  The basic outline of a pupil plane WaveFront Sensor is reviewed taking into
account that the source to be sensed could be different from an unresolved
source, i.e. it is extended, and that it could deploy also in a 3D fashion,
enough to exceed the field's depth of the observing telescope. Under these
conditions it is pointed out that the features of the reference are not
invariant for different position on the pupil and it is shown that the INGOT
WFS is the equivalent of the Pyramid for a Laser Guide Star. Under these
conditions one can imagine to use a Dark WFS approach to improve the SNR of
such a WFS, or to use a corrected upward beam in order to achieve a better use
of the LGS photons with respect to an ideal Shack-Hartmann WFS.
