  We present the optical luminosity function (LF) of gamma-ray bursts (GRBs)
estimated from a uniform sample of 58 GRBs from observations with the Robotic
Optical Transient Search Experiment III (ROTSE-III). Our GRB sample is divided
into two sub-samples: detected afterglows (18 GRBs), and those with upper
limits (40 GRBs). The $R$ band fluxes 100s after the onset of the burst for
these two sub-samples are derived. The optical LFs at 100s are fitted by
assuming that the co-moving GRB rate traces the star-formation rate. The
detection function of ROTSE-III is taken into account during the fitting of the
optical LFs by using Monte Carlo simulations. We find that the cumulative
distribution of optical emission at 100s is well-described with an exponential
rise and power-law decay (ERPLD), broken power-law (BPL), and Schechter LFs. A
single power-law (SPL) LF, on the other hand, is ruled out with high
confidence.
