Patent ID: 7613083

Claim:
An optical scanning device for scanning a record carrier ( 22 ), said record carrier having an outer face ( 24 ), wherein said optical scanning device comprises: a) a radiation source system ( 2 ) arranged to generate radiation; b) an objective system ( 20 ) having an exit face ( 76 ), said objective system being arranged between said radiation source system and said record carrier and providing for evanescent coupling of the radiation across a gap between the exit face of the objective system and the outer face of the record carrier; and c) a radiation detector arrangement for detecting radiation after interaction with the record carrier, characterised in that the radiation detector arrangement is arranged to produce a tilt error signal (α, β) representing a tilt misalignment beteen the exit face of the objective system and the outer face of the record carrier, wherein the radiation detector arrangement is arranged to produce said tilt error signal by detecting information in the radiation which is indicative of a variation in an efficiency of said evanescent coupling across said exit face.