The T-matrix theory was used to compute the backscatter and depolarization of mixed-phase PSC, assuming that particles are solid (NAT or possibly ice) above a threshold radius R and liquid (STS) below, and a single shape is common to all solid particles. We used a dataset of coincident lidar and balloon-borne backscattersonde and OPC measurements. The agreement between modelled and measured backscatter is reasonable and allows us to constrain the parameters R and AR.