Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2002-01-08
2003-01-21
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S013000, C349S112000
Reexamination Certificate
active
06509982
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to holographic diffusers and more specifically to surface holographic diffusers.
2. Background Art
Holographic diffusers of the reflective or transmissive type are well known in the art. Additionally, LCD displays, projection displays, illumination systems, irradiation systems that operate outside of the visible region, beam scanning systems, and other light distribution devices, which can make use of holographic diffusers, and which can be designed to operate for narrow or wide wavelength band monochrome or for color applications, are also well known in the art. For example, an LCD display typically uses a holographic diffuser either to augment the back lighting of the LCD display or to direct the transmitted display light to an observer located within a particular range of viewing angles. To accomplish this the holographic diffuser directs the diffused light in particular paths of propagation designed to fill a specific range of viewing angles.
For example, if an aircraft cockpit display has a holographic diffuser, the head box of the pilot will be the volume that could be occupied by the pilot's eyes from which the pilot can be expected to view the output of the display. Therefore it is advantageous to design the holographic diffuser to direct the light transmitted by the LCD display to the head box of the pilot. Thus, it is known to redirect light using holographic diffusers.
However, it is difficult to maintain uniform luminance over the range of viewing angles that fill the entire volume of the pilot's head box and to produce a sharp luminance fall-off at the edges of the viewing angle range. This difficulty exists because each holographic diffuser design causes display luminance to be a variable function of viewing angle. As a result, display luminance can vary detrimentally when viewed from within the pilot's head box and the luminance cut-off at the fringes of view lacks sharpness. This is generally attributable to two undesirable properties known holographic diffusers. Firstly, as the light's angle of incidence on a holographic diffuser approaches the limits of acceptable angles of incidence consistent with its design, the hologram's diffusion properties begin to break down and the incident light begins to transmit through the hologram without becoming diffused or deviated in propagation angle. Secondly, the corresponding plot of display luminance as a function of viewing angle resembles a bell-shaped curve. This causes the viewed display images to become dim as viewing angles approach the edges of the viewing angle range. Further, considerable wasted light falls outside the useful range of viewing angles owing to lack of sharpness in luminance fall-off at the fringes of the viewing angle range of interest.
FIG. 1
is a side view of a conventional diffusion screen arrangement in the art. With reference to
FIG. 1
, a collimated, or partially collimated, white light input beam
10
illuminates a refractive medium substrate
12
and a holographic film diffuser
13
at normal incidence. The holographic film
13
diffuses the projected output beam
14
over angular range &lgr;. The angle &lgr; shown in
FIG. 2
is the halfpeak full width angle of the luminance angular distribution profile between halfpeaks
20
. Little, or no, color dispersion is noticeable.
Referring to
FIG. 1A
, it is noteworthy that when a collimated, or partially collimated, white light input beam
10
is incident on refractive medium substrate
12
at an angle (p greater or less than 90°, the beam exiting the hologram can be designed to maintain the same (or nearly the same) diffusion angle, &lgr;, as that for normal incidence. Alternatively, referring to
FIG. 1B
, with normal incidence of collimated, or partially collimated, white light input, an output beam with a diffusion angle, &lgr;, can be projected in a direction that is not normal to the substrate. This can improve the luminance of an aircraft cockpit instrument display located below the pilot eye level, and with the instrument display face normal at a considerable (20° to 30° or more) angle to the pilot's direct view line. This can be accomplished by projecting the diffused output beam away from the instrument face normal and toward the center of the pilot's head box.
Also, designs of holographic diffusers are possible in which the input white light collimated, or partially collimated, beam and the propagation direction of the diffused output beam both deviate from the substrate (or instrument display face) normal.
In these prior art diffuser designs, the gradual luminance fall-off at the fringes of the viewing angle range (and at angles beyond those fringes) causes a waste of light resulting in reduced display luminance. Therefore, to minimize wasted light and maximize the light flux captured within the viewing angle range of interest, it is advantageous to maximize the slope at the halfpeak points of the luminance angular distribution curve.
In addition, for vehicle illumination applications, light beyond the pilot's headbox contributes to undesired reflections off of the windows, commonly referred to as canopy reflections, degrading night visibility. Therefore, a sharp cutoff in luminance outside the headbox minimizes the potential for this to occur.
SUMMARY OF THE INVENTION
This invention is particularly useful as a beam deflecting diffusion screen for displays, such as LCD instrument panel modules in aircraft cockpits and heads-up displays although its application is not limited to displays. A set of narrow superimposed deflected diffused beam profiles with sharp luminance cut-offs at their halfpeak full width points forms a composite angular luminance distribution. By concentrating these superimposed light beams that project from a display panel and by capturing them within the pilot's head box, efficiency is improved by minimizing the light wasted by projection outside the pilot's head box. Although an individual projected narrow beam angular profile does not, by itself, render the display luminance uniform as a function of viewing angle, the superposition of a plurality of individual narrow beams can be designed to generate uniform luminance over a wide viewing angle range of interest.
The invention is accomplished with the structure and method of the present invention by sending collimated, or partially collimated, light through a substrate with a film matrix comprising a nested plurality of individual joined geometrically shaped holographic cells. The cells comprising the matrix are subdivided into groups. Each cell within a group contains a uniquely patterned holographic diffuser which may advantageously be a surface holographic diffuser. This generates a diffused narrow light beam projected in a direction diverse from that projected by every other cell in the group. The superposition of the variously directed diffused narrow light beams projected from each cell group produces a combined resultant diffused wide light beam. The resultant light beam has a luminance angular distribution profile with sharply vertical slopes at its halfpeak points and a substantially flat and wide peak over a wide viewing angle range of interest.
The display luminance thus produced is uniform over a wide range of viewing angles that span the dimensions of pilot's head box. This range of angles is centered on a specific beam deflection angle that passes through, or in close proximity to, the midpoint of the pilot's head box. Thus the matrix of cells on the display produces a uniform luminance over the entire surface of that display and when viewed from any point within the pilot's head box. The resulting display luminance is substantially uniform over a wider range of viewing angles than is known in the art and the sharpness of the luminance fall-off at the angular distribution profile halfpeak points is greater than is known in the art.
In the pre
Assaf Fayez
Honeywell International , Inc.
Spyrou Cassandra
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