Precision and errors Optical flat

two λ/10 flats @ 589 nm. although both surfaces have irregularities test shows both flat relative each other. wringing progresses thin fringes widen until single fringe remains.



a thermal image of optical flat after handling few seconds. warmer areas increase thickness of flat on cooler areas, distorting surface accordingly.

counterintuitively, fringes not exist within gap or flat itself. interference fringes form when light waves converge @ eye or camera, forming image. because image compilation of converging wavefronts interfering each other, flatness of test piece can measured relative flatness of optical flat. deviations on flat added deviations on test surface. therefore, surface polished flatness of λ/4 cannot tested λ/4 flat, not possible determine errors lie, contours can revealed testing more accurate surfaces λ/20 or λ/50 optical flat. means both lighting , viewing angle have effect on accuracy of results. when lighted or viewed @ angle, distance light must travel across gap longer when viewed , illuminated straight on. thus, angle of incidence becomes steeper, fringes appear move , change. 0 degree angle of incidence desirable angle, both lighting , viewing. unfortunately, impossible achieve naked eye. many interferometers use beamsplitters obtain such angle. because results relative wavelength of light, accuracy can increased using light of shorter wavelengths, although 632 nm line helium–neon laser used standard.

no surface ever flat. therefore, errors or irregularities exist on optical flat affect results of test. optical flats extremely sensitive temperature changes, can cause temporary surface deviations resulting uneven thermal expansion. glass experiences poor thermal conduction, taking long time reach thermal equilibrium. merely handling flats can transfer enough heat offset results, glasses such fused silica or borosilicate used, have low coefficients of themal expansion. glass needs hard , stable, , thick prevent flexing. when measuring on nanometer scale, slightest bit of pressure can cause glass flex enough distort results. therefore, flat , stable work-surface needed, on test can performed, preventing both flat , test-piece sagging under combined weight, often, precision-ground marble or stone block used work surface, providing steady table-top testing upon. provide flatter surface, test may performed on top of optical flat, test surface sandwiched in middle.