C Holography
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of digital sensor arrays, and the creation of video sequences to see the motion of
objects with no depth of focus problems.
C.4 Some Special Properties of Holograms
In order to make a hologram to form in the first place, the light source must have
spatial and temporal coherence, so that the phase of the scattered light from the
object is correlated with the phase of the reference beam. Temporal coherence can
be achieved by using nearly one frequency of light. The distance over which light
from a given source split into two beams keeps a measurable phase relationship
between the two beams is called the light ‘coherence length’. 2
Light from an incandescent source at 3000 degrees Kelvin has a coherence length
of about 1.2 μm. Laser light can have a coherence length of kilometers, although
typical sources have coherence lengths in centimeters to meters. If the object one
intends to use for a hologram is more distant from the film than the coherence
length, the hologram fails. Also, if any part of the film moves more than a half of
the wavelength of the light during exposure, that part will not record an interference
pattern. For a flexible film, low level sound in the room can ruin the creation of a
hologram. Similarly, the object must be rigidly held to within the same limitations.
The illumination of a hologram after it is made need not be as coherent as the
light that produced the hologram. There are various strategies for using point-like
light sources (even the Sun) and color discriminators to produce 3-D images from a
hologram.
As we noted in the description of optical holograms, they are able to store
information about objects with redundancy spread over an area. The storage includes
not just illumination of the object, but also relative phases between light scattered
from various points on the object, and also, if the object has some transparency, the
distribution of density variations and location of scattering point are recorded.
C.5 Holographic Memory Storage
Note that holograms store the information about an object non locally, i.e. the
change of one point on the object does not produce one local change in the
hologram. Moreover, the information is redundant, in that every finite region of
2 Specifically, a light source is split into two beams and then brought back together to allow the
beams to interfere. One of the beams is now made to travel an extra distance more than the other.
The maximum intensity in the resulting interference fringe pattern minus the minimum divided
the maximum plus the minimum, called the ‘visibility’, is measured. At zero extra distance, the
visibility is one. The extra distance when the visibility drops to one half is defined as the ‘coherence
length’.
639
of digital sensor arrays, and the creation of video sequences to see the motion of
objects with no depth of focus problems.
C.4 Some Special Properties of Holograms
In order to make a hologram to form in the first place, the light source must have
spatial and temporal coherence, so that the phase of the scattered light from the
object is correlated with the phase of the reference beam. Temporal coherence can
be achieved by using nearly one frequency of light. The distance over which light
from a given source split into two beams keeps a measurable phase relationship
between the two beams is called the light ‘coherence length’. 2
Light from an incandescent source at 3000 degrees Kelvin has a coherence length
of about 1.2 μm. Laser light can have a coherence length of kilometers, although
typical sources have coherence lengths in centimeters to meters. If the object one
intends to use for a hologram is more distant from the film than the coherence
length, the hologram fails. Also, if any part of the film moves more than a half of
the wavelength of the light during exposure, that part will not record an interference
pattern. For a flexible film, low level sound in the room can ruin the creation of a
hologram. Similarly, the object must be rigidly held to within the same limitations.
The illumination of a hologram after it is made need not be as coherent as the
light that produced the hologram. There are various strategies for using point-like
light sources (even the Sun) and color discriminators to produce 3-D images from a
hologram.
As we noted in the description of optical holograms, they are able to store
information about objects with redundancy spread over an area. The storage includes
not just illumination of the object, but also relative phases between light scattered
from various points on the object, and also, if the object has some transparency, the
distribution of density variations and location of scattering point are recorded.
C.5 Holographic Memory Storage
Note that holograms store the information about an object non locally, i.e. the
change of one point on the object does not produce one local change in the
hologram. Moreover, the information is redundant, in that every finite region of
2 Specifically, a light source is split into two beams and then brought back together to allow the
beams to interfere. One of the beams is now made to travel an extra distance more than the other.
The maximum intensity in the resulting interference fringe pattern minus the minimum divided
the maximum plus the minimum, called the ‘visibility’, is measured. At zero extra distance, the
visibility is one. The extra distance when the visibility drops to one half is defined as the ‘coherence
length’.