Optical Metrology and Testing
Optical Metrology and Testing
This domain tests quantitative measurement of optical surfaces, wavefronts, and system performance. It includes interferometry, phase measurement, surface figure and roughness, MTF testing, calibration, and uncertainty.
Core rules
- Interference measures optical path difference; one fringe corresponds to (\lambda/2) of surface height in reflection.
- Phase-shifting interferometry converts fringe intensity to phase; calibration of phase step and wavelength is critical.
- Surface figure is often reported as PV and RMS; RMS is more directly tied to optical performance.
- Roughness and mid-spatial-frequency errors scatter light even when low-order figure is good.
- MTF is measured with a slit, edge, or grating target and must be corrected for detector and target effects.
- Uncertainty comes from wavelength, alignment, vibration, air turbulence, detector nonlinearity, and software unwrapping.
Worked example
A two-beam interferometer uses 633 nm light and a small tilt of 1 mrad between wavefronts. Fringe spacing is approximately (\lambda/\theta=633\times10^{-9}/1\times10^{-3}=6.33\times10^{-4}) m, or 0.633 mm. If the tilt is halved to 0.5 mrad, spacing doubles to 1.27 mm. A single fringe shift corresponds to (\lambda/2\approx316.5) nm of optical path difference in reflection.
Traps
Reporting PV alone can hide a large localized defect or overstate typical performance. Ignoring the reflection factor doubles apparent surface error. Vibration and air currents can mimic low-order aberration. MTF measured through a pixelated detector is not the lens MTF unless deconvolved.
Exam focus
State the measurement quantity, its units, and the dominant uncertainty before quoting a result.