 Considering that because of the Bragg condition the simple relation formula_6 holds, where formula_7 is the modulated refractive index in the material (not the base index) at this wavelength, one sees that for typical values (formula_8) one gets formula_9 showing the extraordinary wavelength selectivity of such volume holograms.In the case of a simple grating in the transmission geometry the angular selectivity formula_10 can be estimated as well