Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2000-08-16
2001-11-13
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S641000
Reexamination Certificate
active
06317276
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a lens system for imaging a punctiform radiation source in a diffraction-limited radiation spot in the image plane of the lens system, which system comprises a collimator lens and an objective lens. The invention also relates to an optical scanning device for reading and/or writing information in an information plane, and an apparatus provided with such a device.
Said information plane may be the plane of an optical record carrier in the form of a disc or a tape in which the optically readable information is present or can be stored. Such a record carrier is, for example, the known compact disc (CD) for audio or CD ROM for data or a record carrier derived therefrom, such as the DVD which has a considerably higher storage capacity. The information plane may also be the plane of another object which can be examined, for example, with a scanning microscope.
Notably for optical record carriers, there is a growing need of a scanning device with which increasingly smaller details can still be read in a well separated manner because more and more information is to be stored on the record carrier. The quantity of information which can be stored in an optical record carrier depends, inter alia, on the size of the minimal scanning spot which can be formed by the scanning device on the information plane. As the scanning spot becomes smaller, the information density may become larger. The scanning spot may be reduced by increasing the numerical aperture (NA) to, for example, NA=0.6 at the image side of the objective lens with which this scanning spot is formed. Such an objective lens must be well corrected for various aberrations in order that the scanning spot actually has the desired shape and size.
The objective lens for an optical scanning device must be small and light-weight and therefore preferably consists of a single lens element, one or both refractive surfaces of which are aspherical. An aspherical lens surface is understood to mean a surface whose fundamental shape is regular, for example, spherical or plane, but whose real shape slightly deviates therefrom so as to correct for spherical aberration. Due to the use of the scanning device, namely in consumer apparatuses for, for example, DVDs or DVD-RAMs, a lens element of synthetic material is preferably used for the objective lenses in these apparatuses. A synthetic material lens element is cheaper than a glass lens element and, moreover, has a smaller weight. The problem of a synthetic material lens element is that this element will exhibit spherical aberrations at a change of the temperature, despite the fact that this element is designed in such a way that it is free from spherical aberration at the nominal temperature.
In U.S. Pat. No. 5,474,537, relating to a scanning device for optical records, it is proposed to reduce the spherical aberration due to temperature variations by replacing the collimator lens of a lens system of which also the objective lens forms part by a lens passing a non-parallel, divergent, beam to the objective lens. The relevant lens is referred to as capture lens and is no longer a collimator lens. However, the solution as proposed in U.S. Pat. No. 5,474,537 is not usable in a scanning device or an apparatus in which the objective lens is placed on a movable slide in order to be able to move it rapidly with respect to the record carrier, whereas the collimator lens is stationary. Moreover, since the objective lens is no longer irradiated by a parallel beam, its free working distance is reduced, which is a drawback in practice. The free working distance of an objective lens is understood to mean the distance between the image plane, i.e., for example the information plane of a record carrier, and the surface of the lens located closest to this image plane.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a novel principle of preventing spherical aberration changes due to temperature variations and to provide a lens system of the type described in the opening paragraph which does not have these drawbacks. According to the invention, the lens system is characterized in that the collimator lens comprises a positive lens element of synthetic material and a negative lens element of glass, the absolute value of the power of the positive lens element being larger than the absolute value of the power of the negative lens element.
For the nominal, or design, temperature of the lens system, the collimator lens ensures that the objective lens is irradiated with a parallel beam. When the temperature changes, for example, at an increase of the temperature, the powerful positive lens having a large negative dn/dT renders the beam slightly divergent because the refractive index n decreases at an increase of the temperature (T). Consequently, the focus of the objective lens moves away from this lens. Due to this focus offset, the radiation paths through the objective lens change, on the one hand, and the magnification of the combination of collimator lens and objective lens changes, on the other hand. Both changes result in a typical change of the spherical aberration. Due to the new composition of the collimator lens, both changes of the spherical aberration largely compensate each other so that the combination of collimator lens and objective lens exhibits a negligible aberration change at a change of the temperature.
A preferred embodiment of the lens system according to the invention is characterized in that the material of the negative lens element has a positive temperature coefficient dn/dT the absolute value of which is smaller then the absolute value of the negative temperature coefficient of the positive lens element.
This yields a better correction for temperature changes. Since the collimator lens comprises a synthetic material element and a glass element, this lens may act, in principle, as an achromatic doublet so that it is also possible with this lens to correct for chromatic aberrations of the objective lens. To this end, the wavelength dispersion of the synthetic material element is preferably as small as possible.
The lens system is preferably characterized in that the two refractive surfaces of the positive lens element are aspherical surfaces.
This measure contributes to the fact that the lens system is well corrected for spherical aberration at the design temperature.
The lens system may be further characterized in that the material of the positive lens element is polycarbonate and the material of the negative lens element is the glass SF11.
This glass has a large refractive index n and a relative large positive dn/dT value, while the synthetic material polycarbonate (PC) has a relatively good optical quality.
However, the lens system is preferably further characterized in that the material of the positive lens element is polymethyl methacrylate and the material of the negative lens element is the glass SF 11.
The synthetic material polymethyl methacrylate has a relative low dispersion factor so that its use enables the collimator lens to satisfactorily fulfil the function of an achromatic doublet.
The invention also relates to an optical scanning device for scanning an information plane. This scanning device, which comprises a radiation source for supplying a scanning beam and a lens system for focusing this beam on the information plane, is characterized in that the lens system is the system described hereinbefore.
The invention also relates to an apparatus for reading and/or writing information in an information plane of an optical record carrier. This apparatus, which comprises an optical scanning device for forming a scanning spot on the information plane, a radiation-sensitive detection system for converting radiation from the information plane into electric signals, and means for moving the scanning spot and the information plane with respect to each other, is characterized in that the optical scanning device is the device mentioned above.
These and other aspects of the invention are appare
Belk Michael E.
Schwartz Jordan M.
U.S. Philips Corporation
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