IEEE Consumer Electronics Magazine - September/October 2019 - 101

Figure 2. Optical system for eliminating zero-order
diffraction light from phase-only holographic HUDs.
5,12

diffraction light.
In this method, the SLM displays a half-zone-plate hologram, which is an
amplitude-type hologram whose diffraction angle
is confined by half. The Fourier transform effect
of the first lens divides the modulated light into
þ1, zero, and -1 order diffraction light.13 On the
focal plane, the divided zero and -1 order diffraction light is absorbed by the spatial filter (singlesideband method). However, there is a problem
associated with this method, namely, that the
field of view is reduced by half because half of the
spatial bandwidth is eliminated.
On the other hand, phase-only holograms do
not need to eliminate the À1 order diffraction
light because they can suppress it, as mentioned
above. For this reason, we propose using the
optical system, as shown in Figure 2, to eliminate
the zero-order diffraction light from phase-only
holographic displays. The zero-order diffraction
light cut filter in this optical system absorbs
only the center of the focal plane (focal point).
This means that it can specifically eliminate the
zero-order diffraction light without reducing the
field of view.

Experimental Conditions of a Phase-Only
Holographic HUD System
To confirm the elimination of zero-order diffraction light, we fabricated a phase-only holographic HUD system to eliminate the zero-order
diffraction light. In this experiment, we fabricated a phase-only SLM with driving circuits and
electrodes as projectors. The pixel pitch was
8 mm, so the calculated angle of view was 4.5 .
The focal length of the lenses [f in Figure 2]
was 10 cm. We constructed the zero-order diffraction light cut filter by adhering ink, which
absorbs visible light, onto a glass substrate. The

September/October 2019

Figure 3. Object image and reconstructed images
reproduced by a phase-only holographic HUD. (a)
Object image. (b) Without a zero-order diffraction
light cut filter. (c) With a cut filter.

diameter of the ink was 0.7 mm. Figure 3(a)
shows the object images used in this experiment. We used the angular spectrum method
and the Gerchberg-Saxton method for the optical calculations used in the construction of the
phase-only hologram.13,14 For this experiment,
we calculated the holograms such that they
would form images near the SLM, and at distances of 50 cm. This is because the depth of
the field becomes shallow when we take pictures
of the reconstructed images, and this is suitable
to see the position of the reconstructed images.
In automobile applications, the reconstructed
images would be further away than under our
experimental conditions. To extend the distance
to the reconstructed images, we should lengthen
the propagation distance in the optical calculation used to construct the hologram.

Experimental Results
The images reconstructed by the phaseonly holographic HUD system are shown in
Figures 3(b) and 3(c). Figure 3(b) shows the
results obtained without the zero-order diffraction light cut filter and (c) shows the
results obtained with the cut filter. The white
balance was adjusted using a standard diffusing reflector. As shown in Figure 3(b), when
the cut filter was not inserted, rectangular
zero-order diffraction light the same size as
the SLM appeared on the reconstructed
images. However, as shown in Figure 3(c), in
the case with the cut filter, the zero-order diffraction light was successfully eliminated. In
this experiment, the size of the reconstructed
image was 32.8 Â 32. 8 mm. Therefore, an
angle of view of at least 3.7 was achieved due
to the fact that the entire image was visible at
50 cm. This means that the cut filter

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