Light from an image displayed on a display screen (10) is transmitted to an observer's eye (11) by way of a dynamic optical element (12) (such as a spatial light modulator or an electrically switchable holographic composite) which acts as a lens. Emitters (17) on the display screen (10) emit infrared radiation which is projected by the dynamic lens (12) as a broad wash onto the eye (11). Infrared radiation reflected back from the eye (11) is focused by the dynamic lens (12) onto detectors (18) also provided on the display screen (10). The detectors (18) are thus able to sense the direction of eye gaze, and the dynamic lens (12) is controlled in dependence on this to create an area of high resolution in an area of interest centred on the direction of gaze, which is moved to follow the eye gaze as its direction alters. Other than in the area of interest, the dynamic lens (12) has a relatively low resolution.

Method of and apparatus for viewing an image

A system for producing modulated light is disclosed. The system comprises a spatial light modulator including a light modulating medium switchable between different states so as to act on light in ways which form overall patterns of modulated light. The system also includes an arrangement for switching the modulating medium between the different states in a controlled way and an illumination arrangement for producing a source of light. The system further includes an optics arrangement for directing light from the source of light into the spatial light modulator and for directing light from the spatial light modulator through a predetermined source imaging area. The optics arrangement cooperates with the illumination arrangement and the spatial light modulator so as to produce a real image of the source of light within the source imaging area such that an individual is able to view a virtual image of the overall patterns of modulated light from the source imaging area. A variety of novel optics arrangements are disclosed including specific combinations of different light sources, diffusing plates, polarizers, beam splitters, analyzers, lenses, mirrors, and holographic optical elements which allow the overall optical arrangement to be miniaturized to the same degree and in coordination with the spatial light modulator. The different light sources include using a plurality of light sources, such as LEDs, to form an array of light sources, each of the light sources providing light to a corresponding portion of the spatial light modulator.

Optics arrangements including light source arrangements for an active matrix liquid crystal image generator

Holographic optical elements (HOEs) can be used in systems and methods for providing illumination and for projecting images. The HOEs may be switchable HOEs, whose diffractive properties can be controlled. Described herein is a method of combining light from two or more illumination sources. In one embodiment, a reflection-type HOE is illuminated by the first illumination source. The HOE diffracts light from the first illumination source into an output direction. Light from the second illumination source is transmitted through the HOE and onto a reflective optical element, which reflects the light back through the HOE and into the same output direction. Also described is a projection system that uses two or more HOEs to combine two or more colors of light for use by a single image display. The system includes one or more light sources, an image display (such as a reflective or transmissive LCD display or a MEMS display, for example), and a first and a second HOE. The HOEs are each mounted between a light source and the image display, and configured to provide one color of light to the image display and from the image display into an output direction. In one embodiment, the HOEs are switchable HOEs and can be used for color-sequential display and/or for color balancing.

Holographic projection system

A system for producing spatially modulated monochrome or color light having gray scale includes an active matrix liquid crystal spatial light modulator having light modulating means including (i) a layer of ferroelectric liquid crystal material which is designed to switch between ON and OFF states and (ii) active matrix means including VLSI circuitry for dividing the layer of liquid crystal material into an array of individual liquid crystal pixels and for causing each of the pixels of liquid crystal material to modulate light individually by switching between the ON and OFF states in a way that depends upon the data which the VLSI circuitry is written. The system also includes illumination means having a light source for directing light from the source into the pixel-divided layer of ferroelectric liquid crystal material in a specific way. And finally, the system includes means for writing the VLSI circuitry with preselected data in accordance with a particular data ordering scheme such that the circuitry, in response to the written data, causes the pixels of liquid crystal material to individually switch between their ON and OFF states and therefore modulate light from the source in a way which, depending upon the data, produces a specific overall pattern of gray scale light.

