The spectral properties of La/B, La/B 4C, and LaN/B, LaN/B 4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range based on measured B and B 4C optical constants. Experimentally it is verified to what extent measured and tabulated optical constants are applicable for simulations of the reflectivity of these short period multilayer mirrors. The measured maximum reflectance at various wavelength values around the boron-K absorption edge is compared to calculated values from model systems. The measured reflectance profiles of La/B and La/B 4C show a maximum at a slightly larger wavelength than calculations would predict based on the measured B and B 4C optical constants. This is explained by the influence of a formed boron-lanthanum compound on the wavelength where the multilayer shows maximum reflectance. The maximum reflectance profiles of LaN/B and LaN/B 4C multilayers can be described accurately by using the same boron atomic scattering factors, indicating boron in the LaN/B 4C multilayer to be in a similar chemical state as boron in the LaN/B multilayer. It also indicates that nitridation of the La layer in the multilayer prevents the formation of La-B compounds. We show that the optimal wavelength for boron based optics is about 6.65 nm and depends on the B chemical state. Finally, using the measured B optical constants we are able to calculate the spectral response of the multilayers, enabling the prediction of the optimal parameters for the above mentioned multilayers © 2012 Optical Society of America.