Apparatus derives a sample liquid property and has a container with an outlet section having an overflow edge at a horizontal sample surface. A light source above the surface generates a probe light beam at a non-zero angle β1 to a normal to the surface. A detector above the surface detects intensity of light emitted out through the surface along a first detection axis forming a non-zero angle γ1 with the surface. An optical barrier between the probe light beam and the first detection axis blocks reflected or scattered light. An inlet section receives sample liquid and has an opening to the main section beneath the sample surface. A separating member separates the sample surface of the inlet section from the sample surface of the main section.