Biomedical Engineering Reference
In-Depth Information
Fig. 15
Scheme of Ir.M.A. system
plate is able to record fine interference images. An important condition for hologram
recording and reconstruction is the monochromatic and coherent character of the
light source for maintaining a high quality hologram.
If the image is recorded in the sensitive layer without a reference beam, it is only a
brightness image similar to a photo. The beam phase is recorded by the interference
of the object beam with the reference beam. The resulting interference patterns
represent the intensity and phase component of the object beam. The holographic
object is reconstructed by illuminating the holographic plate with a monochromatic
reconstruction beam with the same geometrical position as the reference beam.
This makes it possible to reconstruct the virtual object at the original distance
from the holographic plate as with the recording. In addition, holography enables
several different objects to be registered on one plate. If two objects at different
distances were to be recorded on a holographic plate, then each object's distance
will be recorded separately. Through reconstruction, the sources are situated in
the same geometrical position as the reference sources recorded. By changing the
reconstruction sources, it is possible to display two separate objects with the stepped
change of virtual distance.
In the case of stimulating the patients' visual apparatus, the distance difference
is in range of metres (for E.M.AN. it is 2.75 m, for Ir.M.A. 1.81 m). Therefore,
space requirements make it impossible to be used in the usual conditions of
ophthalmologist's office. Holographical stimulation used in Ir.M.A. allows space
requirements to be minimised for virtual stimulating objects. Figure
16
gives
an example of reconstructed holographic objects. The real distance between the
holographic plate and the reconstruction beam source is 0.6 m.
A further goal of research is realising a system with moving stimulating objects.
A few objects positioned in the near and far planes will be sequentially reconstructed
to give the impression of moving objects. A moving fixation object will be more
