A proposed method of compressing image data would exploit the well-known capability of a converging lens to generate the Fourier transform of an image by purely optical means, in much less time than is needed to compute the discrete Fourier transform of a sampled image by use of digital electronic circuits. Inasmuch as a transform (whether of the Fourier, discrete-cosine, or other type) is the most computation-intensive part of almost any electronic image-compression scheme, the speedup afforded by the proposed method could make the difference between success or failure in applications in which there are requirements to compress image data at high throughput rates. In addition, because high-speed digital image-processing circuits are typically power-hungry, the use of optical Fourier transformation can reduce power consumption.

The Lens, Lasers, and Beam Splitters would be positioned to generate a hologram containing a Fourier transform of the input image. The optical propagation time to form the hologram would be much shorter than the time needed for digital electronic computation of the Fourier transform.

The Fourier-transform property of a converging lens can be summarized as follows: When the lens is placed at its focal distance from both an input and an output plane, then the image formed by the lens on the output plane is a Fourier transform of the object or image at the input plane. The two-dimensional spatial-frequency vector associated with any given point in the output image is proportional to the position vector from the optical axis to that point.

In the proposed method (see figure) the input image would be generated on a liquid-crystal spatial light modulator illuminated with a readout laser, which would be coherent with a reference laser. (It would be necessary to generate the input image in this way because the coherence of the laser light would be needed to form a hologram described subsequently.) A lens would be located at its focal distance from the input plane as well as from the output plane, where a charge-coupled-device (CCD) or an active-pixel sensor (APS) would be placed to detect the image. As a result, the Fourier transform of the input image would be formed on the image detector.

Capturing the intensity magnitude at the detector is not sufficient for reconstructing the image. For this reason, it would be necessary to write a hologram onto the image detector by means of interference between the lens-transformed image beam and the reference laser beam.

Because most of the information in a typical image is concentrated at low spatial frequencies, the bulk of information in optical Fourier transform would be concentrated about the optical axis. The image detector would sample the Fourier transform. The samples would be digitized, then entropy-coded by use of established digital electronic techniques

This work was done by Deborah Jackson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category.

NPO-20638

Topics:
Imaging and visualization Lasers Logistics Optics Physical Sciences Sensors and actuators
This Brief includes a Technical Support Package (TSP).
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High-Speed Image Compression Via Optical Transformation

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    Overview

    The document discusses a novel method for compressing image data that leverages the optical Fourier transform properties of a converging lens. This approach aims to significantly enhance the speed and efficiency of image compression compared to traditional digital methods, which often rely on power-hungry electronic circuits to compute the discrete Fourier transform.

    The core idea is that a converging lens can generate the Fourier transform of an image optically, which is a computationally intensive step in most image compression algorithms. By using this optical method, the time required to perform the transformation is drastically reduced, allowing for high-throughput image compression. This is particularly beneficial in applications where rapid data transmission is critical, such as in space missions involving fast flybys or probe insertions, where the image content is dynamic and time-sensitive.

    The document outlines the mechanics of how the optical Fourier transform works: when a lens is positioned at its focal distance from both the input and output planes, the image formed at the output plane represents the Fourier transform of the input image. The spatial-frequency vector at any point in the output image correlates with the position from the optical axis, concentrating lower frequency components at the center and higher frequencies outward.

    Additionally, the proposed method not only accelerates the image compression process but also reduces power consumption, making it more efficient for real-time applications. This is particularly relevant in scenarios where power resources are limited, such as in space exploration.

    The document emphasizes the potential of this optical image compression technique to revolutionize data transmission in various fields, particularly in aerospace applications where quick and efficient image processing is essential. It highlights the advantages of using optical methods over traditional digital circuits, paving the way for advancements in image processing technology.

    Overall, the document presents a promising optical approach to image compression that could lead to significant improvements in speed and efficiency, addressing the challenges faced in high-demand applications.