a) Field of the Invention
The invention is directed to a method for determining the image quality of an optical imaging system substantially comprising an illumination device, a sample holder with sample, imaging optics, and at least one spatially resolving detection device. The invention is further directed to the use of the method according to the invention for determining the influence of samples on the amplitude distribution and phase front distribution of the illumination light, of which, in particular, the amplitude distribution is known.
b) Description of the Related Art
In connection with the manufacture of high-quality imaging optics, particularly for use in microscopy, an assessment of the achievable image quality is required.
It is known to determine, at least semi-quantitatively, the image quality of imaging systems which have few lenses and which can also comprise complex optical subassemblies. For this purpose, it is customary to carry out so-called star tests, wherein circular objects below the resolution limit of the specific optics are used as test samples. Based on the behavior of the diffraction patterns in the imaging of these samples with defocusing devices and the symmetries contained therein, the quality of imaging can be determined qualitatively to a degree of accuracy that is usually deficient.
For example, a closed first diffraction ring at the edge of the first Rayleigh region can be considered as a sign of diffraction-limited optics. It is disadvantageous that this assessment must be considered only as integral information. And more specific quantitative information about the distribution of the rest of the imaging errors to different error types, such as a spherical aberration, coma or astigmatism, can also not be gained in this way.
In another procedure, the fit of the individual optical components is checked by interferometry to arrive at assertions concerning the geometric errors, e.g., of a lens body, which can then be converted to system-dependent imaging errors.
In this connection, system-dependent influencing factors are also already detected insofar as the measurement wavelength of the interferometer conforms to the working wavelength or wavelength spectrum of the illumination light. In more complicated optical systems, specially adapted interferometers are also occasionally used to check the image quality under given constraints and at the correct working wavelength with respect to the total imaging system.
This is applied, for example, in imaging objectives for steppers or scanners to be used in semiconductor microlithography. This procedure requires a relatively high technical complexity and is therefore very cost-intensive and not usually employed in connection with microscope manufacture.
Further, it is known to measure the wavefront of optical imaging systems with so-called Hartmann or Shack-Hartmann wavefront sensors or with sensors operating on similar principles. This also calls for relatively elaborate technology and, for that reason, the corresponding measuring systems are usually only designed to provide measurements only for different subsystems which, however, have similar interfaces, e.g., microscope objectives for microscopy.
In this connection, a continual problem, particularly for microscope producers, is the lack of an available general testing procedure that would make it possible to determine the image quality as accurately as possible for the different optical imaging systems which differ from one another with respect to optical, geometric and mechanical parameters.
Also, this problem exists not only during the manufacture and adjustment processes, but also in connection with quality control of imaging systems that are already in use by the customer.
Further, the determination of the image quality for a plurality of field positions of the imaging system is uneconomical or inaccurate in all of the previously known procedures.
The following source literature is cited in this connection: Joseph Geary, “Wavefront sensors”, SPIE Press 1995, and Daniel Malacara, “Optical Shop Testing”, Wiley Verlag 1992.