Digital Holographic Microscopy (DHM) for Measuring Biophysical Parameters of Living Cells - Biomedical Optical Phase Microscopy and Nanoscopy - page 70

5.2 Technical Introduction

5.2.1 Classical Holography

Holography techniques were developed by Gabor in 1948 with the aim of improving the
detection of spatial resolution in the X-ray wavelength by exploiting its lensless imaging
capabilities. This resulted in the possibility of generating, during the illumination of the
recorded hologram (reconstruction process), an exact replica, with a specific
magnification, of the full object wave front created by the observed specimen [7,83] .
However, as already identified by Gabor, the imaging possibilities of holography are
greatly reduced in quality because of the presence of different diffraction orders in the
propagation of the diffracted wave front by the hologram when illuminated during the
reconstruction process. This was resolved by Leith and Upatneiks [84,85] , who proposed
the use of a reference wave of a slightly different propagation direction than the object
wave. This method, referred to as off-axis geometry, has been analyzed from a
computational point of view [86] with a formalism based on diffraction, and is used in the
first quantitative phase measurements [87] .

However, the first developments in off-axis configuration were performed through a
common-path configuration due to the coherence limitation of the light sources. Moreover,
the emergence of laser light sources, enabling very long coherence lengths with high power,
takes advantage of the versatility of “interferometric configuration.” Note that short
coherence length sources have been investigated recently in various cases [88 90] . Shorter

coherence lengths have the capability to improve the lateral resolution as well as to
decrease the coherence noise which could limit the quality of the reconstructed image,
particularly the phase sensitivity. However, these low coherence implementations require
more complicated arrangements where the coherence zone (in both spatial and temporal
domains) must be adapted to ensure optimal interference [91] .

5.2.2 From Classical to Digital Holography

The use of digital means in holography gradually occurred at the end of 1960s, when
Goodman and Lawrence [92] used a vidicon detector to encode a hologram, which could be
reconstructed on a computer. However, the interest in digital holography rose with the
availability of cheaper digital detectors and charge-coupled device (CCD) cameras. The use
of CCD cameras for holographic applications was validated in the mid 1990s, in the case of
reflection macroscopic holograms [93] and microscopic holograms in endoscopic
applications [94] .

Another approach for hologram reconstruction was developed by taking advantage of the
capability of digital detectors to rapidly record multiple frames, through the use of a
phase-shifting technique [95] as developed first for interferometry [96,97] .