Short-term variability of the irradiance at 535 nm as observed by PREMOS (in black) and by SORCE (in red) from July 2011 to July 2012 (panel a)). The difference (in ppm) between PREMOS and SORCE is also plotted (panel b)) for the corresponding period. Thin vertical lines indicate the passage of a sunspot. Panel c) shows the probability density function (PDF) of the difference between both instruments (solid line) along with the respective Gaussian fit (dashed line).