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The commonly and widely
used way of imaging of the reality is the photography. A photograph is basically the
recording of the differing intensities of the light reflected by the object  and
imaged by a lens. However, information about dimensions of the object contained not only
in amplitude (intensity), but also in a phase of light waves.
A great difference
between holography and photography is the information recorded. This difference is why
photographs are two dimensional (2-D) images while holograms are three dimensional (3-D)
images. Photographs contain only one view point of an object. Our eyes need a minimum of
two view points in order to see depth. Vision using two viewpoints of an object is called
stereoscopic vision. Each eye receives a slightly different view point of an object, our
brain combines the two and we perceive depth. We can fool our eyes into seeing photographs
in three dimensions by taking two slightly different views of an object and allowing each
eye to see only one image, the right image for the right eye and the left image for the
left eye. We can do this with a stereoscope (for pictures) or with polarized glasses (for
movies). The shortcoming of stereoscopic images is that when we move our head from side to
side or up and down, we still only see the same two view points, whereas we should be
seeing continuously changing viewpoints of the object. The image therefore doesn't quite
appear to be three dimensional. In order to make a record of a three dimensional object we
need to record this continuous set of viewpoints of the object.
Estimating sizes of the
objects and considering shape  and direction of shadows from these objects, we can
create in our mind general representation about volumetric properties of the scene,
represented in a photo. But, if sizes of the objects are identical and there are no
shadows, volumetric content of the photographed scene is completely lost. For example, we
can not define in the photo of snowflakes on a dark background, which of them is closer,
and which of them is farther.
Holography is the only
visual recording and playback process that can record our three-dimensional world on a
two-dimensional recording medium and playback the original object or scene to the unaided
eyes as a three dimensional image. The image demonstrates complete parallax and
depth-of-field and floats in space either behind, in front of, or straddling the recording
medium.
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