Loughborough University
Leicestershire, UK
LE11 3TU
+44 (0)1509 263171
Loughborough University

Loughborough University Institutional Repository

Please use this identifier to cite or link to this item: https://dspace.lboro.ac.uk/2134/21628

Title: Focus variation microscope: linear theory and surface tilt sensitivity
Authors: Nikolaev, N.I.
Petzing, Jon N.
Coupland, Jeremy M.
Keywords: Microscopy
Optical transfer functions
Issue Date: 2016
Publisher: © Optical Society of America
Citation: NIKOLAEV, N.I., PETZING, J.N. and COUPLAND, J.M., 2016. Focus variation microscope: linear theory and surface tilt sensitivity. Applied Optics, 55(13), pp. 3555-3565.
Abstract: In a recent publication [3rd International Conference on Surface Metrology, Annecy, France, 2012, p. 1] it was shown that surface roughness measurements made using a focus variation microscope (FVM) are influenced by surface tilt. The effect appears to be most significant when the surface has microscale roughness (Ra ≈ 50 nm) that is sufficient to provide a diffusely scattered signal that is comparable in magnitude to the specular component. This paper explores, from first principles, image formation using the focus variation method. With the assumption of incoherent scattering, it is shown that the process is linear and the 3D point spread characteristics and transfer characteristics of the instrument are well defined. It is argued that, for the case of micro-scale roughness and through the objective illumination, the assumption of incoherence cannot be justified and more rigorous analysis is required. Using a foil model of surface scattering the images that are recorded by a FVM have been calculated. It is shown that for the case of through the objective illumination at small tilt angles, the signal quality is degraded in a systematic manner. This is attributed to the mixing of specular and diffusely reflected components and leads to an asymmetry in the k-space representation of the output signals. It is shown that by using extra-aperture illumination or at tilt angles greater than the acceptance angle of aperture (such that the specular component is lost), the incoherent assumption can be justified once again. The work highlights the importance of using ring-light illumination and/or polarizing optics, which are often available as options on commercial instruments, as a means to mitigate or prevent these effects.
Description: This paper was accepted for publication in the journal Applied Optics and the definitive published version is available at http://dx.doi.org/10.1364/AO.55.003555
Sponsor: This work was funded by the European Metrology Research Programme under the grant IND59 “Microparts”.
Version: Accepted for publication
DOI: 10.1364/AO.55.003555
URI: https://dspace.lboro.ac.uk/2134/21628
Publisher Link: http://dx.doi.org/10.1364/AO.55.003555
ISSN: 1559-128X
Appears in Collections:Published Articles (Mechanical, Electrical and Manufacturing Engineering)

Files associated with this item:

File Description SizeFormat
NIKOLAEV_AO_2016_55_3555_3565.pdfAccepted version1.19 MBAdobe PDFView/Open


SFX Query

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.