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29.1 Introduction
To situate the interest of digital holographic microscopy (DHM)
in biology, let us recall that, in the past, phase contrast micros-
copy
(Zernike 1937, 1942, 1949, Françon 1949) has played a domi-
nant role in the observation of living cells and has permitted the
rst precise determination of the refractive index of living cells
(refractometry) (Barer and Joseph 1954, Barer 1957). At that time,
the measuring procedure was quite complex and usually required
the adjustment of the refractive index of an external medium in
contact with the cell. ese determinations have allowed estab-
lishing a precise relationship between the ...