Patent ID: 7221627

Claim:
An optical device for reading information on a track ( 1 ) on a surface of an optically readable information carrier ( 2 ) to be accommodated in the device, the device comprising: rotation means ( 3 ) for accommodating and rotating the information carrier ( 2 ) with a rotational speed ω, which rotational speed ω is dependent on a rotation control signal (RS), a parameter p being proportional to the rotational speed ω; a controller ( 9 ) for controlling the rotational speed ω by controlling the rotation control signal (RS); a radiation source ( 4 ) for generating a radiation beam; a carriage ( 5 ) which can be moved in a first direction (D) transverse to the track ( 1 ) and along the surface; an objective ( 6 ) for focusing the radiation beam on the track ( 1 ), the objective ( 6 ) being connected to the carriage; an actuator ( 7 ) for moving the objective ( 6 ) relative to the carriage ( 5 ) in the first direction (D) in dependence on an actuator control signal (AS) which has an amplitude U; a radial controller ( 8 ) for keeping the radiation beam in position on the track ( 1 ) in the first direction (D) by generating the actuator control signal (AS), the radial controller ( 8 ) having a bandwidth f b , and detection means ( 20 ) for determining a mass-unbalance of the information carrier ( 2 ), characterized in that the detection means ( 20 ) are capable of carrying out the following steps: a) making the information carrier ( 2 ) rotate at a rotational speed ω 1 with the aid of the rotation means ( 3 ) and having the radiation beam follow the track ( 1 ) with the aid of the actuator ( 7 ) and the carriage ( 5 ) such that a maximum value U max1 of the amplitude U occurs and the parameter p has a value p 1 ; b) making the information carrier ( 2 ) rotate at a rotational speed ω 2 with the aid of the rotation means ( 3 ) and having the radiation beam follow the track with the aid of the actuator ( 7 ) and the carriage ( 5 ) such that a maximum value U max2 of the amplitude U occurs and the parameter p has a value p 2 ; and c) comparing a first relation between U max1 and p 1 with a second relation between U max2 and p 2 for determining the mass-unbalance.