  A dense layer of nitrogen-vacancy (NV) centers near the surface of a diamond
can be interrogated in a widefield optical microscope to produce spatially
resolved maps of local quantities such as magnetic field, electric field and
lattice strain, providing potentially valuable information about a sample or
device placed in proximity. Since the first experimental realization of such a
widefield NV microscope in 2010, the technology has seen rapid development and
demonstration of applications in various areas across condensed matter physics,
geoscience and biology. This Perspective analyzes the strengths and
shortcomings of widefield NV microscopy in order to identify the most promising
applications and guide future development. We begin with a brief review of
quantum sensing with ensembles of NV centers, and the experimental
implementation of widefield NV microscopy. We then compare this technology to
alternative microscopy techniques commonly employed to probe magnetic materials
and charge flow distributions. Current limitations in spatial resolution,
measurement accuracy, magnetic sensitivity, operating conditions and ease of
use, are discussed. Finally, we identify the technological advances that solve
the aforementioned limitations, and argue that their implementation would
result in a practical, accessible, high-throughput widefield NV microscope.
