ABSTRACT Stream-stream collisions play an important role in the circularization of highly eccentric streams that result from tidal disruption events (TDEs). We perform three-dimensional radiation hydrodynamic simulations to show that stream collisions can contribute significant optical and ultraviolet light to the flares produced by TDEs, and can explain the majority of the observed emission. Our simulations focus on the region near the radiation-pressure-dominated shock produced by a collision and track how the kinetic energy of the stream is dissipated by the associated shock. When the mass flow rate of the stream is a significant fraction of the Eddington accretion rate, 2% of the initial kinetic energy is converted to radiation as a result of the collision. In this regime, the collision redistributes the specific kinetic energy into the downstream gas and more than 16% of the mass can become unbound. The fraction of unbound gas decreases rapidly as drops significantly below the Eddington limit, with no unbound gas being produced when drops to 1% of Eddington; we find, however, that the radiative efficiency increases slightly to 8% in these cases of low . The effective radiation temperature and size of the photosphere are determined by the stream velocity and , and we find them to be a few times 104 K and 1014 cm in our calculations, comparable to the values inferred for some TDE candidates. The size of the photosphere is directly proportional to , which can explain its rapidly changing size as seen in TDE candidates such as PS1-10jh.