The detection and characterization of antineutrino emissions from nuclear reactors is a topic of great interest to the fundamental physics and nuclear nonproliferation communities. Reactor antineutrino spectra contain fuel-composition information, and can be used to infer fundamental physical parameters. The detection of reactor antineutrinos using the inverse beta decay (IBD) reaction has been proposed as a means of long-standoff reactor monitoring using kiloton-scale water-Cherenkov detectors. Detector fill materials capable of improved energy resolution and vertex reconstruction in comparison to water are an area of vigorous research, as they may lower detection thresholds and reduce backgrounds. One such candidate material is water-based liquid scintillator (WbLS), a hybrid detector medium which promises water-like attenuation and scattering behavior, while offering tuneable scintillation light yield. This dissertation focuses on the development of an optical property measurement device for IBD detector media including WbLS. The horizontal, adjustable path-length “long-arm” attenuation system developed for this application provides complete isolation from the atmosphere, a high degree of vibration insensitivity, and a simple and reliable method of calibration. Scattering measurements are available at multiple angles and with adjustable beam polarization, enabling both reliable quantitative scatter measurements and scattering phase-function reconstruction. The scattering and attenuation measurement performance of the device was benchmarked using deionized water. Subsequently, the system was used to quantify the attenuation, scattering and depolarization of diverse candidate fill materials throughout the visible spectrum. This work focused in large part on accurately characterizing WbLS produced by Brookhaven National Laboratory, which is a leading candidate fill material for nonproliferation-focused antineutrino detectors. This work represents the first high-sensitivity measurements of WbLS optical properties, and provides insight into the viability of Cherenkov-scintillation separation at scale. Additional work has been performed to characterize leaching of stainless steel components by Gd-loaded and non-Gd WbLS which may impact optical properties.