The j(O1D) filter radiometer (Metcon GmbH) provides a measurement of solar actinic radiation integrated over selected wavelength ranges by means of a filtered photomultiplier. The instrument consists of a 2-pi receiver optic which is essentially a frosted quartz dome combined with a horizontal shadow ring which limits the field of view to one hemisphere. The collected radiation is then guided towards the photomultiplier which is housed in a water-tight aluminium cylinder suitable for outdoor operation and equipped with a cell containing a drying agent to ensure proper operation of optical and electronic components. The power supply, housed indoors, is connected to the instrument by 10 m or 30 m cables (depending where the instrument is to be operated). The output of the filter radiometer is proportional to the corresponding photolysis frequencies and absolute calibration which was determined during an intercomparison exercise when the instrument was run alongside a reference spectroradiometer. The final output from the instrument are analogue voltages in a 0–10V range that can be logged continuously.