  Structured illumination microscopy (SIM) provides images of fluorescent
objects at an enhanced resolution greater than that of conventional
epifluorescence wide-field microscopy. Initially demonstrated in 1999 to
enhance the lateral resolution two-fold, it has since been extended to enhance
axial resolution two-fold (2008), applied to live-cell imaging (2009) and
combined with myriad other techniques, including interferometric detection
(2008), confocal microscopy (2010) and light sheet illumination (2012). Despite
these impressive developments, SIM remains, perhaps, the most poorly understood
`super-resolution' method. In this article, we provide answers to the 13
questions regarding SIM proposed by Prakash et al., along with answers to a
further three questions. After providing a general overview of the technique
and its developments, we explain why SIM as normally used is still
diffraction-limited. We then highlight the necessity for a non-polynomial, and
not just non-linear, response to the illuminating light in order to make SIM a
true, diffraction-unlimited, super-resolution technique. In addition, we
present a derivation of a real-space SIM reconstruction approach that can be
used to process conventional SIM and image scanning microscopy (ISM) data and
extended to process data with quasi-arbitrary illumination patterns. Finally,
we provide a simple bibliometric analysis of SIM development over the past two
decades and provide a short outlook on potential future work.
