A calibration method and also a calibration device are proposed to enable a spatial position andor orientation of a surgical referencing unit of a surgical navigation system fitted with at least one inertial sensor to be specified in relation to a spatial coordinate system. The zirconia-mullite refractory raw material comprises crystalline zirconia and mullite as primary components, with the remainder being corundum andor a matrix glass, wherein the crystalline zirconia includes a eutectic zirconia crystal having a grain size of 1. 7 gcm3 and which has a high density layer having a density of 0. A layer of thin-film semiconductor material is deposited onto the substrate. Also included is the use of a ratioing technique to correlate the spectral radiance values rather than spectral reflectance values to the surface parameters of interest; this simplifies both the filed instrumentation requirements and post processing procedures.