misfigure

To misfigure is to deviate from the intended shape or prescription during the grinding, polishing, or coating of an optical surface. The result is a lens, mirror, or prism that does not meet its design specification and introduces optical aberrations: spherical aberration, astigmatism, coma, or other wavefront errors that degrade image quality or system performance.

Misfiguring occurs across the full range of optical manufacturing. A lens blank may be ground to the wrong radius of curvature, drift during polishing, or suffer localized wear that creates a hill or valley in the surface. A mirror substrate may warp during the polishing process or pick up residual stress during coating. Precision aspherical surfaces are especially vulnerable because they demand tight tolerances across the entire aperture; a deviation of even a few nanometers in the sag profile can exceed allowable wavefront error budgets.

Common causes include inadequate process control during long polishing runs, thermal expansion of the substrate during wet processing, incorrect dwell times in CNC grinding, or contamination of the optical surface. Worn machine tools, unstable chuck holding, and temperature fluctuations in the workshop all contribute. Detection relies on interferometry (Fizeau or Twyman-Green setups), optical profilometry, or phase-shifting methods to reveal the surface shape and compare it to the design drawing.

The term reflects the industrial distinction between acceptable scatter or microroughness, which is cosmetic, and systematic shape error, which is optical failure. A misfigured surface breaks the intended wave propagation; a rough but correctly figured surface scatters light but preserves phase. When discovered before final assembly, a misfigured optical element is reworked, downgraded to lower-specification use, or scrapped. The cost of rework must be weighed against the loss of material and machine time.