FEM-based modeling of microsphereenhanced interferometry

To improve the lateral resolution in microscopic imaging,microspheres are placed close to the object’s surface in order to support the imaging process by optical near-field information.Although microsphere-assisted measurements are part of various recent studies,no generally accepted explanation for the effect of microspheres exists.Photonic nanojets,enhancement of the numerical aperture,whispering-gallery modes and evanescent waves are usually named reasons in context with microspheres,though none of these effects is proven to be decisive for the resolution enhancement.We present a simulation model of the complete microscopic imaging process of microsphere-enhanced interference microscopy including a rigorous treatment of the light scattering process at the surface of the specimen.The model consideres objective lenses of high numerical aperture providing 3D conical illumination and imaging.The enhanced resolution and magnification by the microsphere is analyzed with respect to the numerical aperture of the objective lenses.Further,we give a criterion for the achievable resolution and demonstrate that a local enhancement of the numerical aperture is the most likely reason for the resolution enhancement.

Electromagnetic modeling of interference,confocal,and focus variation microscopy

We present a unified electromagnetic modeling of coherence scanning interferometry,confocal microscopy,and focus variation microscopy as the most common techniques for surface topography inspection with micro-and nanometer resolution.The model aims at analyzing the instrument response and predicting systematic deviations.Since the main focus lies on the modeling of the microscopes,the light–surface interaction is considered,based on the Kirchhoff approximation extended to vectorial imaging theory.However,it can be replaced by rigorous methods without changing the microscope model.We demonstrate that all of the measuring instruments mentioned above can be modeled using the same theory with some adaption to the respective instrument.For validation,simulated results are confirmed by comparison with measurement results.

Comparison of a new optical biometer and a standard biometer in cataract patients

Background:Cataract surgery is the most common surgical procedure in ophthalmology.Biometry data and accurate intraocular lens(IOL)calculations are very important in achieving the desired refractive outcomes.The aim of this study was to compare measurements using a new optical low coherence reflectometry(OLCR)biometer(OA-2000)and the gold standard partial coherence interferometry(PCI)optical biometer(IOLMaster 500).Methods:Ocular biometry of cataract patients were measured by the OA-2000 and IOLMaster 500 to compare keratometry(K),axial length(AL),anterior chamber depth(ACD),white-to-white(WTW)diameter,and IOL power using the SRK/T formula.Results:One hundred and two eyes of 68 cataract patients were evaluated with the two optical biometers.The mean values of the AL,K,ACD,and WTW differed very little(OCLR biometer,23.12 mm,44.50 diopters(D),3.06,and 11.64 mm,respectively;PCI biometer,23.18 mm,44.6 D,3.15,and 11.86 mm,respectively),but the differences were significant(all,p≤0.05).The AL,K,and ACD showed excellent correlations(r=0.999,0.980,and 0.824,respectively;all p<0.001);however,there was a weak correlation of the WTW diameter between the two devices(r=0.256).The IOL powers using the SRK-T formula derived from both instruments were very similar,with an excellent correlation(r=0.989).The mean difference between the two instruments was 0.32 D.Conclusions:The OLCR biometer showed very a strong agreement with the standard PCI optical biometer for almost all ocular biometry measurements,except for the WTW diameter.Trial registration:TCTR20160614003;date 06/09/2016;‘retrospectively registered’.

Route to OCT From OFS at University of Kent

A review is presented of several technical solutions developed by the Applied Optics Group （AOG） in the field of low coherence interferometry applied to optical fiber sensors （OFS） that subsequently allowed AOG to quickly progress in the field of optical coherence tomography （OCT）.

Precision Measurement of Complex Optics Using a Scanning‑Point Multiwavelength Interferometer Operating in the Visible Domain

The growing demands of optical systems have led to increasingly complex aspheres and freeforms.In this paper,an established measurement system in asphere production,which is also a promising approach in high-precision freeform metrology,is presented.It is based on a scanning-point multiwavelength interferometer approach.The scanning principle enables great fexibility,reduces setup time and costs,and has almost no limitations in spherical departure.Due to the absolute measurement capability,the utilized multiwavelength approach is benefcial for segmented,annular,and discrete surfaces,which are common designs of modern applications’optical elements.The metrology system enables measurements on a nanometer scale on a great variety of apertures—from 1 to 1000 mm.Recently,a new short-coherence multiwavelength interferometer operating in the visible domain has been developed.It enables the high-precision measurement on silicon-coated surfaces,which can be found in space applications and are also commonly used for extreme-ultraviolet(EUV)lithographic setups,which will be used as an example throughout this work.Owing to the large absolute measurement capability,applied grating structures for suppressing infrared light in the EUV process can easily be measured.This paper summarizes the basic working principles of the proposed metrology system,explains the special requirements for diferent felds of application,highlights the capabilities of the visible multiwavelength approach,and shows the measurement results.