Flatness Reference Chart

Surface Flatness is the maximum deviation from a fully flat face along the same lateral dimensions specified in fraction of reference wavelength. This specification is uniquely influenced by geometric surface deviations of the optic compared with an optical flat element. It should be considered that this specification only defines a single surface and there is usually one single flatness filter specification provided, which means that the same flatness value must be achieved on the first and second optical surface of the optic.
 
Typically, deviation is measured using a high-quality flat optical system that works as a flat reference to compare the flatness of a test piece (mirror, plano- lens, window, optical filter, or prism). When the test optic’s surface is placed against the reference surface, the shape of fringes that appear indicates the surface flatness of the tested piece. If both elements are evenly spaced straight and parallel, our optic is at least as flat as the reference optical flat. On the contrary, flatness errors appear when fringes are curved.
 
Usually, surface figure is specified on optical drawing specifications and waves are the predominant unit value used in technical data sheets. A key piece to remember when determining the surface flatness is needed is the wavelength of interest, because the surface figure is measured in fractions of a reference wavelength at 632.8 nm, the HeNe laser wavelength, if not mentioned the contrary in the Surface Flatness specification.

Working with larger wavelengths than 633 nm intrinsically makes this specification to be more relaxed. In other words, a surface flatness measured for visible wavelengths at the IR acts as higher quality and distorts less to the incident beam.

The optical flat of reference helps to determine the surface flatness a customer needs relying on the purpose which it is used for. The surface accuracy of windows and mirrors is extremely critical to the performance of an optical system. For any laser application where you are transmitting light (window) or reflecting light (mirror), the smallest imperfections can scatter light in unintended directions causing distortion, degradation of the overall signal, or image quality. Even if a glass object is visually smooth, on a microscopic level there are tiny peaks and valleys.