(a) Field of the Invention
The present invention relates to an optical read-out lens system to be used with an information reading device for optically reading out the signals recorded on optically recorded disks such as digitally recorded audio disks, video disks, etc.
(b) Description of the Prior Art
For an optical read-out lens system for optically recorded disks, high resolving power is required because it is used for reading out the very small signals which are recorded with high density on a disk. Besides, a reasonably large field angle is required in order to obtain a favourable spot of light even when there is something imperfect in an automatic focusing mechanism to be used for continuously focusing the light onto the optically recorded disk, which rotates at high speed, or when there is inclination of a read-out lens caused due to tolerance in mounting accuracy of the read-out lens. Moreover, for some tracking methods, a reasonably large field angle is required in order to obtain the tracking signal.
To make the automatic focusing mechanism and tracking mechanism compact and simple, the lens system itself should be light in weight.
Furthermore, if the optical read-out lens for optically recorded disks comes into contact with the optically recorded disk, the lens and disk are broken. To prevent the above, the optical read-out lens system for optically recorded disks should have a long working distance.
For playback of optically recorded disks, monochromatic light is used generally. Therefore, to increase the signal-to-noise ratio of signals amplified after being detected by a light detector, it is preferable to use lens materials which are as transparent as possible for the light of the wavelength to be used.
Due to the reasons described in the above, for an optical read-out lens system for optically recorded disks, the number of lenses constituting the lens system should be limited to the minimum required number and lens surfaces should be provided with antireflection coatings which ensure sufficient effect.
Moreover, in these days, players for optically recorded disks are being put to practical use and it is required to make them smaller in size and lower in price. Therefore, light sources are now being switched over from He-Ne laser tubes to semiconductor lasers. This requirement for making the divices smaller in size is applied also to optical systems. To make N.A. large and to make the diameter of entrance light pencil small, optical read-out lens systems for optically recorded disks now show such tendency that the focal length is made short. On the other hand, however, it is required to keep the working distance at a reasonably large value.