Optical system comprising ans optical multiplexer

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 (A) between the measurement plane and the image sensor plane;
• a reference sensor;
• and
• an optical multiplexer configured to receive incoming light along the optical axis (A) 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 defines a first area for receiving the first portion of the incoming light and a second area for receiving the second portion of the incoming light, the second area radially surrounding the first area in a projection onto a plane that is perpendicular to the optical axis (A), and
• the optical multiplexer is arranged in the optical system in such a manner that the optical multiplexer directs the first portion of the incoming light, which impinges on the first area, to the image sensor and the second portion of the incoming light, which impinges on the second area, to the reference sensor.