  Generally, turn-to-turn power fluctuations of incoherent spontaneous
synchrotron radiation in a storage ring depend on the 6D phase-space
distribution of the electron bunch. In some cases, if only one parameter of the
distribution is unknown, this parameter can be determined from the measured
magnitude of these power fluctuations. In this Letter, we report an absolute
measurement (no free parameters or calibration) of a small vertical emittance
(5--15 nm rms) of a flat beam by this method, under conditions, when it is
unresolvable by a conventional synchrotron light beam size monitor.
