IEEE Consumer Electronics Magazine - September/October 2019 - 66

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

conditions. This is determined by two fundamental
parameters for HUDs: The adaptation of the eye to
the surrounding brightness and the contrast ratio.
For automotive displays this adaptation
requires a very large dimming range from high
maximum luminance at sunlight conditions (otherwise the display appears too dark) to low luminance (a few cd/m2) in night driving conditions
so the driver is not dazzled (display appears too
bright).
The most appropriate way to elaborate the
required luminance for a HUD is via contrast
ratio. The readability for text, icons, etc., is
expressed by the contrast ratio ðCR Þ defined as
the luminance of the image (foreground, AR
content) divided by the luminance of the road
or surroundings (AR overlay) as background for
projection
CR %

LForegound
þ 1:
LBackground

(1)

Typical daylight background luminance
LBackground values range from 100's (road) to
10,000's cd/m2 (bright sky as background for
AR content). The (background) luminance of
roads (look-down angle) depends on their reflectivity. On a bright sunny day with 50,000 lx
illuminance, a standard dry road has about
5,000 cd/m2 (reflectance $ 20%), a wet road
exhibits 10 000 cd/m2 (reflectance $ 40%) and
a snow-covered road reaches 20 000 cd/m2
(reflectance $ 90%)-similar to white buildings
in sunlight for AR-HUD background.
To achieve sufficient readability, the minimum contrast ratio CR according to equation (1)
should be 3:1. Therefore the (white) luminance
of the HUD (LForeground) must reach at least twice
the background luminance. This results in about
40 000 cd/m2 for HUDs displaying AR information
on a bright background. For night driving, this
must be dimmed down to $ 10 cd/m2 for rural
roads! Requirements for high luminance are one
of the reasons why consumer AR goggles fail in
outdoor use situations.
Besides contrast ratio the perception of color
is significantly reduced by the bright background of HUDs. Therefore, AR HUDs require
light sources with narrow half-width such as
Quantum Dot enhanced LCD backlights or lasers,

see the paper by Blankenbach and Buckley.11
They show a gamut which is twice the value of
today's HUD light sources for LCDs.

CONCLUSION
AR HUDs will provide advanced information
in terms of safety and infotainment compared to
today's HUDs with low FOV. A striking benefit of
AR-HUDs is building trust in autonomous driving
by visualization of lanes, cars, pedestrians and
as well by projecting the intended path of the
car. The FOV should be 60 (h) Â 40 (v) for
urban use cases and a reasonable readability
requires a luminance of 40,000 cd/m2. New
approaches like holographic devices and DMD
PGUs promise to reach these values for an
acceptable volume and power consumption. The
fundamental methods of AR-HUDs can be transferred to consumer applications. As in consumer
electronics,12 evaluation standards have to be
established for automotive use.

ACKNOWLEDGMENTS
This work was supported by AR-HUD specialists from BMW and CONTINENTAL. Special gratitude to the members of the automotive displays
platform of the German Flat Panel Forum (www.
displayforum.de) for valuable discussions.

& REFERENCES
1. Y. Yuan, "Changing the world with virtual\/augmented
reality technologies," IEEE Consum. Electron. Mag.,
vol. 6, no. 1, pp. 40-41, Jan. 2017.
2. Y. Hwang, B.-J. Park, and K.-H. Kim, "Effects of
augmented-reality head-up display system use on risk
perception and psychological changes of drivers,"
ETRI J., vol. 38, no. 4, pp. 757-766, 2016.
^te,
3. P. Coni, J. L. Bardon, A. Gueguen, and M. Grossete
"Development of a 3D HUD using a tunable bandpass
filter for wavelength multiplexing," in Proc. Soc. Inf.
Display, Int. Symp., Digest Tech. Papers, 2017,
pp. 876-879.
4. J. H. Seo, C. Y. Yoon, J. Ho Oh, and S. B. Kang, "A
study on multi-depth head-up display," in proc. Soc.
Inf. Display, Digest Tech. Papers, 2017, pp. 883-885.
5. R. Noguchi, T. Daimon, K. Kasazumi, and T. Mori,
"Influence of image position and visual target on depth
perception when using an automotive 3D head-up
display," in Proc. Int. Display Workshop/Asia Display,
2016, pp. 933-936.

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IEEE Consumer Electronics Magazine - September/October 2019

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