A photometric analysis of the sub-structure of a granular light bridge in a large solar pore is performed. The data consist of a 66 min time series of white light images (lambda = 5425 Å,+/- 50 Å) of an active region NOAA 7886 obtained at the Swedish Vacuum Solar Telescope on La Palma, Canary Islands. The light bridge can be resolved into an assembly of small grains embedded in a diffuse background with an intensity of about 85% of the mean photospheric intensity (Iphot). Following the temporal evolution of these sub-structures in their irregular motions inside the light bridge, proper motions with velocities up to 1.5 km s-1 can be detected. Their lifetime distribution shows a maximum at 5 min and a second peak at approximately 20 min. The origin and the decay of these sub-structures is very similar to those of granules, i.e. fragmentation, merging and spontaneous origination from, and dissolution into, the background can be observed. Some of them are able to escape from the light bridge into the umbra where they cannot be distinguished from adjacent umbral dots. Generally, this study presents evidence that the observed phenomenon represents convective motions.