Abstract: For the positive real function image encoded by double random-phase,the first random-phase mask placed in the blank can not serve as the key when the decrypted image is detected by intensity detector in the decrypting process.In connection with the defects,an improved encryption system illuminated with spherical wave is proposed.The input image is not nestled closely to the first random-phase mask,so the first random-phase mask can be used as the key for positive function image and the position of the first random-phase mask provides an additional key.Computer simulation indicates the feasibility of the proposed technique.