Solar radiation is the energy source for all photosynthetic life-forms. Due to their individual pigment compositions only a specific part of the spectrum can be used by the single species for photosynthesis. Therefore multispectral radiation measurements are helpful for their investigation and monitoring. With the background of increasing ultraviolet radiation due to the decreasing ozone layer in the atmosphere, the measurement of the UV-A and UV-B part of the spectrum is of increasing interest to people. In the RAMSES project (Radiation Measurement Sensor with Enhanced Spectral resolution), founded by the German Ministry of Research and Technology, a new subminiature multispectral radiometer is under development. This new instrument allows spectral measurements in the range from 250 to 720 nm. A modular concept of the instrument, combined with very low power consumption, allows to use the instrument in profiling systems as well as in stand-alone applications, such as moorings, monitoring systems or weather stations. Sensor heads with different detection characteristics are available, and up to 16 of them can be combined to a single instrument for simultaneous measurement, e.g. for up-welling and down-welling light. Typical applications of this instrument are satellite calibration and validation, UV measurements, biology, ecosystem modelling, water quality and even climatology.