A stabilized transfer-standard system for spectral irradiance

High-power quartz-halogen lamps have been in use as transfer standards for a long time. However, even the best lamps available are not stable enough to match the increasing requirements stemming from the progress of measuring techniques. In particular, long-term stability in the ultraviolet-B region has to be improved. In order to achieve higher stability, it is necessary to change the operating conditions from constant-current to a detector-controlled system. The new OMTec transfer-standard system, consisting of a control unit including power supply and a lamp detector unit, is described. Its main features are: long-term stability with a deviation of the spectral radiant flux from the calibration values of less than 3 × 10 -5 h -1 during nearly 400 hours of operating time throughout the spectral range from the ultraviolet-B to the near-infrared; continuous check on stability by monitoring and viewing the measured detector photocurrents; reduction of the required seasoning time by a factor of three thus enhancing the lifetime of the lamp; storage of lamp and calibration data in the lamp detector unit for external use.