An inflatable light concentrating mirror (10) comprising a first transparent sheet (1) and a second reflective sheet (2), wherein the first sheet and second sheet are connected or sealed to each other, whereby a void is provided there-between, the void being adapted to receive a gas so as to inflate the light concentrating mirror, characterized in that the mirror further comprises tensioning means (14) adapted to produce a defined longitudinal tension (11) in at least the second sheet (2) such that wrinkles or creases therein are significantly reduced. The tensioning means comprises one or more tensioning devices (14) adapted to be attached to at least one mirror end or one free end of the second sheet, the tensioning device is configured to provide a pulling force (11) on the second sheet so as to provide the longitudinal tension.