Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2001-11-30
2002-09-17
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S837000, C359S833000, C359S834000
Reexamination Certificate
active
06452734
ABSTRACT:
TECHNICAL FIELD
This invention provides an electrophoretically switchable reflective image display which retro-reflects incident light rays with respect to three mutually perpendicular axes.
BACKGROUND
U.S. Pat. No. 6,304,365 (the '365 patent), which is incorporated herein by reference, describes a reflective (front-lit) image display for viewing images in a preferred direction. The display has parallel, macroscopically planar, structured surface (preferably prismatic), light deflecting and reflecting portions which are longitudinally symmetrical in mutually perpendicular directions, both of which are perpendicular to the preferred viewing direction. A liquid electro-phoretic medium containing a particulate suspension contacts the light reflecting portion. A controller applies an electromagnetic force to selectively electrophoretically move the particles into the evanescent wave region adjacent the light reflecting portion to frustrate total internal reflection (TIR) of light rays at selected points on the light reflecting portion.
The structured surfaces on the light deflecting portion deflect light rays incident in the preferred viewing direction toward the light reflecting portion by imparting to the rays a directional component in the direction of longitudinal symmetry of the light reflecting portion. The structured surfaces on the light reflecting portion totally internally reflect the deflected light rays toward the light deflecting portion at points other than the selected points at which TIR is frustrated. Then, the structured surfaces on the light deflecting portion again deflect the totally internally reflected light rays, cancelling the directional component therefrom, such that the deflected totally internally reflected light rays emerge from the display in a direction substantially parallel to the preferred viewing direction.
The directional characteristic of any light ray can be described in terms of three vectors corresponding to three mutually perpendicular axes. For a light ray to undergo “full retro-reflection”, all three vectors must undergo a directional inversion. An odd number of reflections from a planar reflector oriented perpendicular to a given axis directionally inverts (i.e. reverses the sign of) the component of the ray's direction vector for that axis. Full retro-reflection requires an odd number of reflections in each one of the three mutually perpendicular directions.
FIGS. 1A and 2
pictorially illustrate two optical geometries disclosed in the '365 patent. In
FIG. 1A
, outward and inward thin sheets
10
,
12
are separated by fluidic gap
14
. Prisms
16
are formed on the inward surface of outward sheet
10
, which has a flat outward surface. Prisms
18
are formed on the inward surface of inward sheet
12
, which also has a flat outward surface. Prisms
16
,
18
extend longitudinally in mutually perpendicular directions: prisms
16
extending substantially parallel to the X axis, and prisms
18
extending substantially parallel to the Y axis. The preferred Z axis viewing direction is mutually perpendicular to both the X and Y axes. A low refractive index medium (not shown) is maintained in gap
14
to reduce the extent to which light rays entering inward sheet
12
are refracted, thus maintaining a high effective refractive index for inward sheet
12
. In the
FIG. 2
geometry, sheet
20
is formed with mutually perpendicular, longitudinally extending outward prisms
22
and inward prisms
24
on opposite sides of a single sheet
20
; prisms
22
extending substantially parallel to the X axis, and prisms
24
extending substantially parallel to the Y axis. An electrophoresis medium (not shown) is maintained in contact with inward prisms
24
.
The
FIG. 1A
geometry is better suited to use by viewer
32
with light rays which are incident in the Z axis direction
34
A perpendicular to the flat outward surface of outward sheet
10
. Both the X axis vector component and the Z axis vector component of a light ray incident on the
FIG. 1A
geometry are directionally inverted, but the incident ray's Y axis vector component is not inverted. The
FIG. 1A
geometry inverts the vector components of incident light rays in two of the three mutually perpendicular X, Y and Z directions, namely the X and Z directions; without inverting the vector component in the third (Y axis) direction.
FIG. 1B
depicts inversion of the X and Z components of light rays incident on sheet
12
, viewed in cross-section along the Y axis.
FIG. 1C
depicts inversion of the Z component, but not the Y component, of a light ray incident on sheet
12
, viewed in cross-section along the X axis.
In applications such as variable retro-reflectivity image displays, directional inversion of X, Y and Z components is desirable, rendering the
FIG. 1A
geometry inadequate for such applications. Highly retro-reflective sheets consisting of glass beads or corner-cube structures are currently in widespread use as retro-reflective signs. In the latter case, reflection is caused by TIR in the cube structures, but it is impractical to modulate TIR in such sheets to produce a variable retro-reflectivity image display. For example, it is impractical to fabricate such sheets using materials of sufficiently high refractive index for TIR to occur when the material contacts a suitable electrophoretic medium.
The
FIG. 2
geometry, which similarly inverts the X and Z components without inverting the Y component, is better suited to use by viewer
32
with light rays which are incident in direction
34
B inclined at 45° to the macroscopic plane of sheet
20
. Electrophoretically switchable image displays incorporating
FIG. 2
type geometric structures are easily fabricated, work well within a reasonably wide angular range of incident light, and are amenable to achieving full retro-reflection in the X, Y and Z directions in accordance with this invention.
SUMMARY OF INVENTION
The invention provides a three sheet reflective variable image display for retro-reflecting light with respect to mutually perpendicular X, Y and Z axes. The display has a preferred Z axis viewing direction. The first (outermost) sheet is a light deflecting/recombining transmitter sheet which is longitudinally symmetrical with respect to the X axis. The second (innermost) sheet is an X and Z vector components inverting reflector sheet with light deflecting and light reflecting portions. The light deflecting portion is longitudinally symmetrical with respect to the X axis. The light reflecting portion is longitudinally symmetrical with respect to the Y axis. The X and Z vector components inverter sheet is substantially macroscopically parallel to the light deflecting/recombining transmitter sheet. The third (intermediate) sheet is a Y vector component inverting transmitter sheet having longitudinal symmetry with respect to the X axis. The Y vector component inverter is substantially macroscopically parallel to and positioned between the light deflecting/recombining transmitter sheet and the X and Z vector components inverter sheet. The Y vector component inverter has a plurality of microstructure reflector elements with their surface normal substantially parallel to the Y axis. Each element has a height H. Adjacent pairs of elements are spaced apart by a separation distance D.
Light rays within about 25° of perpendicular incidence to the light deflecting/recombining transmitter (hereafter called “approximately perpendicular” light rays) are transmitted by the light deflecting/recombining transmitter toward the Y vector component inverter and X and Z vector components inverter at an angle &thgr; of about 30° to 60° with respect to the Z axis. The rays are reflected by the X and Z vector components inverter toward the Y vector component inverter and light deflecting/recombining transmitter at the same angle &thgr; with respect to the Z axis. The vector components of substantially all of the light rays reflected by the X and Z vector components inverter are directionally inverted with respect to the X and Z ax
Mossman Michele Ann
Whitehead Lorne A.
Oyen Wiggs Green & Mutala
Sikder Mohammad
The University of British Columbia
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