  Based on the measurement of quantum correlation functions, the quantum
statistical properties of spectral measurements are studied for broadband
radiation fields. The spectral filtering of light before its detection is
compared with the direct detection followed by the spectral analysis of the
recorded photocurrents. As an example, the squeezing spectra of the atomic
resonance fluorescence are studied for both types of filtering procedures. The
conditions for which the detection of the nonclassical signatures of the
radiation is possible are analyzed. For the considered example, photocurrent
filtering appears to be the superior option to detect nonclassicality, due to
the vacuum-noise effects in the optical filtering.
