The aim of the VIRGO investigation (Variability of solar IRradiance and Gravity Oscillations) on SOHO (SOlar and Heliospheric Observatory) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. VIRGO contains two different active-cavity radiometers for monitoring the sol- ar 'constant' (DIARAD and PM06-V), two three-channel sunphotometers (SPM) for the measurement of the spectral irradiance at 402, 500, and 862 nm with a bandwidth of 5 nm, and a low-resolution imager (Luminosity Oscillation Imager, LOI) with 12 'scientific' and 4 guiding pixels, for measuring the radiance dis- tribution over the solar disk: at 500 nm. The instrumentation has been described in detail by Frohlich et al. (1995). In addition, the observed in-flight performance and operational aspects of the irradiance observations are described by Frohlich et al. (1997), and those of the LOI by Appourchaux et al. (1997).
