Rear View Virtual Image Displays

Rear-view virtual image displays
Edward Buckley, Lilian Lacoste, Dominik Stindt
Light Blue Optics Ltd.
nd
2 Floor Platinum Building, St. John’s Innovation Park, Cowley Road, Cambridge, CB4 0WS, United Kingdom
{edward, lilian.lacoste, dominik.stindt}@lightblueoptics.com

Abstract: Light Blue Optics’ holographic laser projection shown [7], the necessity to re-accommodate focus when
technology can be utilised to create a virtual image display transitioning between the outside world and
which, with a volume enclosing less than 700cc, exhibits a instrumentation displays, and the amount of visual
form-factor consistent with integration into a rear-view scanning required to absorb this information, imposes a
mirror. By combining the visual accommodation and significant time penalty on the driver’s ability to
concomitant reaction time benefits of a head-up display recognise and act on such information. For these reasons,
with the ability to present high resolution safety-critical it seems that integrating a conventional display into a
information in a rear-view off-axis configuration with large rear-view mirror would negate any potential BSW safety
eyebox, significant potential safety benefits can result. benefits.

1 Introduction To reconcile the conflicting requirements of presenting
blind-spot information with accordingly minimal visual re-
In 2003, Najm et al. [1] determined that lane change and accommodation time, the authors propose the integration of
merge crash types accounted for 9% of all light vehicle a virtual image display into either the side or front rear-
crashes; a subsequent US Department of Transportation view mirror. Virtual images are typically presented at an
study found that this equated to some 630,000 collisions apparent distance of greater than 2 m from the viewer’s
every year [2]. eyes, reducing the need to re-accommodate focus when
transitioning between cluster imagery and the outside
In an attempt to mitigate the lane change collision threat, world. This presentation of imagery within the visual
a number of manufacturers have developed blind-spot accommodation of the driver enables recognition and
warning (BSW) systems. Such systems typically employ action upon information far more quickly than would be
ultrasonic, laser or radar sensing technologies to detect possible using the equivalent head-down configuration. The
the presence of an object in the vicinity of the car and, overall benefit time, which has been estimated at 777 ms
since the relative speed of lane change/merge collisions is (corresponding to a distance of 21.6 m whilst driving at a
relatively low, the sensors tend to be short range. This speed of 100 km/h) clearly indicates the safety benefits of
restriction to warning of blind-spot occupancy limit the projecting a virtual image within the visual field of the
utility of such systems somewhat; other systems which driver. Not only would this proposed scheme offer a
exploit detection of the turn signal in order to reduce the significant benefit in reducing blind spot-related accidents,
probability of false alarm [3] are compromised by the it also gives designers the opportunity to redundantly
surprisingly high average failure rate of turn signal usage present blind spot warning visual data on both the head-up
prior to a lane change [2]. display (HUD) and rear-view mirrors, ensuring the driver
has access to safety-critical information.
According to Mazzae et. al. [4], there are a number of
desirable features that determine the ultimate utility of Until now, both the size [8] and inefficiency [9] of
BSW systems. Of particular interest are those which LED/TFT-based HUD solutions has precluded the use of
provide a visual display located on or near the line of such a solution. Light Blue Optics (LBO) has been
sight of the appropriate side-view mirror, as auditory developing laser-based virtual image displays that have
stimuli would tend to interfere with those already reserved the potential to be significantly smaller than current HUD
for the turn signal (for example.) Some commercially designs and, additionally, are capable of displaying high
available systems, such as Iteris’ AutoVue [5], already brightness symbology at high resolution and in
provide this functionality. full-colour. In this paper, the authors present a virtual
image display architecture consisting of LBO’s
A 1997 study took this concept to its ultimate conclusion holographic laser projector and a novel projection
in suggesting that the integration of a visual display element which forms a full-colour virtual image in a
system into either rear-view mirror [6] would promote package size and form-factor consistent with the
mirror check behaviour that could potentially reduce lane integration of a virtual image display into a rear-view
change accidents. However, as has previously been mirror.

