A spectrometer has: Accumulation means to obtain a data set containing N data points, repeating the measurement M times to obtain M spectral data sets or time-domain data sets S1 (d1 to dN) to SM (d1 to dN), and accumulating the M spectral data sets or time-domain data sets. Means for creating sets S1 (dn) to SM (dn) of the data points contained in the M spectral data sets or time-domain data sets S1 (d1 to dN) to SM (d1 to dN). Correlation computing means for finding correlations. Computing means for finding either the product of an accumulated or anticipated spectrum.