  The use of Immersed Gratings offers advantages for both space- and
ground-based spectrographs. As diffraction takes place inside the high-index
medium, the optical path difference and angular dispersion are boosted
proportionally, thereby allowing a smaller grating area and a smaller
spectrometer size. Short-wave infrared (SWIR) spectroscopy is used in
space-based monitoring of greenhouse and pollution gases in the Earth
atmosphere. On the extremely large telescopes currently under development,
mid-infrared high-resolution spectrographs will, among other things, be used to
characterize exo-planet atmospheres. At infrared wavelengths, Silicon is
transparent. This means that production methods used in the semiconductor
industry can be applied to the fabrication of immersed gratings. Using such
methods, we have designed and built immersed gratings for both space- and
ground-based instruments, examples being the TROPOMI instrument for the
European Space Agency Sentinel-5 precursor mission, Sentinel-5 (ESA) and the
METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European
Extremely Large Telescope.
  Three key parameters govern the performance of such gratings: The efficiency,
the level of scattered light and the wavefront error induced. In this paper we
describe how we can optimize these parameters during the design and
manufacturing phase. We focus on the tools and methods used to measure the
actual performance realized and present the results. In this paper, the
bread-board model (BBM) immersed grating developed for the SWIR-1 channel of
Sentinel-5 is used to illustrate this process. Stringent requirements were
specified for this grating for the three performance criteria. We will show
that -with some margin- the performance requirements have all been met.
