  Aims. We aim to study the 250 micron luminosity function (LF) down to much
fainter luminosities than achieved by previous efforts.
  Methods. We developed a modified stacking method to reconstruct the 250
micron LF using optically selected galaxies from the SDSS survey and Herschel
maps of the GAMA equatorial fields and Stripe 82. Our stacking method not only
recovers the mean 250 micron luminosities of galaxies that are too faint to be
individually detected, but also their underlying distribution functions.
  Results. We find very good agreement with previous measurements in the
overlapping luminosity range. More importantly, we are able to derive the LF
down to much fainter luminosities (around 25 times fainter) than achieved by
previous studies. We find strong positive luminosity evolution \propto (1 +
z)^4.89\pm1.07 and moderate negative density evolution \propto (1 +
z)^-1.02\pm0.54 over the redshift range z=[0.02, 0.5].
