th order. Experiments with scintillating crystals yielded data about the use of the new instrument as a quantitative sensing device for nuclear radiation. The experimental instrument gave satisfactory performance as to zero stability, independence from leakage currents and the exclusion of dark current without cooling. The use of a.c. amplification assured convenient range-switching and the possibility to amplify the signal further for registration or control purposes. The new ultra-photometer will now be investigated in conjunction with spectrophotometry, astronomic and other low-level photometry as well as the indirect measurement of X-ray and nuclear radiation.