The apparatus for deriving at least one property like turbidity or fluorescence of a sample liquid from measuring emitted light emitted from said sample liquid upon irradiation of said sample liquid with a probe light beam comprises a container for containing said sample liquid, said sample liquid forming a sample surface. It furthermore comprises a light source for generating said probe light beam directed, in an angle β1≠0° with respect to a sample surface normal, at said sample surface, and a detector adapted to detecting an intensity of said emitted light emitted through said sample surface out of said sample liquid generally along a first detection axis, said first detection axis forming an angle γ1≠0° with a sample surface normal. It also comprises an optical barrier arranged between said probe light beam and said first detection axis adapted to blocking a propagation of light originating from reflection or scattering of said probe light beam at said sample surface generally along said first detection axis. The apparatus may comprise an outlet section for removing sample liquid from said container, having at least one opening to said main section located beneath said sample surface and a first separating member for substantially separating a portion of said sample surface belonging to said outlet section from a portion of said sample surface belonging to a main section, where said light enters and exits the sample liquid.