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Dimensional measurements

Areal calibration set

Calibration of surface topography measuring instruments

This traceable high precision areal standard, is designed to calibrate the metrological characteristics of areal surface topography measuring instruments. The NPL areal standard enables users to:

  • Adopt good measurement practice and meet the requirements of the forthcoming ISO standards for determination of 3-D surface texture in a cost effective way
  • Calibrate common optical instruments equipped with 10×, 20× and 50× magnification lenses, and contact stylus instruments
  • Automate calibration procedures due to unique combination of alignment markers and a comprehensive range of calibration features on a single plate
  • Validate instrument performance using two complementary electroformed areal irregular pattern artefacts, AIR-B40 and AIR-B70.

NPL areal standard
The NPL areal standard is a 50 mm square toughened glass plate bearing a silicon wafer of 10 mm square in the middle of the plate. The silicon wafer contains geometrical patterns, designed for the calibration of areal surface topography measuring instruments, including vertical and lateral scale calibration, flatness deviation and testing the lateral resolution. The patterns on the NPL areal standard that are calibrated are listed in the table below.

Name Description Feature Size
Type PGR Groove, Rectangular (profile) Step height 2 µm
1 µm
500 nm
200 nm
100 nm
50 nm
Type ACG Cross grating (areal) Pitch 100 µm
50 µm
20 µm
Type AFL Flat Flatness Sz < 20 nm
Type ASG Star shape grooves (areal) Resolution Range Depth
0.7 µm to 50 µm 200 nm
0.7 µm to 25 µm 200 nm
0.7 µm to 25 µm 50 nm
0.2 µm to 6 µm 50 nm
Type AIR Irregular (areal) Sa, Sq, Sz, Ssk, Sku 40 µm autocorrelation length
70 µm autocorrelation length

Influence parameters and metrological characteristics
The metrological characteristics of an areal surface texture instruments are shown in the table below. The NPL areal standard has been designed to directly address each of the metrological characteristics.

Metrological Characteristics Symbol* Main Potential Error along Calibrated patterns that addresses characteristic
Amplification coefficient ax , ay , az x, y, z Type PGR
Type ACG
Linearity lx , ly , lz x, y, z
Perpendicularity ΔPERxy x, y
Flatness deviation ZFLT z Type AFL
Measurement noise NM z
Lateral period limit DLIM z Type ASG

*As per ISO DIS 25178-600:2016, Date: 20-02-2016

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