Coded aperture

Coded Apertures or Coded-Aperture Masks are grids, gratings, or other patterns of materials opaque to various wavelengths of light. The wavelengths are usually high-energy radiation such as X-rays and gamma rays. By blocking and unblocking light in a known pattern, a coded "shadow" is cast upon a plane of detectors. Using computer algorithms, properties of the original light source can be deduced from the shadow on the detectors. Coded apertures are used in X- and gamma rays because their high energies pass through normal lenses and mirrors.

Rationale

Image formation, normally done at optical wavelengths by lenses and mirrors, must be done for non-focusable wavelengths via image modulation. The pinhole camera is the most basic form of such a modulation imager, but a single aperture mask can contain many holes, in one of several particular patterns, to improve the throughput for hard X-rays and γ-rays, for example. Multiple masks, at varying distances from a detector, add flexibility to this tool. Specifically the modulation collimator, invented by Minoru Oda, was used to identify the first cosmic X-ray source and thereby to launch the new field of X-ray astronomy in 1965. Many other applications in other fields, such as tomography, have since appeared.

In a coded aperture more complicated than a pinhole camera, images from multiple apertures will overlap at the plate or detector array. It is thus necessary to use a computational algorithm (which depends on the precise configuration of the aperture arrays) to reconstruct the original image. In this way a sharp image can be achieved without a lens. The image is formed from the whole array of sensors and is therefore tolerant to faults in individual sensors; on the other hand it accepts more background radiation than a focusing-optics imager (e.g., a refracting or reflecting telescope, and therefore is normally not favored at wavelengths where these techniques can be applied.

The coded aperture imaging technique is one of the earliest forms of computational photography and has a strong affinity to astronomical interferometry

Well known types of masks

Different mask patterns exhibit different image resolutions, sensitivities and background-noise rejection, and computational simplicities and ambiguities, aside from their relative ease of construction.

• URA
• MURA
• Levin
• Veeraraghavan

Coded-Aperture Space Telescopes

• Rossi X-ray Timing Explorer- ASM
• BeppoSAX- Wide Field Camera
• INTEGRAL- IBIS and SPI
• Swift- BAT
• In addition, the SAS-3 and RHESSI missions detect radiation based on a combination of masks and rotational modulation

See Also

• Pinhole camera
• Deconvolution
• Rotational Modulation Collimator
• Computational_photography
• Tomographic reconstruction and X-ray computed tomography

External links

• Coded Aperture Imaging in High-Energy Astronomy [1]
• In the news: [2]