Homography based identification for automatic and robust calibration of projection integral imaging displays.

Recent advances in the creation of microlens arrays as holographic optical elements allow the creation of projector-based see-through light field displays suitable for augmented reality. These systems require an accurate calibration of the projector with relation to the microlens array, as any small misalignment causes the 3D reconstruction to fail. The methods reported so far require precise placement of the calibration camera w.r.t. the lens array screen, which affects the display configuration. We propose a calibration approach which is more robust, and which allows free camera placement. Hence, it does not limit the capabilities of the system. Both a homography-based technique and structured light play a central role in realizing such a method. The method was tested on a projection-based integral imaging display system consisting of a consumer-grade projector and a digitally designed holographic optical element based micromirror array screen. The calibration method compensates for the lens distortion, intrinsics, and positioning of the projector with relation to the screen. The method uses a single camera and does not require the use of obtrusive markers as reference. We give an in-depth explanation of the different steps of the algorithm, and verify the calibration using both a simulated and a real-world setup.

Digitally designed holographic optical element for light field displays.

Concave micro-mirror arrays fabricated as holographic optical elements are used in projector-based light field displays due to their see-through characteristics. The optical axes of each micro-mirror in the array are usually made parallel to each other, which simplifies the fabrication, integral image rendering, and calibration process. However, this demands that the beam from the projector be collimated and made parallel to the optical axis of each elemental micro-mirror. This requires additional collimation optics, which puts serious limitations on the size of the display. In this Letter, we propose a solution to the above issue by introducing a new method to fabricate holographic concave micro-mirror array sheets and explain how they work in detail. 3D light field reconstructions of the size 20 cm×10 cm and 6 cm in depth are achieved using a conventional projector without any collimation optics.

Copying of holograms by spot scanning approach.

To replicate holograms, contact copying has conventionally been used. In this approach, a photosensitive material is fixed together with a master hologram and illuminated with a coherent beam. This method is simple and enables high-quality copies; however, it requires a large optical setup for large-area holograms. In this paper, we present a new method of replicating holograms that uses a relatively compact optical system even for the replication of large holograms. A small laser spot that irradiates only part of the hologram is used to reproduce the hologram by scanning the spot over the whole area of the hologram. We report on the results of experiments carried out to confirm the copy quality, along with a guide to design scanning conditions. The results show the potential effectiveness of the large-area hologram replication technology using a relatively compact apparatus.

Integral three-dimensional television with video system using pixel-offset method.

Integral three-dimensional (3D) television based on integral imaging requires huge amounts of information. Previously, we constructed an Integral 3D television using Super Hi-Vision (SHV) technology, with 7680 pixels horizontally and 4320 pixels vertically. We report on improved image quality through the development of video system with an equivalent of 8000 scan lines for use with Integral 3D television. We conducted experiments to evaluate the resolution of 3D images using an experimental setup and were able to show that by using the pixel-offset method we have eliminated aliasing produced by full-resolution SHV video equipment. We confirmed that the application of the pixel-offset method to integral 3D television is effective in increasing the resolution of reconstructed images.

Projection-type see-through holographic three-dimensional display.

Owing to the limited spatio-temporal resolution of display devices, dynamic holographic three-dimensional displays suffer from a critical trade-off between the display size and the visual angle. Here we show a projection-type holographic three-dimensional display, in which a digitally designed holographic optical element and a digital holographic projection technique are combined to increase both factors at the same time. In the experiment, the enlarged holographic image, which is twice as large as the original display device, projected on the screen of the digitally designed holographic optical element was concentrated at the target observation area so as to increase the visual angle, which is six times as large as that for a general holographic display. Because the display size and the visual angle can be designed independently, the proposed system will accelerate the adoption of holographic three-dimensional displays in industrial applications, such as digital signage, in-car head-up displays, smart-glasses and head-mounted displays.

Cross talk elimination using an aperture for recording elemental images of integral photography.

A major problem with integral photography using a lens array is overlapping recordings (cross talk) between elemental images. Another problem is the decrease in the number of pixels in the elemental images. We describe two methods (including analyses) of manipulating the aperture of a telecentric optical system to improve these problems. The first method locates the aperture on the focal plane of a field lens. The advantage of this method is that cross talk can be reduced without changing the size of the whole optical system. The second method establishes a telecentric optical system between objects and the lens array. The advantage of this method, even though the whole optical system becomes bigger, is that cross talk can be completely eliminated. In addition, the number of pixels in the elemental images can be increased by varying the aperture position sequentially with respect to time. We also describe how cross talk is reduced in both methods by taking diffraction into consideration. Experimental results are presented to verify this reduction.

