Interferometer system and lithographic apparatus

An interferometer system comprising: a first detector for receiving a first measurement beam traveling to a reference surface; a second detector for receiving a second measurement beam adapted to travel across the target surface; a reference variable delay path and/or a measurement variable delay path and a delay path controller for adapting the delay length. Under the reference coherence device, a reference spectral coherence pulse appears at the first detector; and under a measurement coherence device, a measurement spectral coherence pulse appears at the second detector. A control unit receives a reference coherent signal from the first detector and a measurement coherent signal from the second detector, and determining a zero position of the target surface based on the reference coherence signal and the measurement coherence signal, and based on a delay path difference between the reference coherence device and the measurement coherence device and/or between the reference coherence device and the measurem