If you look at
a beam of light whether projected from your enlarger's negative
carrier or a shaft of light shinning across a room you will notice
lots of particals floating in the air. Dust over the course of time
will settle on the upper surface's of your enlarger and lens.
It does not take
long for a considerable amount of dust to build up on the surface of
your lens greatly reducing the brilliance of light passing through
it. You should make it a rule that once or twice a year you clean the
lens thoughly. It is also a good idea to cover the enlarger at the
end of each session with its cover if it has one or a black plastic
bag this will greatly reduce the dust build up.
If you have a
window in your darkroom that needs to be blacked out it is not a good
idea to use curtains as these are dust traps. You are better off
using a wooden shutter made of hard board painted matt black or a
roller blind made of light tight material that has some way of being
sealed along the sides.
What you have on
the floor is also important. If it is carpet it needs to be removed,
as this is a dust trap too. It should be an easy clean material like
lino if you have floor boards or garage floor paint for concrete
floors these keep the dust at bay making it easy to mop the floor
with water, it also means that you do not have to vacuum very often
leaving more time to print.