  This paper considers some simple surface brightness (SB) estimates for
galaxies in the Automated Plate Measuring Machine (APM) catalogue in order to
derive homogeneous SB data for a very large sample of faint galaxies. The
isophotal magnitude and area are used to estimate the central surface
brightness and total magnitude based on the assumption of an exponential SB
profile. The surface brightness measurements are corrected for field effects on
each UK Schmidt plate and the zero-point of each plate is adjusted to give a
uniform sample of SB and total magnitude estimates over the whole survey.
Results are obtained for 2.4 million galaxies with blue photographic magnitudes
brighter than b_J = 20.5 covering 4300 deg^2 in the region of the south
galactic cap. Almost all galaxies in our sample have central surface brightness
in the range 20 to 24 b_J mag per arcsec^2. The SB measurements we obtain are
compared to previous SB measurements and we find an acceptable level of error
of +/- 0.2 b_J mag per arcsec^2. The distribution of SB profiles is considered
for different galaxy morphologies for the bright APM galaxies. We find that
early-type galaxies have more centrally concentrated profiles.
