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Optical surface profilometer

1. Why use white light interferometry? WLI 3D microscope provides: non-contact fast accurate repeatable area topography, including shaping, waviness and roughness 2. Interference objectives 2.1 Michelson (1.5x ~ 5x) low magnification large field of view 2.2 Mirau (10x ~ 100x) medium to high magnification medium to small field of view 2.3 Linnik (any magnification) 3. […]

1. Why use white light interferometry?

WLI 3D microscope provides:

  • non-contact
  • fast
  • accurate
  • repeatable
  • area topography, including shaping, waviness and roughness

2. Interference objectives

2.1 Michelson (1.5x ~ 5x)

  • low magnification
  • large field of view

2.2 Mirau (10x ~ 100x)

  • medium to high magnification
  • medium to small field of view

2.3 Linnik (any magnification)

3. Measurement modes

3.1 Phase shift interferometry

  • Used for testing smooth objects with very high precision (0.1 nm)

3.2 White light interferometry

  • True 3D measurement of surface area.
  • Ability to measure rough surfaces.
  • Good results with low contrast fringes.
  • Results independent of intensity variations across field of view.
  • Vertical height limited only by scanner and objective working distance.
  • No step height ambiguities.
White light interferometry Phase shifting interferometry
Vertical resolution ~3 nm Vertical resolution < 0.1 nm
Steps or surface variations up to 10 mm Smoothly varying surfaces
Surfaces with rough/steep surfaces Polished materials, small height differences

4. Some notes

  • "Surface roughness", Ra (Sa) is the average of the absolute value of profile heights over a given length (area).

    ASME B46.1-2019: Surface Texture (Roughness, Waviness, Lay)

  • The Root Mean Square roughness (Rq) is the root mean square average of the roughness profile ordinates.

  • Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length.

Reference

3D Optical Profilometry | An Introduction to Non Destructive 3D Surface Texture Studies | Bruker

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