National Physical Laboratory

Determination of the transfer function for optical surface topography measuring instruments.

Foreman, M R, Giusca, C L, Leach, R K, Coupland, J M*
NPL Report ENG 36, July 2012
NPL Doc. Ref:
PDB: 6483 | DDB: 5169
Document Type:
NPL Report

Note: An asterisk after an author's name indicates a non-NPL author.


A significant number of areal surface topography measuring instruments, largely based on optical techniques, are commercially available. However, implementation of optical instrumentation into production is problematic due to the lack of understanding of the complex interaction between the light and the component surface - studying the optical transfer function of the instrument could solve this. The purpose of this report is to review the optical transfer function measurement techniques. Starting from the basis of a spatially coherent, monochromatic confocal scanning imaging system the theory of optical transfer functions in 3D imaging is presented. Further generalisations are reviewed allowing extension of the theory to the description of conventional and interferometric 3D imaging systems, over a range of spatial coherence. Polychromatic transfer functions are also briefly considered. Specialisation to the measurement of surface topography is further made. Following presentation of theoretical results, experimental methods to measure the optical transfer function of each class of system is presented, with a focus on suitable methods for establishment of calibration standards in 3D imaging and surface topography measurements.

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