2
of the ith displayed image is I xy = Fxy )
(i
then the time-
2 Holographic Laser Projection Technology
averaged percept over N subframes is
LBO’s holographic laser projection engine takes a unique 1 N
(i ) 2
approach to the formation of video images by exploiting V xy =
N
∑F
i =1
xy (2)
the physical process of two-dimensional diffraction. A
typical imaging projection system works by displaying the which is noise-free, as illustrated in Figure 2.
desired image Fxy on a microdisplay, which is usually
sequentially illuminated by red, green and blue light to
form colour. In this case, the microdisplay simply acts to
selectively block (or amplitude modulate) the incident
light; after passing through some magnification optics, the
projected image Fxy appears. Conversely, holographic laser
projection forms the image Fxy by illuminating a diffraction
(or hologram) pattern huv by laser light of wavelength λ. If Hologram Video Subframe Video Frame
the hologram pattern is represented by a display element 1 N
(i ) 2
with pixel size ∆ then the image Fxy formed in the focal huv = exp jϕuv I xy = F (i ) 2
xy
Vxy =
N
∑F
i =1
xy

plane of the lens is related to the pixellated hologram Figure 2 - The relationship between hologram huv , subframe Fxy
pattern huv by the discrete Fourier transform F [·], and is and frame Vxy in LBO’s holographic projection
written as technology.
Fxy = F [huv ] (1)
LBO has developed and patented proprietary algorithms for
as shown in Figure 1 below. the purposes of calculating sets of N holograms huv from the
desired image Fxy both efficiently and in real time, as first
demonstrated in 2004 [11]. Crucially, such algorithms can
be efficiently implemented in a custom silicon chip.

A practical realisation is shown in the schematic of Figure
3. A desired image is converted into sets of holograms by
LBO’s proprietary algorithms and displayed on a phase-
modulating microdisplay which is time-sequentially
illuminated by red, green and blue laser light. The
subsequent diffraction patterns pass through a
demagnification lens pair L1 and L2, which are chosen to
provide a projection angle of 90°. Due to the nature of
Figure 1 – The relationship between hologram huv and image Fxy Fraunhofer diffraction, the image remains in focus at all
present at the back focal plane of a lens of focal length distances from the lens L2.
f, illuminated by coherent monochromatic light of
wavelength λ.
L2
The crucial efficiency advantage of LBO’s system occurs L1
because the hologram huv is quantised to a set of phase only
values ϕuv, where huv = exp jϕuv, so that the incident light is
steered into the desired image pixels – without blocking – Microdisplay

by the process of coherent interference, and the resultant
instantaneous projected image appears as a direct
consequence of Fourier optics. To achieve video-rate
holographic display, a dynamically-addressable display
element is required to display the hologram patterns;
LBO’s system uses a ferroelectric liquid crystal on silicon
(LCOS) microdisplay. Lasers

To achieve high image quality, a fast microdisplay is used
to display N holograms per video frame within the 40 ms Figure 3 - A schematic diagram of LBO’s holographic laser
temporal bandwidth of the eye, each of which produces an projection technology illustrating lasers, phase-
image Fxy exhibiting quantisation noise [10]. If the intensity modulating microdisplay and demagnification lens
pair L1, L2.

