Abstract
The errors in the Vickers diagonal length measurement are discussed from the point of view of the design of microscope. Effects of the numerical aperture (NA) of an objective lens are evaluated through the experiment. With smaller NA, the resolving power of the microscope gets worse and the depth of focus gets deeper as predicted Rayleigh’s criterion. The relationship between the repeatability of diagonal length and the objective NA follows the theory. In addition, the variation of NA relates the measured diagonal length. Those effects are much significant than the image resolution or the minimum count of a measuring system.
Highlights
Consider establishing a national primary Vickers hardness standard where any reference materials are not available, the error in diagonal length measurement is one of the most significant sources of measurement uncertainty as well as testing force, testing cycle, uniformity of reference blocks or repeatability of measurement
To evaluate the repeatability of focusing, a displacement transducer, Mitutoyo LGH-110, with 0.01 μm resolution was attached on the microscope column and the position of the microscope tube was recorded corresponding to every shot of image
Pupil numerical aperture (NA) diameter, mm Resolving power δ, μm Maxium expansion ε, μm Images were taken in a format of 2592×1944 pixels and diagonal lengths of indentation were measured on a computer screen
Summary
Consider establishing a national primary Vickers hardness standard where any reference materials are not available, the error in diagonal length measurement is one of the most significant sources of measurement uncertainty as well as testing force, testing cycle, uniformity of reference blocks or repeatability of measurement. The ISO standard [1] requires to consider “both the resolution of the length measurement indicating instrument and the optical resolution of the measuring microscope”, most of discussions are focused on the indicating instrument G., the discussion in the EA committee) and quite few studies concerned the resolving power of the microscope [2]. Possible errors caused by the design of an optical microscope are discussed
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