Coded aperture
Encyclopedia
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.
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-ray
s 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
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
Pinhole camera
A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
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-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s and γ-rays
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
, 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
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
in 1965.
Many other applications in other fields, such as tomography
Tomography
Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science,...
, have since appeared.
In a coded aperture more complicated than a pinhole camera
Pinhole camera
A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
, 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
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
, 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
Computational photography
Computational imaging refers to any image formation method that involves a digital computer. Computational photography refers broadly to computational imaging techniques that enhance or extend the capabilities ofdigital 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 ExplorerRossi X-ray Timing ExplorerThe Rossi X-ray Timing Explorer is a satellite that observes the time structure of astronomical X-ray sources. The RXTE has three instruments—the Proportional Counter Array, the High-Energy X-ray Timing Experiment , and one instrument called the All Sky Monitor...
- ASM - BeppoSAXBeppoSAXBeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts , the most energetic events known in the universe...
- Wide Field Camera - INTEGRALINTEGRALThe European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.INTEGRAL is an ESA mission in...
- IBIS and SPI - SwiftSwift Gamma-Ray Burst MissionThe Swift Gamma-Ray Burst Mission consists of a robotic spacecraft called Swift, which was launched into orbit on 20 November 2004, 17:16:00 UTC on a Delta II 7320-10C expendable launch vehicle. Swift is managed by the NASA Goddard Space Flight Center, and was developed by an international...
- BAT - In addition, the SAS-3Third Small Astronomy SatelliteThe Small Astronomy Satellite 3 was a NASA X-ray astronomy space telescope. It functioned from May 7, 1975 to April 1979. It covered the X-ray range with four experiments on board...
and RHESSI missions detect radiation based on a combination of masks and rotational modulationRotational Modulation CollimatorRotational Modulation Collimators are a specialization of the modulation collimator, an imaging device invented by Minoru Oda. Devices of this type create images of high energy X-rays . Since high energy X-rays are not easily focused, such optics have found applications in various instruments...
See Also
- Pinhole cameraPinhole cameraA pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
- DeconvolutionDeconvolutionIn mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...
- Rotational Modulation CollimatorRotational Modulation CollimatorRotational Modulation Collimators are a specialization of the modulation collimator, an imaging device invented by Minoru Oda. Devices of this type create images of high energy X-rays . Since high energy X-rays are not easily focused, such optics have found applications in various instruments...
- Computational photographyComputational photographyComputational imaging refers to any image formation method that involves a digital computer. Computational photography refers broadly to computational imaging techniques that enhance or extend the capabilities ofdigital photography...
- Tomographic reconstructionTomographic reconstructionThe mathematical basis for tomographic imaging was laid down by Johann Radon. It is applied in computed tomography to obtain cross-sectional images of patients...
and X-ray computed tomography
External links
- Coded Aperture Imaging in High-Energy Astronomy http://astrophysics.gsfc.nasa.gov/cai/
- In the news: http://news.bbc.co.uk/1/hi/technology/7553622.stm