Lexan and Polycarbonate are the same thing. Lexan is a brand name and Polycarbonate is the chemical Name.
Polycarbonate was developed separately by Bayer and General Electric (became GE Plastics and then sold to Sabic). They both filed for patents in 1955. Bayer sold it under the name "Makrolon" and GE under the name "Lexan". Because of the success of the products the quality of the material name "Lexan" became tightly identified with products like "Tissue" or "Band-Aid."