  Hypertemporal visible imaging of an urban lightscape can reveal the phase of
the electrical grid granular to individual housing units. In contrast to
in-situ monitoring or metering, this method offers broad, persistent,
real-time, and non-permissive coverage through a single camera sited at an
urban vantage point. Rapid changes in the phase of individual housing units
signal changes in load (e.g., appliances turning on and off), while slower
building- or neighborhood-level changes can indicate the health of distribution
transformers. We demonstrate the concept by observing the 120 Hz flicker of
lights across a NYC skyline. A liquid crystal shutter driven at 119.75 Hz
down-converts the flicker to 0.25 Hz, which is imaged at a 4 Hz cadence by an
inexpensive CCD camera; the grid phase of each source is determined by analysis
of its sinusoidal light curve over an imaging "burst" of some 25 seconds.
Analysis of bursts taken at ~15 minute cadence over several hours demonstrates
both the stability and variation of phases of halogen, incandescent, and some
fluorescent lights. Correlation of such results with ground-truth data will
validate a method that could be applied to better monitor electricity
consumption and distribution in both developed and developing cities.
