The excitation filters on our Biorad 600 show discoloration in the
middle.  We have used a standard Ar-ion laser for about 6 years. Looking
through the filters one can see no difference in
transmission across the diameter.  We borrowed a new filter set and
noticed bigger fluorescence signals.  There may be a problem with both
excitation and emission filters.  I was told that dichroic mirrors are
much longer lived than filters. Can anyone confirm that?
Milton Charlton