IEEE Consumer Electronics Magazine - September/October 2019 - 70

Special Section on Human Centric AR&VR Display and Human Interface Technologies for Automobile

Table 1. HUD size and volume.
FOV

Width

Depth

High

Volume
(liter)

7Â4

200

125

200

5

14Â6

325

175

250

14

35Â20

600

400

350

84

contributes to the user experience. Also of intertendue": the
est is the notion of "geometric e
tendue can be roughly described as the prode
uct of the EMB (surface) by the FOV (solid
tendue provides a good measure of
angle). The e
the intrinsic complexity of the optical system
required to achieve simultaneously FOV and
EMB.
According to the principle of conservation of
tendue, the value of the e
tendue in the
the e
pilot's space (FOV Â EMB) is identical to the
tendue in the imager space (surface of imager
e
Â the angle of aperture of the optical system).
As the optical aperture is necessarily limited by
geometry (strictly <90 ), one way to increase
the EMB for a given FOV is to work with a larger
imager, or at least a larger intermediate image
(large PGU).
The collimating distance: In order to display a
virtual image superimposed on the external scene,
the collimation distance shall match with the distance of objects of the scene, to avoid parallax
error and accommodation-vergence conflicts.2
All these critical parameters have a strong
impact on the HUD FOV. Increasing the vertical
and/or the horizontal FOV will adversely affect
the size of optical components, resulting in a
large HUD box needed to host the large mirrors,
as demonstrated on document.3
Depending of the HUD needs in term of FOV,
EMB, and collimation distance, three categories
of HUD are identified:
The legacy HUD: Displays only graphics symbols and parameters, avoiding the need of looking down the cockpit display system. For this
application, the required FOV is not more than
7 Â 4 , with a collimation distance about 2-3 m.
The Augmented Reality HUD (ARHUD): The
goal is to display symbols superimposed on the
real world as a synthetic vision of the road, directions to take, security distance from the front

70

vehicle, warning on pedestrian crossing, etc. In
this case, a FOV at least 14 Â 6 is needed, with a
collimation distance not less than 10 m.
The Full Windshield HUD (FWHUD): This is the
ultimate HUD, giving the maximum usable EMB
(500 mm and more), an FOV adapted to the
human eye capability (>35 Â 20 , see Figure 2,
corresponding to the part of the human eye FOV
where symbols can be assessed or recognized
unambiguously) and a variable collimation distance between 2 m to infinite
If we consider that the most dimensioning
parameter is the FOV, the relationship given by
the document4 could be used to estimate the
impact on the HUD box size and volume (Table 1).
A FWHUD using an architecture based on
classic optics is not achievable, regarding the
huge volume needed to host the optical elements (84 liters).

DIRECT WINDSHIELD PROJECTION
For a better user experience, recent Automotive HUD uses the windshield as the last optical
element. But the windshield needs to have specific properties to reflect images with an acceptable level of quality. In this case, the windshield
is used as a partially reflective mirror, with a
high transmission rate, meaning that the reflection rate is very poor (Typically, only 20% of the
image luminance is reflected).
An intermediate approach is to integrate in
the windshield the optical function of a concave
mirror, as presented by Okumura et al.5,6 This
will result in larger field-of-view capability for
the HUD within a constrained space envelope.
The Fresnel reflector can be seen as an intermediate step toward a fully holographic combiner:
the active pattern dimensions of the Fresnel
component are sufficiently small to minimize
see-through adverse effects, but sufficiently
large to work only in classical Snell-Descartes
reflection and not in diffraction mode.

HOLOGRAPHIC APPROACH
The use of Holographic components is not
new: today, holographic diffusers are used in
backlight systems, but also in projection systems, like the PGU, used to display the intermediate image on the HUD box.

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