  We study the covariant holographic entropy bound from an operational
standpoint. Therefore we consider the physical limit for observations on a
light-sheet. A light-sheet is a particular null hypersurface, and the natural
measuring apparatus is a screen. By considering the physical properties of the
screen - as dictated by quantum mechanics - we derive an uncertainty relation.
This connects the number of bits of decoded information on the light-sheet to
two geometric uncertainties: the uncertainty on the place where the bits are
located and the uncertainty on the local expansion of the light-sheet. From
this relation we can argue a local operational version of the (generalized)
covariant entropy bound: the maximum number of bits decoded on a light-sheet
interval goes like the area difference (in Planck units) of the initial and
final surface spanned by the light rays.
