Have managed to get round this problem by modifying grub as follows:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux
acpi_backlight=vendor"
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Public bug reported:
Grub menu displayed normally. On boot screen appears dark but brightness
can be increased by laptop fn keys.
ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: xorg 1:7.7+1ubuntu4
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic i686
.tm
