Abstract: Refractive index distribution standard in the form of a three-dimensional object which contains in its volume a base medium and regions of variable size and distance with a refractive index other than that of the base medium, characterised in that the difference between the refractive index of said regions and the refractive index of the base medium is not greater than 0.04, at least one of the regions is a set of at least two prisms or cylinders or coaxial rings of variable size and distance, having a dimension in at least one direction similar to the resolving power of the measurement system under assessment and at least one of the regions is sphere-like or ellipsoid-like in shape.