  The Hanle depolarization signals of Sr 4607 A have been used to estimate the
unsigned magnetic flux and magnetic energy existing in the quiet Sun
photosphere. However, the Sr 4607 A Hanle signals are not sensitive to the
unsigned flux and energy. They only bear information on the fraction of
photosphere occupied by magnetic field strengths smaller than the Hanle
saturation, which do not contribute to the unsigned flux and energy. We deduce
an approximate expression for the relationship between magnetic fill factor and
Hanle signal. When applied to existing Hanle depolarization measurements, it
indicates that only 40% of the quiet Sun is filled by magnetic fields with a
strength smaller than 60 G. The remaining 60% of the surface has field
strengths above this limit. Such constraint will be needed to determine the
distribution of magnetic field strengths existing in the quiet Sun.
