Biomedical Engineering Reference
In-Depth Information
9.2.6 Going Digital
By digital holography, we refer to all holographic techniques in which the image reconstruction is per-
formed numerically, from a digital hologram. With this definition, we exclude the field of computer-
generated holograms, in which holograms are digitally synthesized and exported to physical form for
optical image reconstruction.
Digital hologram recording, and subsequent numerical image reconstruction, is attributed to
Goodman and Lawrence (1967) and was first demonstrated in 1967. Their work was motivated by the
desire to obtain an electronic detection of holograms at the earliest possible opportunity, that is, when
the signal-to-noise ratio is the highest, to provide the high sensitivity needed to detect very weak objects
of small angular subtense.
Because digital holography differs from its classical counterpart on both hologram recording and
reconstruction steps, it is no surprise that it offers unique advantages and drawbacks related to these two
key steps of the holographic process. Compared to classical holography, digital holography offers many
advantages for hologram recording. For one, digital format makes possible stable, long-term storage of
holograms.
Much more importantly though, the time for hologram recording and reconstruction has dropped
by a few orders of magnitude. Where it took hours, or at least minutes, to correctly develop and opti-
cally reconstruct a photographic plate, transferring a frame from the sensor to the computer, which
makes the numerical reconstructions, is now a matter of milliseconds. Thanks to the advent of personal
computers, made more accessible in the 1980s, and to the emergence of integrated solid-state digital
sensors, live, video-rate reconstruction of holograms is now easily achievable. With these technological
advances, digital holography has seen an increasing interest that fueled the development of the field and
really allowed it to take off.
The ease of changing the exposure time of the sensor should not be neglected, as it gives, in combi-
nation with live histogram display, the tools to set the right exposure level. This is rather important as
best results are obtained for highest possible intensities that do not result in overexposure (saturation)
of some pixels. Overexposure should be avoided, because it introduces nonlinearities in the signal by
adding harmonics into the hologram spectrum. It is considered as a noise source which can fortunately
be avoided by appropriate exposure adjustment, which is made easy by the digital technology.
The one big challenge intrinsic to digital holography is the adequate separation of diffraction orders
(imaging and zero-order terms). Digital sensors are, to this day, essentially 2D detectors that cannot
record volume holograms. All information about the object wavefront therefore has to be recorded on
this 2D surface which possesses relatively large pixel pitches that limit the maximum off-axis angle still
resulting in appropriately sampled interference fringes. Overall, these characteristics of digital sensors
limit the separation power of diffraction orders in digital holography. Fortunately, judicious choices in
the optical configuration may still allow separation of diffractions orders by off-axis scheme. Other ded-
icated methods, such as phase-shifting interferometry, were also developed to overcome the problem.
Overall, the digital era, by making holography capable of high-speed imaging, has made it very
appealing for a wide range of new applications requiring fast frame rates. In principle, it also made
holography capable of truly exploiting the ultrafast nature of nonlinear light-matter interactions, which
is of great interest for SHG imaging.
9.3 Recording of Digital SHG Holograms
As proposed by Goodman, digital holography involved recording of holograms on an orthicon, that is,
a video camera tube similar in operation to a cathode ray tube. Such pixel-scanning recording devices
convert light intensity to an electrical signal that can in turn be digitalized. For example, Goodman's
orthicon converted the optical hologram to a 256 × 256 pixels digital hologram with intensity values
quantized to eight gray levels (3-bits) only.
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