Adrian Travis
Wedge displays as cameras
1
Sophie Boual , Timothy Large1, Mark Buckingham1, Adrian Travis2 and Simon Munford2
1
Cambridge Flat Projection Displays Ltd, Manor House, Fenstanton PE28 9JQ, UK
2
Engineering Department, Cambridge University, Cambridge CB2 1PZ, UK
Abstract
2.
Wedge projection enables big slim screens at low cost, but wedge
panels also work in reverse as cameras. Eye-limited resolution
can be seen through a panel and since cameras are not subject to
the étendu limit of projectors, there is the potential to make panels
particularly slim.
.
1.
Introduction
Wedge projection1 has been developed as a way of allowing rear
projection televisions (RPTV’s) to be made as slim as flat panel
displays (FPD’s). The technology allows one to get the form
factor of FPD’s but with the high image quality of RPTV.
Manufacturing cost has the potential not only to be as low as for
RPTV but perhaps to be lower for two reasons. Firstly, the folded
light-guide allows fan-out angles to be as little as 300 so that
lenses need less curvature and can be made at less expense than
for conventional minimal thickness RPTV’s. Secondly, the
Wedge panel itself is sufficiently stiff that there is no need for a
large, costly cabinet for the folding optics.
Principle
Shine a ray of light into the thick end of a transparent wedge and
it will typically be guided by total internal reflection towards the
thin end. Each time the ray reflects off one material/air interface,
the angle of the ray relative to the opposite interface will alter
until it reaches the critical angle and emerges into air. Angle of
injection is therefore turned into point of emission, so when a
video projector is pointed into the thick end of a transparent
wedge, a picture emerges from the side as shown in Figure 2.
Figure 2: Photograph of an image with 16” diagonal projected
through a 30 mm thick Wedge panel.
The principle of Wedge projection works in reverse: if a ray of
light travelling through air is shone through a wedge panel
contrary to the axis of taper and at glancing incidence, the ray will
be captured by the wedge and guided towards the thick end. The
angle at which it emerges from the thick end will be determined
by the point at which it was injected.
Position of
ray output
Position where
ray originates
Figure 1: Wedge projection offers the potential for large displays
to be made at low cost.
Optical systems are in principle reversible and Wedge panels are
no exception. This article describes how instead of coupling light
from a projector to a screen, wedge panels have been developed to
work in reverse so as to couple light from a screen to a camera.
Angle at
which ray is
injected
Angle at
which ray
emerges
Figure 3: A wedge panel can convert angle of ray injection to
position of ray output, or do the reverse.
1
The on-screen image can be captured by placing a camera at the
thick end of the Wedge panel as shown in Figure 4.
3.
Imaging with no projector
Green LED
Fresnel Lens
diffuser
Mirrors
Power
Supply
Wedge
Camera
Computer
Figure 5: In the first experimental set-up, targets were placed on
the Wedge screen and illuminated diffusely from the rear.
Figure 4: Place a video camera at the thick end of a wedge panel
and one can image pictures or patterns of light on the screen.
Tests were performed on a series of Wedge panels machined from
off-the-shelf 30 mm thick acrylic (poly-methyl methacrylate) with
a screen diagonal of 16”. Light from a green LED with a
wavelength of 530 nm was collimated by a Fresnel lens then
diffused and homogenized by side mirrors in such a way as to
provide even, diffuse illumination of a target placed over the
Wedge panel. Targets for resolution, contrast and uniformity were
placed in turn on ten randomly selected panels, and the targets
were imaged through the Wedge panel with a digital camera
which had a resolution of 640 by 480 pixels. Figure 6 shows the
image captured by the camera when a resolution chart was placed
on the surface of a Wedge panel.
Place a piece of paper on the screen and while rays from the
screen may travel in any direction, some rays will travel towards
the camera much as if the camera were pointing at the screen
through free space.
It should be noted that although Wedge panels are slim, there is
no fundamental penalty in efficiency of the capture of light. It is
of course true that if light coming from the object placed against
the screen of the Wedge panel is diffuse, then most rays will miss
the entrance of the camera and therefore be lost to the system. But
this is a property of all conventional cameras: their entrance pupil
is small and collects only a tiny fraction of the light scattered from
a scene. The light-sensitive film or sensors inside a camera are
nevertheless sensitive enough to form good images in short
exposure times.
Figure 6: Through-wedge image when a resolution chart was
placed on the screen of a Wedge panel.
2
Adrian Travis
Figure 7: Distortion through a Wedge panel acting as a camera.
Significant reductions in distortion have since been shown.
Resolution was found to be camera-limited in all the panels, so the
panel was inspected by eye. Resolution was found to be as fine as
the eye could see, even when the target was exchanged for laser
printed text on white paper illuminated by white light.
Figure 8: An application of Wedge panels in baggage inspection
Contrast was measured by imaging a four by four chequer-board
and finding the ratio of image intensities at the centre of one
transparent and one opaque square, in a manner similar to the
ANSI standard for projection. The average contrast was 200:1.
Uniformity was measured by finding the percentage difference
between intensities at the centre and corner of the image in the
absence of a target, weighted by the same intensities measured
when the camera was pointed directly at the diffuser. Variations in
uniformity were found to be less then 10% on average.
Place a phosphor screen against a Wedge panel with camera and
the result is particularly useful for taking real-time X-ray video in
confined spaces, something which until now has required
expensive special purpose TFT arrays. The panels have recently
been accepted for use in scanners designed to inspect unattended
baggage which is typically left adjacent to walls.
4.
Transparent display
In effect, Wedge panels replace the space between a screen and a
point optic such as a camera or projector and there is therefore a
great variety of potential applications for the technology. In
particular, diffusers designed to work for bulk optic projection
should work equally well with Wedge projection and the authors
decided to test a transparent diffuser in order to illustrate the
point.
The results are shown in Figure 9. The projected image is an array
of white lines, while the hand placed behind the screen is visible
through it.
Figure 9: A transparent wedge display with a hand behind.
3
5.
Conclusions
We report the operation of a Wedge projection panel in reverse as
a slim light-guide between a camera and its target. On-screen
resolution is as fine as the eye can see, and thickness is limited in
the first instance by the pupil of the camera rather than by lamp
étendu as in a system used for projection.
6.
References
1. Travis, A., “The focal surface of a wedge projection display”,
Society for Information Display, 2005 International
Symposium, Digest of Technical Papers, Volume XXXVI,
Boston, 2005, paper P-156, pp896-897
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2. Travis, A., Buckingham, M. and Large, T., “Manufacturing
requirements for high quality flat projection displays”, Society
for Information Display, International Display Manufacturing
Conference 2005, Taipei International Convention Center,
Taiwan, February 21-24, 2005, Invited paper Thu-14-01,
pp229-232
3. Travis, A., Large, T. and Buckingham, M., "Image quality in
flat projection wedges", Society for Information Display, 2004
International Symposium, Digest of Technical Papers, Volume
XXXV, Washington State Convention and Trade Center,
Seattle, Washington, May 26-27 2004, paper 20.4, pp850-853