Active Matrix Liquid Crystal Image Generator

We discuss the polarization properties and first-order diffraction efficiencies of volume phase holographic (VPH) transmission gratings, which can be exploited to improve the throughput of modern spectrographs. The wavelength of peak efficiency can be tuned by adjustment of the incidence angle. We show that the variation of the Kogelnik efficiency versus Bragg angle depends only on one parameter, given by Ptune = (Δnd)/(nΛ), where Δn is semiamplitude of the refractive index modulation, n is the average index, d is the thickness of the active layer, and Λ is the grating period. The efficiency has a well-defined dependence on polarization. In particular, it is possible to obtain theoretical 100% diffraction efficiency with one linear polarization at any angle, or to obtain 100% efficiency with unpolarized light at specific angles. In the latter case, high efficiency is the result of aligning the peaks of the s- and p-polarization efficiency-versus-thickness curves. The first of these "s-p-phased gratings" for astronomy is in use with the 6dF spectrograph. Consideration of polarization is particularly important for high spectral resolution, which requires large incidence angles. We also discuss the possibility of separating polarization states for improved throughput along the entire optical train of a spectrograph.

https://www.jstor.org/stable/10.1086/383622

Volume Phase Holographic Gratings: Polarization Properties and Diffraction Efficiency

Volume holographic optical elements exhibit a property that we call bidiffringence, in which the diffraction efficiency of the element is strongly dependent on the polarization of the incident beam. Dualelement, volume holographic polarization separators utilizing bidiffringence can provide high extinction ratios, and they can provide greater angular separation of the polarized beams and greater flexibility in the beam-separation parameters than conventional birefringent elements. We designed and constructed holographic polarization separators using dichromated gelatin as the holographic medium. Experimental results are compared with the results predicted by the Kogelnik coupled wave theory.

Holographic polarization-separation elements.

Large area volume phase holographic (VPH) gratings have been made for use in spectrographs attached to large telescopes and for scanning LIDAR systems. Examples of the transmitted wavefronts, the spectral efficiency measurements and other parameters such as uniformity, scatter, absorption and Q have been gathered and presented. Two exposure layouts have been used and are described along with some discussion of modulation and bulk index of processed DCG. A discussion of thickness regimes is given. A special case (Dickson) design is presented with examples of performance and some intrinsic properties.

/pdf/fabrication-and-testing-of-large-area-vph-gratings-55khhdx5u1.pdf

Fabrication and testing of large area VPH gratings

Dichromated gelatin exhibits variable changes in effective refractive index (n) from 1.54 before exposure to less than 1.25 as it expands during processing. This aerogel like effects causes aberrations in diffractive optics and Kogelnik's theory predicts strong polarization separation in gratings at many different angles other than 90 degrees. The diffraction efficiency of both S and P polarizations at any angle is dependent on the product of thickness and index modulation while the angle inside the medium is dependent on n. We investigated predicted conditions where only one polarization would be diffracted and subsequently proved n varies from about 1.4 to 1.2 after processing and depends on the film thickness and processing procedures. Transmission gratings made at angles from 36 to 66 degrees were fit to mathematical models as proof of the phenomena, some performed with extinction ratios greater than 100:1. We were also able to demonstrate a similar range in conformal reflection structures and to design a novel polarizer. The calculation of exposure geometries for display holograms becomes more accurate when index change is included in the formulas but some results remain hard to explain.

Polarization properties of gelatin holograms

A hologram system is comprised of two identical volume holograms. Both holograms have Bragg planes oriented perpendicular to the surface of the hologram. The first volume hologram separates a single beam into two orthogonal polarization component beams. The second volume hologram diffracts one of the polarization component beams such that a desired separation angle is achieved between the two beams. This separation angle may be precisely adjusted during manufacture by orienting the Bragg planes of the second volume hologram at a rotational angle with respect to the Bragg planes of the first volume hologram.

Hologram system having hologram layers with rotationally offset Bragg planes

In the early seventies a number of researchers noted that DCG was in some cases an ideal material for HOE fabrication. B.J. Chang and Andres Graube both did extensive characterizations of the material and many others have contributed since then. A large number of HOES have been made since then and a few are in industrial or commercial machines already. It would take a very long time to describe all the variations and applications that have been reported. In this paper we discuss only a few applications where non-standard practices may have been employed in meeting the design objectives. The emphasis is on the fabrication solutions and on any unusual or previously thought unlikely results.

Richard D. Rallison

Papers

Holographic polarization-separation elements.

Fabrication and testing of large area VPH gratings

Polarization properties of gelatin holograms

Hologram system having hologram layers with rotationally offset Bragg planes

Holographic Optical Elements (HOES) In Dichromated Gelatin (DCG):Progress