Amplified optical window for three-dimensional images.

A proposed amplified optical window can form an observable three-dimensional image of an object in darkness. The window comprises two gradient-index (GRIN) lens arrays with an image intensifier between them. The length of the individual GRIN lenses that constitute the arrays is three fourths of the cycle of the meandering optical path on the input side and one fourth of the cycle on the output side. A primitive experimental result proved that the method produces three-dimensional images to be observed. This device would be used as a viewer for observing three-dimensional objects in a dark space without a camera and display equipment.

Calculation of holograms from elemental images captured by integral photography.

We describe a method in which holograms can be produced by calculation from images captured by integral photography (IP). We present a basic algorithm obtained by simulating IP reconstruction, in which conditions are set so as not to cause aliasing in the holograms after the calculations. To reduce the calculation load, we also propose a way to limit the range of calculation considering the distribution of light and a way to shift the optical field on the exit plane of microlenses in a lens array. Finally, by optical experiments, we confirm that three-dimensional images can be reconstructed from holograms calculated from an integral photograph of a real object captured with an IP camera.

Optical screen for direct projection of integral imaging.

We describe a way to display three-dimensional images by integral imaging using an ordinary projector. We first explain a method that uses a large-aperture converging lens, then we explain the proposed method that uses two sets of lens array. Based on the principle of this new approach, front projection as well as rear projection is possible. Only a proper viewing area can be formed on the optical screen by this method, which improves the brightness of images on the screen. The projector itself does not need an additional optical system. We report on the results of an experiment carried out to confirm the validity of the proposed method.

Integral three-dimensional television using a 2000-scanning-line video system.

We have developed an integral three-dimensional (3-D) television that uses a 2000-scanning-line video system that can shoot and display 3-D color moving images in real time. We had previously developed an integral 3-D television that used a high-definition television system. The new system uses -6 times as many elemental images [160 (horizontal) x 118 (vertical) elemental images] arranged at -1.5 times the density to improve further the picture quality of the reconstructed image. Through comparison an image near the lens array can be reconstructed at -1.9 times the spatial frequency, and the viewing angle is -1.5 times as wide.

Moire fringe reduction by optical filters in integral three-dimensional imaging on a color flat-panel display.

We propose a method to reduce the color moire fringes that are attributable to the structure of a color flat-panel display in integral three-dimensional imaging. The method uses two types of optical low-pass filter, diffuser and defocus. The effectiveness of the method was confirmed in an experiment. We describe a way to design these filters with moire's residual energy and video signal energy as indices and demonstrate the validity of the model, which combines two filters to reduce moire fringes.

Viewing-zone enlargement method for sampled hologram that uses high-order diffraction.

We demonstrate a method of enlarging the viewing zone for holography that has holograms with a pixel structure. First, aliasing generated by the sampling of a hologram by pixel is described. Next the high-order diffracted beams reproduced from the hologram that contains aliasing are explained. Finally, we show that the viewing zone can be enlarged by combining these high-order reconstructed beams from the hologram with aliasing.

Geometrical effects of positional errors in integral photography.

The effects of misarrangement of elements (elemental lenses and elemental images) that construct three-dimensional (3-D) images in integral photography are presented. If the lens arrays of the capturing system and the display system are not aligned accurately, positional errors of elements may occur, causing the 3-D image to be reconstructed in an incorrect position. The relation between positional errors of elements and the reconstructed image is derived. As a result, it is shown that a 3-D image is separated by local positional errors and blurred by global positional errors. In both local and global positional errors, 3-D images reconstructed far from the lens array are greatly affected.

Effects of focusing on the resolution characteristics of integral photography.

The effects of focusing on the resolution characteristics of integral photography (IP) are analyzed. First, there is an attempt to obtain the resolution characteristics of capture and display systems as the product of their modulation transfer functions (MTFs). Next, the relationship between this overall MTF and focusing during the capture is studied. The results show that, with focusing set at infinity, IP can provide three-dimensional images without remarkable resolution degradation over a wide range of depth.