  In this paper, we report measurements of diffraction efficiency and angular
dispersion for a large format (~ 25 cm diameter) Volume-Phase Holographic (VPH)
grating optimized for near-infrared wavelengths (0.9 -- 1.8 micron). The aim of
this experiment is to see whether optical characteristics vary significantly
across the grating. We sampled three positions in the grating aperture with a
separation of 5 cm between each. A 2 cm diameter beam is used to illuminate the
grating. At each position, throughput and diffraction angle were measured at
several wavelengths. It is found that whilst the relationship between
diffraction angle and wavelength is nearly the same at the three positions, the
throughputs vary by up to ~ 10% from position to position. We explore the
origin of the throughput variation by comparing the data with predictions from
coupled-wave analysis. We find that it can be explained by a combination of
small variations over the grating aperture in gelatin depth and/or refractive
index modulation amplitude, and amount of energy loss by internal absorption
and/or surface reflection.
