Despite the large interest in the application of ${\rm LaBr}_{3}$ ${:}$${\rm Ce}^{3+}$, little is known yet about its optical properties, as measurements are hampered by the hygroscopicity of ${\rm LaBr}_{3}$ ${:}$${\rm Ce}^{3+}$ and because it is not trivial to produce crystals with optically polished surfaces. Here, the absorption and scattering lengths as well as the refractive index of ${\rm LaBr}_{3}$ ${:}$$5\%{\rm Ce}^{3+}$ are determined experimentally for the first time. The refractive index is found to vary from 2.25 to 2.40 in the ${\rm Ce}^{3+}$ emission wavelength region depending on crystal orientation. Furthermore, a model of the ${\rm Ce}^{3+}$ absorption and emission probability as a function of wavelength, ${\rm Ce}^{3+}$ concentration, and scintillation photon traveling distance is developed. This model is used in combination with the measured absorption and scattering lengths to obtain the intrinsic emission spectrum of ${\rm LaBr}_{3}$ ${:}$${\rm Ce}^{3+}$ from measured emission spectra. Additionally, the model is used to illustrate the importance of the investigated crystal properties for scintillation detector design. It is demonstrated that for crystals with dimensions in the order of a few centimeters, the fraction of scintillation photons undergoing scattering and/or absorption before reaching the photosensor can be several tens of percents depending on the ${\rm Ce}^{3+}$ concentration. Finally, it is shown that self-absorption and re-emission of the scintillation photons can have a non-negligible effect on the timing resolution of ${\rm LaBr}_{3}$ ${:}$${\rm Ce}^{3+}$ scintillator detectors.