  Grisms are dispersive transmission optics that find their most frequent use
in instruments that combine imaging and spectroscopy. This application is
particularly popular in the infrared where imagers frequently have a cold pupil
in their optical path that is a suitable location for a dispersive element. In
particular, several recent and planned space experiments make use of grisms in
slit-less spectrographs capable of multi-object spectroscopy. We present an
astronomer-oriented general purpose introduction to grisms and their use in
current and future astronomical instruments. We present a simple, step-by-step
procedure for adding a grism spectroscopy capability to an existing imager
design. This procedure serves as an introduction to a discussion of the device
performance requirements for grisms, focusing in particular on the problems of
lithographically patterned silicon devices, the most effective grism technology
for the 1.1-8 micron range. We begin by summarizing the manufacturing process
of monolithic silicon gratings. We follow this with a report in detail on the
as-built performance of parts constructed for a significant new space
application, the NIRCam instrument on JWST and compare these measurements to
the requirements.
