IEEE Consumer Electronics Magazine - September/October 2019 - 71

Figure 3. Snell-Descartes reflections laws lead to
an large mirror.

In some existing aircraft HUDs, Holographic
combiners are also used to reflect the light
toward the eyebox. Such HUDs usually present a
large off-axis of the combiner, resulting in a very
large astigmatism, which can be partly corrected
through the use of Holographic Optical Elements
(HOEs).
But in these two cases, Holographic components represent only a part of the optical system,
and not the whole system, as proposed in this
paper.
Instead of using conventional refractive or
reflective optics, Diffractive or HOEs could be
used. By using one specific order of diffraction,
it is possible to reflect light in a given direction,
and/or add optical power.
One possible application is direct projection
on windshield. With only one HOE, installation
constraints are reduced, and only one small projector is required to display the HUD symbology.
Wu et al.7 presents such a system, using a small
UV laser projector, and a windshield including a
layer with transparent phosphor.
With the use of HOE, we take the benefit of
the spectral selectivity of Bragg gratings, providing an efficient reflection when using a narrow
bandwidth illumination source.
When the HUD illumination source match with
the HOE reflection peak, most of the energy coming from the projector will be reflected towards
the pilot, with limited loss in transmission.
One remaining issue is to achieve, when
required, the "reflective-diffusive" function of
the screen. This can again be done through the
use of HOE.8
At each point on the screen, a specific optical
function enables the incoming light to be
spread over the required solid angle at a given

September/October 2019

Figure 4. HUD mirror size reduced by using HOE
first order of reflection.

wavelength. This function is performed by a surface grating computed and simulated by software (Computer Generated Hologram).

DISCUSSION
The image is formed on the windshield, near
the driver's eyes. It raises several issues, particularly the mismatch between real world and
images presented at short distance from the
eyes (Vergence-Accomodation Conflict).2
Taking into account this limitation, such a
system could still be used for displaying non
conformal information, such as speed or vehicle
parameters.
Today, ARHUD requires conformal image,
and henceforth, collimated images as most of
the external world is far away from the eye. This
is performed with a HUD box as described in
Figure 3, and needs very large mirror(s).
Again, in this case, HOE could solve this issue.
The mirror sizes typically depend on ray's incidence that follows Snell-Descartes reflection laws.
But, with a HOE, it is possible to use a diffraction order that will not follow the Cartesian
condition, and reflect the light at different incidences. Figure 4 shows a simple schematic
example, showing an outstanding reduction of
the mirror size.
This is a schematic principle; in fact, chromatic
aberration occurs on such architecture, and a
more complex optical system is needed to compensate this effect. But, the result remains the same;
the HUD box size will be dramatically reduced.
In this example, the HOE is a Bragg grating
and is only used as an angle deflector, but HOE

IEEE Consumer Electronics Magazine - September/October 2019

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