/gps/particle opticalphoton
/gps/energy 3.0 eV
/gps/direction 0.0 -1.0 0.0
/gps/polarization 1.0 1.0 0.0

/gps/pos/type   Plane
/gps/pos/shape  Circle
/gps/pos/radius 20.0 cm

/gps/pos/rot1   1.0 0.0 0.0
/gps/pos/rot2	0.0 0.0 -1.0
/gps/pos/centre  0.0 90.0 150. cm