Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2000-12-20
2002-07-16
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S013000, C349S112000
Reexamination Certificate
active
06421148
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to volume holographic diffusers.
BACKGROUND OF THE INVENTION
Holographic diffusers are well known in the art. Additionally, LCD displays, which make use of holographic diffusers, are also well known in the art. Typically, an LCD display 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 area 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 desired range of viewing angles 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 of 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 volume 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 &phgr; greater or less than 90°, the beam exiting the volume 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 volume 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.
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 volume 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. This range of angles is centered on a specific beam deflection angle that passes through 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 preferred embodiment, a non-alternating, single image is also provided for both eyes rather than alternating a separate right eye image with a separate left eye image.
This invention is most useful for applications where collimated, or partially collimated, light is incident on a display and a need exists to project the light transmitted by the display into a wider and more diffuse beam. A further enhancement of its usefulness occurs when the projected diffuse beam is uniformly distributed over a desired wide range of viewing angles and with sharp luminance cutoffs at the edges of that range. The projected diffuse beam can also have an asymmetric (i.e. different angular widths in orthogonal profile planes) output beam envelope, which has a high efficiency and little or no color dispersion. It may also be desired to have the option of deflecting the axis of this output beam envelope at a different angle from the input
Honeywell International , Inc.
Schuberg Darren
Yeadon Loria B.
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