Data processing apparatus for processing intensity values received from a broadband scanning interferometer measuring a surface of a sample, said data processing apparatus comprising: first processing means (34a, 340) configured for carrying out processing on the intensity values as the intensity values are received by receiving means during a measurement operation of the broadband scanning interferometer to produce data indicating the position of a coherence peak; and second processing means (346; 350) configured for, after completion of a measurement operation, using the data produced by the first processing means to obtain data indicative of a height of the measured surface region, wherein: one of the first and second processing means comprises correlating means (44; 440), configured for correlating the intensity values with correlation function data representing a correlation function to provide correlation data for the measured sample surface region to enable a position of a coherence peak in the intensity values for the measured sample surface region to be identified; and the second processing means comprises surface height determining means (35; 350), configured for determining a height of the measured sample surface region from the correlation data related to the position of the coherence peak, wherein, in the correlating means, the correlation function comprises first and second periodically varying wave functions with the first and second wave functions being of different phase and being bounded by an envelope. first processing means (34a, 340) configured for carrying out processing on the intensity values as the intensity values are received by receiving means during a measurement operation of the broadband scanning interferometer to produce data indicating the position of a coherence peak; and second processing means (346; 350) configured for, after completion of a measurement operation, using the data produced by the first processing means to obtain data indicative of a height of the measured surface region, wherein: one of the first and second processing means comprises correlating means (44; 440), configured for correlating the intensity values with correlation function data representing a correlation function to provide correlation data for the measured sample surface region to enable a position of a coherence peak in the intensity values for the measured sample surface region to be identified; and the second processing means comprises surface height determining means (35; 350), configured for determining a height of the measured sample surface region from the correlation data related to the position of the coherence peak, wherein, in the correlating means, the correlation function comprises first and second periodically varying wave functions with the first and second wave functions being of different phase and being bounded by an envelope. one of the first and second processing means comprises correlating means (44; 440), configured for correlating the intensity values with correlation function data representing a correlation function to provide correlation data for the measured sample surface region to enable a position of a coherence peak in the intensity values for the measured sample surface region to be identified; and the second processing means comprises surface height determining means (35; 350), configured for determining a height of the measured sample surface region from the correlation data related to the position of the coherence peak, wherein, in the correlating means, the correlation function comprises first and second periodically varying wave functions with the first and second wave functions being of different phase and being bounded by an envelope.