A requirement for a minimum viewing box brightness of 3000 cd m(-2) fo
r reading mammograms has been widely advocated. Some recent work has c
hallenged that opinion by reporting no significant variation in visibi
lity of low contrast and fine detail objects over a wide range of brig
htness levels. This paper provides further experimental evidence to su
pport the latter conclusion, at least over the range 1340-4190 cd m(-2
), and suggests that the currently recommended minimum viewing box bri
ghtness levels need to be revised. The importance of reducing room lig
hting levels is fully confirmed.