Mirrors and plates with plane or spherical surfaces are a basic part of most optical setups. Mirrors are used to deflect or to focus light beams. Polished plates can serve as windows, beamsplitters or etalons.

Some important parameters determining the quality of mirrors and plates are introduces in the following.

Shape deviation

The tolerance for the shape deviation defines the allowed difference of the real surface with respect to the ideal shape (plane or sphere). In optics the tolerance is usually given in units of the wavelength λ (e.g. λ/4). To convert this specification to length units it is essential to know the reference wavelength λref. The specifications in this catalog are referenced to a wavelength of λrefnm unless otherwise noted (d.h. λ/10 entspricht 54,6 nm).

A further parameter for the specification of the shape deviation is the method used to evaluate the measured data. With this respect the difference of the extreme values (peak-to-valley, PV) as well as the quadratic average of the deviation (root mean square, RMS) are commonly used. To calculate the PV value only the locations with the largest deviation are regarded whereas the RMS values characterizes the average deviation over the entire surface. Both values have their motivation of use. In this catalog the values specified for the PV deviation unless otherwise noted. Although there is no general relation between the PV and the RMS deviation one can assume the RMS deviation to be lower than the PV value by a factor of 3-5 for most surfaces occurring in practice. For example a value of λ/10 (PV) corresponds to λ/30-λ/50 (RMS).

The form deviations specified in this catalog are valid for an area of at least 90% (typ. >95%) of the outer dimensions of the element. If only a smaller part of the clear aperture is used the corresponding shape deviation will be significantly lower.

Micro roughness

The micro roughness describes deviation of the surface with lateral dimensions been of the order of the wavelength of light or lower (i.e. typ. <1µm). These deviations is mostly responsible for scattered light.

The micro roughness is usually specified by the quadratic average Rq of the surface deviation. The polishing techniques used in our workshop allow to obtain values of Rq <1nm (typ. Rq <0,5 nm).