yafray caustic and pure white plane


i unsuccesfully try to have simultaneously caustics (from a cube) and a white plane. my problem is that the plane is not white but grey or rose.

my scene is simple , a transparent cube (ior set) a white plane , a photon lamp, a GI with emit pwr= 0.87.

if i put the plane to shadeless, the caustics disapear.
if i put a lamp in the same layer of the plane, the caustics also isn’t visible.
since i spend too many hours to fix the parameters, i am completly lost on the parameters i have to change.

anybody can sent me a blend i can
thank you