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2.4 RSD (retinal scanning display)
RSD scans the light rays coming from an object into a raster structure, pixel
by pixel, directly to the optic nerve through the cells in the retina of the
person who is looking at the object. This gives high-quality digital images,
wide and bright, without additional displays.
RSD is made of one light emitter, one modulator, two scanners (horizontal
and vertical) and optical components [2]. The light modulator regulates the
photon stimulation intensity of retinal receptors. The scanners are synchronized
with the light modulator by means of some electronic video components.
The optical components magnify images and make the scans converge to
achieve an optical focus [3].
RSD can be very effective in AR. Its direct projection gives the highest
image definition and a wide visual field. A RSD device is very light and
cheap. Display brightness is directly regulated by the scanned ray and,
therefore, it can be used even in bright environments as, for instance, a sunny
place.
2.5 HHD (handheld display)
HHDs are unlikely to be compliant with AR philosophy and need drawing our
attention more to the reality around than to the devices we use.
HHDs can be considered as advanced personal digital assistants (PDAs).
They are equipped with LCD display and embedded video camera to get real
video superimposed by digital images.
In spite of their non-compliance with AR philosophy, HHDs can help AR
diffusion, because their technology is already available in PDAs and smart
phones which are likely to be equipped with AR software in the near future.
2.6 SAR (spatially augmented reality)
A real environment can be augmented by integrating virtual objects in it. For
instance, digital projectors can create images on walls and digital displays
can be installed anywhere.
The main advantage of SAR, in comparison with HMD and HHD, is the
capability of freeing users from carrying unusual hardware with them, thus
leaving people to immerse themselves in AR and interact with it.
Several SAR systems have been tested so far with 2D and 3D floating
images projected on planes or irregular real objects. Some tests were
carried out with one projector, others with more than one. The simplest one
is with one projector without any concern with user position. SAR suffers
from alignment problems when 3D images are projected from different
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