Patent ID: 7149040

Claim:
An optical scanning device for scanning an information layer of an optical record carrier, the information layer being covered by a transparent layer of thickness t d and refractive index n d , the device comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam on the information layer, the objective system being characterised in comprising a lens comprising a synthetic resin on a substrate, the total thickness t of the lens satisfying the condition: 0.8 < t - 1.1 ⁢ ϕ + 1.1 1.18 - 2.28 ⁡ [ FWD + t d n d ] < 1.2 where FWD+t d /n d <0.51, and FWD is the free working distance between the lens and carrier and φ is the entrance pupil diameter of the lens, where t, t d , φ and FWD are expressed in millimetres.