A device for scanning a first and second type of optical record carriers (2,
40) generators a first and a second radiation beam for scanning the first and
second type of record carriers, respectively. The information layers (4, 42)
of the first and second type of record carriers is scanned through transparent
layers (3, 41) of different thickness. The first radiation beam (17)
has a first wavelength and a first numerical aperture NA-;1. The second
radiation beam (46) has a different, second wavelength and an effective
second numerical aperture NA2 smaller NA1. The rays of the
second radiation beam having an NA smaller than NA2 form a central sub-beam
(48), the rays having a larger NA form an outer sub-beam (49). The
device includes a non-periodic phase structure that does not affect the first radiation
beam. The phase structure introduces an amount of spherical aberration in the central
sub-beam (48). The phase structure is transparent for the central and outer
sub-beam (48, 49). The introduced spherical aberration compensates the difference
in spherical aberration caused by the difference in thickness of the transparent
layer (3, 41) of the first and second type of record carriers (2, 40).
To reduce the amount of stray light falling on the detection system (25)
from rays in the outer sub-beam (49), the phase structure introduces an
amount of defocus in the second radiation beam (17). The defocus displaces
the focus of the central sub-beam with respect to the focus of the outer sub-beam,
causing the intensity distribution of the central and outer sub-beam split in two
separate distributions. If the position and size of the detection system are properly
chosen, the detection system will capture mainly rays from the central sub-beam
and not from the outer sub-beam. Hence, the displacement of the foci allows spatial
filtering in the plane of the detection system (25) of the desired and undesired
rays of the second radiation beam.