3 Rear-view virtual image display using it is critical to maintain reliable operation despite potential
holographic laser projector pixel failures. Due to the diffractive nature of the
technology, LBO’s system does not exhibit a one-to-one
As has previously been shown, LBO’s laser-based phase- correspondence between microdisplay pixels and projected
modulating holographic projection display has a number image pixels, as imaging systems do. In fact, each pixel on
of features and benefits that are uniquely suited for use as the microdisplay contributes to every pixel in the image, so
light engines in vehicular applications and particularly for that faulty display element pixels do not correspond to
virtual image displays [12]. ‘dead’ image pixels. This allows the realisation of a truly
fault-tolerant display with built-in redundancy, since
Of principal concern in an automotive virtual image display multiple microdisplay pixel failures can be tolerated
is obtaining an acceptable image brightness level of without compromising the integrity of the displayed data.
5,000 cd/m2 (typically requiring an image source
luminance of 25,000 cd/m2) [13] when displaying sparse Despite the commonalities in terms of light engine
imagery. Due to the blocking nature of conventional HUDs performance and functionality, the virtual image display
employing amplitude modulating TFT panels, large arrays requirements for a rear-view display as described
of LEDs [14, 15] must be employed as a backlight to previously differ markedly from a conventional HUD,
provide the required image source luminance. The use of a principally because the image must be viewed off-axis. In
phase-modulating projection technology, however, common with HUDs, however, the eyebox must be large
provides a highly power-efficient method of projection enough to allow free head movement by the driver and the
since, unlike such imaging displays, no light is blocked in light must be directed accurately in the direction of the
the system. This property becomes especially driver to preserve the optical efficiency of the display.
advantageous when displaying symbology and night-vision
imagery, which have average pixel intensities of less than The approximate geometry is shown in Figure 4, in which
10% and 25% respectively [16], allowing brightness targets the eyebox (1) is located at approximately driver head-
to be met using just three small laser sources. Furthermore, height and the virtual image (3) in this particular case
since laser sources can provide wide color gamut images appears to come from greater than two metres behind the
due to their narrow spectral bandwidth, the Helmoltz- position of the rear-view mirror (2). In this way, the driver
Kohlrausch effect [17] can further increase perceived is able to view the image without re-accommodating
brightness due to the psychophysical effects of highly focus or becoming distracted. Realising such a geometry
saturated primaries. represents a challenging design task which cannot easily
be satisfied by conventional HUD optical subsystems.
For safety critical applications such as a virtual image
displays in which blind spot information is to be presented,

Figure 4 – Image viewing geometry for a rear-view virtual image display, showing the eye-box (1), rear-view mirror surface (2) and
apparent virtual image position (3). The image display geometry allows the display of a virtual image within the visual
accommodation of the driver whilst providing a large enough eyebox to allow realistic head movement.

The requirements were achieved in the context of a rear- the increasing attention of designers to the human-
view virtual image display by combining LBO’s miniature machine interface (HMI) [18] – and the off-axis viewing
light engine and a novel eyebox expansion optic, the last configuration minimises light pollution and hence the
element of which forms the surface of the rear-view mirror. potential for distracting other drivers.
In such a configuration, LBO’s projection engine is imaged
onto the surface of a diffusing screen and the resultant
image is transferred via some fold optics to the rear of the
pupil expansion optic as shown in
Figure 5(a). Since the pupil expander is semi-transparent,
the virtual image appears on the front surface as shown in
Figure 5(b).

Figure 6 – Integration of LBO’s virtual image display into a
rear-view mirror. Note that the final surface of the
pupil expansion optic is itself the rear-view mirror
surface.

4 Initial performance evaluation
Some preliminary results for the virtual image display are
presented in Table 1 below; the average pixel intensity in
(a) Virtual image display - rear view
the virtual image was approximately 10%.

Parameter Value
Total volume (cc) < 700
Eyebox size (mm) 190 × 60
Field of view (deg) 8 × 3.5
Virtual image distance (m) >2
Virtual image brightness (cd/m2) ~6,000
Virtual image resolution 850 × 480

(b) Virtual image display - front view Table 1 – Specifications for prototype rear-view mirror virtual
image display system.
Figure 5 – Front (a) and rear (b) views of the LBO projection
engine, optical path (including diffusing screen and An example image, captured in daylight, is shown in
fold optics) and pupil expander. Figure 7 below; the high brightness and wide colour
gamut due to the use of the LBO light engine are both
The concept presented in Figure 5 confers several evident.
advantages. First, the total volume enclosed by this initial
mechanical design, including the diffusing screen, fold
optics, pupil expander, 30cc light engine and electronics
(including hologram computation, video interface and
laser drive circuitry) is less than 700 cc. As a result, the
entire assembly is comfortably small enough to integrate
into a rear-view mirror as shown in Figure 6, and with
aggressive design optimisation a target volume of 500 cc
should be achievable. In addition, the rear-projection
configuration provides the off-axis virtual image formation
of Figure 4 and presents a reduced sensitivity to solar
loading compared to conventional HUDs since the imager
cannot be directly exposed to external lighting. Finally, it
should be noted that the rear-view virtual image display is Figure 7 - Sample image demonstrating the performance of
invisible when not operating - which is desirable given LBO's full-colour rear-view virtual image display.

5 Conclusion [6] L. Tijerina and S. Hetrick, “Analytical evaluation
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