Radius-of-curvature measurement by spectrally-controlled interferometry
The ROC value of a test surface is measured with a single spectrally-controlled interferometric measurement using a reference source of known ROC. The test surface is placed at the confocal position of the reference surface and the light source is modulated so as to produce localized interference fringes at the location of the test surface. The interference fringes are then processed with conventional interferometric analysis tools to establish the exact position of the test surface in relation to the reference surface, thereby determining the distance between the test surface and the reference surface. The radius of curvature of the test surface is obtained simply by subtracting such distance from the known radius of curvature of the reference surface.
1. A method for measuring a radius of curvature of a test surface with a single confocal measurement, the method comprising the following steps:
providing an interferometer with a broadband light source capable of spectral modulation to produce a light beam with a varying spectral distribution, said beam having a temporal coherence such that interference fringes are detectable within a measurement space of the interferometer, and said interferometer including a curved reference surface with a known radius of curvature;
placing the test surface at a substantially confocal position of the reference surface;
spectrally modulating the light source according to spectrally controlled interferometric principles so as to produce multiple localized correlograms at said confocal position of the reference surface;
processing said multiple localized correlograms to establish an exact position of the test surface in relation to the reference surface, thereby determining a distance between the test surface and the reference surface; and
obtaining said radius of curvature of the test surface by subtracting said distance from said known radius of curvature of the reference surface.
2. The method of claim 1 , wherein said multiple localized correlograms are produced by changing a period of modulation of the light source.
3. The method of claim 1 , wherein said multiple localized correlograms are produced by changing a phase of modulation of the light source.
4. The method of claim 1 , wherein said multiple localized correlograms are produced by changing a mean wavelength of the light source.
5. The method of claim 1 , wherein each of said multiple localized correlograms is produced by sinusoidal spectral modulation of the light source.
6. The method of claim 1 , wherein said placing, modulating, processing and obtaining steps are repeated with successive test surfaces.
7. The method of claim 6 , wherein said multiple localized correlograms are produced by changing a period of modulation of the light source.
8. The method of claim 6 , wherein said multiple localized correlograms are produced by changing a phase of modulation of the light source.
9. The method of claim 6 , wherein said multiple localized correlograms are produced by changing a mean wavelength of the light source.
10. The method of claim 6 , wherein each of said multiple localized correlograms is produced by sinusoidal spectral modulation of the light source.
11. The method of claim 1 , wherein said light source is adapted to produce multiple beams having different mean wavelengths to produce said multiple localized correlograms, and said processing step is carried out by applying multi-wavelength interferometric analysis to said multiple localized correlograms.