A measurement device for determining optical properties of a sample surface, the measurement device comprising: at least one light source configured to illuminate a measurement spot on the sample surface along at least one illumination direction; and an optical system comprising an image sensor, the image sensor defining an image sensor plane, the optical system being configured to image a measurement plane onto the image sensor plane, the optical system defining an optical axis between the measurement plane and the image sensor plane, the optical axis being oriented along an observation direction, the optical system being arranged to receive, as its incoming light, light that has been reflected from the measurement spot into the observation direction, a reference sensor; and an optical multiplexer (300; 400; 400') configured to receive incoming light along the optical axis and to direct a first portion of the incoming light to the image sensor and a second portion of the incoming light to the reference sensor, characterized in that the optical multiplexer (300; 400; 400') defines a first area (330; 430; 440) for receiving the first portion of the incoming light and a second area (320; 420; 450) for receiving the second portion of the incoming light, the second area (320; 420; 450) radially surrounding the first area (330; 430; 440) in a projection onto a plane that is perpendicular to the optical axis, and the optical multiplexer (300; 400; 400') is arranged in the optical system in such a manner that the optical multiplexer (300; 400; 400') directs the first portion of the incoming light, which impinges on the first area (330; 430; 440), to the image sensor and the second portion of the incoming light, which impinges on the second area (320; 420; 450), to the reference sensor. at least one light source configured to illuminate a measurement spot on the sample surface along at least one illumination direction; and an optical system comprising an image sensor, the image sensor defining an image sensor plane, the optical system being configured to image a measurement plane onto the image sensor plane, the optical system defining an optical axis between the measurement plane and the image sensor plane, the optical axis being oriented along an observation direction, the optical system being arranged to receive, as its incoming light, light that has been reflected from the measurement spot into the observation direction, a reference sensor; and an optical multiplexer (300; 400; 400') configured to receive incoming light along the optical axis and to direct a first portion of the incoming light to the image sensor and a second portion of the incoming light to the reference sensor, characterized in that the optical multiplexer (300; 400; 400') defines a first area (330; 430; 440) for receiving the first portion of the incoming light and a second area (320; 420; 450) for receiving the second portion of the incoming light, the second area (320; 420; 450) radially surrounding the first area (330; 430; 440) in a projection onto a plane that is perpendicular to the optical axis, and the optical multiplexer (300; 400; 400') is arranged in the optical system in such a manner that the optical multiplexer (300; 400; 400') directs the first portion of the incoming light, which impinges on the first area (330; 430; 440), to the image sensor and the second portion of the incoming light, which impinges on the second area (320; 420; 450), to the reference sensor.