7.25 Optical Holography
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It is also possible to generate a similar hologram by a computer calculation
(‘digital hologram’) of what the interference pattern should be and then printing
that pattern on a two-dimensional surface. At the simplest level, scratching a surface
with arcs of a compass whose center is placed at various points of an object will
make the equivalent of a Huygens-constructed interference pattern. Illuminating
those scratches with a point-like source filtered for one color reveals a holographic
image. This construction is essentially what the computer algorithm does.
The fact that a three-dimensional (3-D) image can be recorded on a twodimensional surface can be thought of in following way: Imagine light passing
through a window. One can look through the window and see a three-dimensional
world beyond. Now imaging recording all the information that passes through the
window in a very short time. Then that information should be capable of reproducing
the three-dimension view one would have seen in that short instance of time. Indeed,
in viewing a hologram, one does not look at the surface where the interference
pattern exists, but rather one looks ‘through’ the film at the image beyond (i.e. the
eyes must focus beyond the film of the hologram). Moving one’s head to a new
position while looking through the hologram will allow viewing the object from a
different angle than before. Also, if part of the window becomes obscured, one can
still see objects through the unobscured part of the window. Holograms have the
same property. This means they have significant redundancy, and store delocalized
information. (See Appendix C for a description of how wave interference creates a
hologram.)
Digital Holographic Microscopy
If monochromatic light scattered from regions on and within a three-dimensional
object is used to create a recorded interference pattern on a two-dimensional
digital sensor array, the system is called a digital holographic microscope. Unlike
traditional photographs recording just the intensity of light on a surface, information
stored holographically contains the intensity and phase information sufficient to
reconstruct a three-dimensional representation of the object, and, due to its speed of
recording, time sequences can be stored (sometimes referred to as a 4-D imaging).
A ‘digital in-line holographic microscope’ (DIHM) is constructed using a simple
LED laser, with its light passing through a pinhole (about a micron in diameter)
into a medium with objects or microbes of interest (about a millimeter or more
from the hole), and then to a CCD attached to a recorder. Image reconstruction is
performed by a computer (rather than secondary illumination of a hologram). Apart
from the computer, the device itself can be made relatively cheaply and compactly.
DIHMs have successfully and non-invasively recorded, in situ and in real time, live
microscopic marine organisms, with both lateral and depth resolution far better than
microscopes with just lenses.
255
It is also possible to generate a similar hologram by a computer calculation
(‘digital hologram’) of what the interference pattern should be and then printing
that pattern on a two-dimensional surface. At the simplest level, scratching a surface
with arcs of a compass whose center is placed at various points of an object will
make the equivalent of a Huygens-constructed interference pattern. Illuminating
those scratches with a point-like source filtered for one color reveals a holographic
image. This construction is essentially what the computer algorithm does.
The fact that a three-dimensional (3-D) image can be recorded on a twodimensional surface can be thought of in following way: Imagine light passing
through a window. One can look through the window and see a three-dimensional
world beyond. Now imaging recording all the information that passes through the
window in a very short time. Then that information should be capable of reproducing
the three-dimension view one would have seen in that short instance of time. Indeed,
in viewing a hologram, one does not look at the surface where the interference
pattern exists, but rather one looks ‘through’ the film at the image beyond (i.e. the
eyes must focus beyond the film of the hologram). Moving one’s head to a new
position while looking through the hologram will allow viewing the object from a
different angle than before. Also, if part of the window becomes obscured, one can
still see objects through the unobscured part of the window. Holograms have the
same property. This means they have significant redundancy, and store delocalized
information. (See Appendix C for a description of how wave interference creates a
hologram.)
Digital Holographic Microscopy
If monochromatic light scattered from regions on and within a three-dimensional
object is used to create a recorded interference pattern on a two-dimensional
digital sensor array, the system is called a digital holographic microscope. Unlike
traditional photographs recording just the intensity of light on a surface, information
stored holographically contains the intensity and phase information sufficient to
reconstruct a three-dimensional representation of the object, and, due to its speed of
recording, time sequences can be stored (sometimes referred to as a 4-D imaging).
A ‘digital in-line holographic microscope’ (DIHM) is constructed using a simple
LED laser, with its light passing through a pinhole (about a micron in diameter)
into a medium with objects or microbes of interest (about a millimeter or more
from the hole), and then to a CCD attached to a recorder. Image reconstruction is
performed by a computer (rather than secondary illumination of a hologram). Apart
from the computer, the device itself can be made relatively cheaply and compactly.
DIHMs have successfully and non-invasively recorded, in situ and in real time, live
microscopic marine organisms, with both lateral and depth resolution far better than
microscopes with just lenses.