For comparing first optical properties of a first fluid with second optical properties of a second fluid a first transparent grating having a grating constant is made of the first liquid, and a second transparent grating also having the grating constant is made of the second liquid. The second transparent grating is arranged at a lateral offset of less than 45% of the grating constant with regard to the first transparent grating such that grating bars of the first and second transparent gratings are arranged side by side. Coherent light is directed onto the first and second transparent gratings such that light which passed through the grating bars of the first and second transparent gratings forms a diffraction pattern comprising intensity maxima. Two light intensities of two intensity maxima of a same order higher than zero are measured and compared to each other.