  The generation of a streaking spectrogram is based on energy absorption from
the streaking laser. Investigating this absorption we show rigorously under
which condition the measured time shift is independent of properties of the
streaking light. In this case it provides the Wigner-Smith time delay. The
latter is infinite for systems with a long-range potential tail, such as
Coulomb systems. Here, we suggest to determine the time delay relative to pure
hydrogen for meaningful results. Finite delays obtained so far for Coulombic
systems without the hydrogen reference are the consequence of a finite
streaking frequency and depend on its value as well as on the electron's excess
energy. Our analysis also suggests a time-delay measurement technique that
avoids the record of a complete streaking scan.
