Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2002-04-25
2004-06-29
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S566000, C359S015000, C359S013000
Reexamination Certificate
active
06757105
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to an optical device and, more particularly, to an optical device having a wide field-of-view and which is capable of providing multicolor images.
Miniaturization of electronic devices has always been a continuing objective in the field of electronics. Electronic devices are often equipped with some form of a display, which is visible to a user. As these devices reduce in size, there is an increase need for manufacturing compact displays, which are compatible with small size electronic devices. Besides having small dimensions, such displays should not sacrifice image quality, and be available at low cost. By definition the above characteristics are conflicting and many attempts have been made to provide some balanced solution.
An electronic display may provide a real image, the size of which is determined by the physical size of the display device, or a virtual image, the size of which may extend the dimensions of the display device.
A real image is defined as an image, projected on or displayed by a viewing surface positioned at the location of the image, and observed by an unaided human eye. Examples of real image displays include a cathode ray tube (CRT), a liquid crystal display (LCD), or an organic light emitting diode array (OLED). Typically, desktop computer systems and workplace computing equipment utilize CRT display screens to display images for a user. The CRT displays are heavy, bulky, and not easily miniaturized. For a laptop, a notebook, or a palm computer, flat-panel display is typically used. The flat-panel display may use LCD technology implemented as passive matrix or active matrix panel. The passive matrix LCD panel consists of a grid of horizontal and vertical wires. Each intersection of the grid constitutes a single pixel, and is controlled by a LCD element. The LCD element either allows light through or blocks the light. The active matrix panel uses a transistor to control each pixel, and is more expensive.
An OLED flat panel display is an array of light emitting diodes, made of organic polymeric materials. Existing OLED flat panel displays are based on both passive and active configurations. Unlike the LCD display, which controls light transmission or reflection, an OLED display emits light, the intensity of which is controlled by the electrical bias applied thereto.
The flat-panels are also used for miniature image display systems because of their compactness and energy efficiency compared to the CRT displays. Small size real image displays have a relatively small surface area on which to present a real image, thus have limited capability for providing sufficient information to the user. In other words, because of the limited resolution of the human eye, the amount of details resolved from a small size real image might be insufficient.
By contrast to a real image, a virtual image is defined as an image, which is not projected onto a viewing surface, and no light ray connects the image and an observer. A virtual image can only be seen through an optic element, for example a typical virtual image can be obtained from an object placed in front of a converging lens, between the lens and its focal point. Light rays, which are reflected from an individual point on the object, diverge when passing through the lens, thus no two rays share two endpoints. An observer, viewing from the other side of the lens would perceive an image, which is located behind the object, hence enlarged. A virtual image of an object, positioned at the focal plane of a lens, is said to be projected to infinity.
Conventional virtual image displays are known to have many shortcomings. For example, such displays have suffered from being too heavy for comfortable use, as well as too large so as to be obtrusive, distracting, and even disorienting. These defects stem from, among other things, the incorporation of relatively large optics systems within the mounting structures, as well as physical designs which fail to adequately take into account important factors as size, shape, weight, etc.
Recently, holographic optical elements have been used in portable virtual image displays. Holographic optical elements serve as an imaging lens and a combiner where a two-dimensional, quasi-monochromatic display is imaged to infinity and reflected into the eye of an observer. A common problem to all types of holographic optical elements is their relatively high chromatic dispersion. This is a major drawback in applications where the light source is not purely monochromatic. Another drawback of some of these displays is the lack of coherence between the geometry of the image and the geometry of the holographic optical element, which causes aberrations in the image array that decrease the image quality.
New designs, which typically deal with a single holographic optical element, compensate for the geometric and chromatic aberrations by using non-spherical waves rather than simple spherical waves for recording; however, they do not overcome the chromatic dispersion problem. Moreover, with these designs, the overall optical systems are usually very complicated and difficult to manufacture. Furthermore, the field-of-view resulting from these designs is usually very small.
U.S. Pat. No. 4,711,512 to Upatnieks, the contents of which are hereby incorporated by reference, discloses a head-up display based on planar optics technique, by the use of relatively thick volume holograms. Collimated light wavefronts of an image enter a glass plate, located in an aircraft cockpit between the pilot and the aircraft windscreen, through an input diffraction grating element, are transmitted through the glass plate by total internal reflection and are coupled out in a direction of an eye of a pilot, by means of another diffractive element.
One ordinarily skilled in the art would appreciate that although thick volume holograms provide high diffraction efficiency, the maximal field-of-view which may be obtained, as well as the multicolor bandwidth of the images is substantially narrow.
U.S. Pat. No. 5,966,223 to Friesem et. al., the contents of which are hereby incorporated by reference, discloses a holographic optical device similar to that of Upatnieks, with the additional aspect that the first diffractive optical element acts further as the collimating element that collimates the waves emitted by each data point in a display source and corrects for field aberrations over the entire field-of-view. Indeed, according to Friesem et. al., the obtained field-of-view is ±6°, which improves on the Upatnieks' device. However, the diffractive collimating element is known to narrow spectral response, and although Friesem et. al. observed low chromatic sensitivity over wavelength shift of ±2 nm around a 632.8 nm wavelength, chromatic aberrations would be rather considerable at spectral ranges of ±20 nm or ±70 nm.
There is thus a widely recognized need for, and it would be highly advantageous to have, an optical device capable of providing a wide field-of-view of multicolor images devoid of the above limitations.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a diffractive optical element for optimizing a field-of-view for a multicolor spectrum, comprising a linear grating being formed in a light-transmissive substrate, wherein: (a) the linear grating is characterized by a pitch, d, selected so as to allow total internal reflection of at least a first portion of a light striking the diffractive optical element at a first field-of-view angle, &agr;
−
FOV
, the first portion having a shortest wavelength of the spectrum, &lgr;
B
; and (b) the light-transmissive substrate is characterized by an index of refraction, n
s
, larger than a minimal index of refraction, n
MIN
, the minimal index of refraction is selected so as to allow total internal reflection of at least a second portion of the light striking the diffractive optical element at a second field-of-view angle, &agr;
+
FOV
, the s
Niv Yehuda
Nivon Uzia
Chang Audrey
G.E. Ehrlich (1995) Ltd.
Lavarias Aynel C.
Planop Planar Optics Ltd.
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