Frontside Illumination means the photons are indicident
upon the electrode (gate) side of the CCD.   Polysilicon
layer has a typical thickness of 0.5 microns and hence
photons with wavelength shorter than this (i.e. Blue Band)
will interact with this layer producing an interference
pattern.  The preferential absorption of blue photons means
low QE in the blue.

This can be circumvented using backside illumination where
the photons are incident on the substrate side.  In order
for blue photons to reach the gates, however, the CCD must
be thinned down a layer of about 10 microns in thickness.
The thinning must be uniform.  For todays 2048x2048 we
are talking about an active area of 51200x51200 microns all
uniformly thinned.  This is non-trivial.