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