  A broad variety of light sources exhibit photon-count fluctuations that
display inverse-square spectral behavior at extremely low frequencies. These
sources include light-emitting diodes, superluminescent diodes, laser diodes,
incandescent sources, and betaluminescent sources. In a series of experiments
carried out over an 18-month period, the photon-count fluctuations for these
sources were found to exhibit a $1/f^2$ spectral signature over the frequency
range $1.0 \times 10^{-6} \le f \le 5.0 \times 10^{-4}$ Hz, corresponding to
$33$ min $ \le T_f \le 11.6$ d, where $T_f \equiv 1/f$. The lower time limit is
established by the photodetector noise floor while the upper time limit is
determined by the duration of the individual experiments. Scalograms computed
from our data are consistent with the periodograms. The universal character of
this inverse-square baseband spectral behavior stands in sharp contrast to the
optical spectra of these sources, which differ markedly. Time traces of the
photon-count fluctuations are examined to shed light on the origin of this
enigmatic spectral behavior.
