  The Visible Integral Field Replicable Unit Spectrograph (VIRUS) is an array
of at least 150 copies of a simple, fiber-fed integral field spectrograph that
will be deployed on the Hobby-Eberly Telescope (HET) to carry out the HET Dark
Energy Experiment (HETDEX). Each spectrograph contains a volume phase
holographic grating as its dispersing element that is used in first order for
350 nm to 550 nm. We discuss the test methods used to evaluate the performance
of the prototype gratings, which have aided in modifying the fabrication
prescription for achieving the specified batch diffraction efficiency required
for HETDEX. In particular, we discuss tests in which we measure the diffraction
efficiency at the nominal grating angle of incidence in VIRUS for all orders
accessible to our test bench that are allowed by the grating equation. For
select gratings, these tests have allowed us to account for > 90% of the
incident light for wavelengths within the spectral coverage of VIRUS. The
remaining light that is unaccounted for is likely being diffracted into
reflective orders or being absorbed or scattered within the grating layer (for
bluer wavelengths especially, the latter term may dominate the others).
Finally, we discuss an apparatus that will be used to quickly verify the first
order diffraction efficiency specification for the batch of at least 150 VIRUS
production gratings.
