  We have obtained Gemini Planet Imager (GPI) J-, H-, K1-, and K2-Spec
observations of the iconic debris ring around the young, main-sequence star HR
4796A. We applied several point-spread function (PSF) subtraction techniques to
the observations (Mask-and-Interpolate, RDI-NMF, RDI-KLIP, and ADI-KLIP) to
measure the geometric parameters and the scattering phase function for the
disk. To understand the systematic errors associated with PSF subtraction, we
also forward-modeled the observations using a Markov Chain Monte Carlo
framework and a simple model for the disk. We found that measurements of the
disk geometric parameters were robust, with all of our analyses yielding
consistent results; however, measurements of the scattering phase function were
challenging to reconstruct from PSF-subtracted images, despite extensive
testing. As a result, we estimated the scattering phase function using disk
modeling. We searched for a dependence of the scattering phase function with
respect to the GPI filters but found none. We compared the H-band scattering
phase function with that measured by Hubble Space Telescope STIS at visual
wavelengths and discovered a blue color at small scattering angles and a red
color at large scattering angles, consistent with predictions and laboratory
measurements of large grains. Finally, we successfully modeled the SPHERE H2 HR
4796A scattered phase function using a distribution of hollow spheres composed
of silicates, carbon, and metallic iron.
