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We have tested many of the systems on the market including the SOLA, Colibri
and LED120. We work a lot with live cells and in my opinion these sources
are all way brighter than they need to be. We could not image our live cells
without attenuation because they do not move! We use ND filters to work with
0.1% of the blue excitation for EGFP for example.

So in my experience they are plenty bright. 

We find longer exposures with lower power (even for fixed cells) gets much
more light out of the fluorophores because of the reduced bleaching.