Shack-Hartmann
Encyclopedia
A Shack–Hartmann wavefront sensor (SHWFS) is an optical instrument used to characterize an imaging system. It is a wavefront sensor
commonly used in adaptive optics
systems. It consists of an array of lenses (called lenslets) of the same focal length. Each is focused onto a photon sensor (typically a CCD
array or quad-cell). The local tilt of the wavefront
across each lens can then be calculated from the position of the focal spot on the sensor. Any phase aberration
can be approximated to a set of discrete tilts. By sampling an array of lenslets all of these tilts can be measured and the whole wavefront approximated.
Since only tilts are measured the Shack–Hartmann can not detect discontinuous steps in the wavefront.
The design of this sensor was based on an aperture array
that had been developed in 1900 by Johannes Franz Hartmann
as a means to trace individual rays of light through the optical system of a large telescope, thereby testing the quality of the image. In the late 1960s, Roland Shack and Ben Platt modified the Hartmann screen by replacing the apertures in an opaque screen by an array of lenslets. The terminology as proposed by Shack and Platt was Hartmann screen. The fundamental principle seems to be documented even before Huygens
by the Jesuit philosopher, Christopher Scheiner, in Austria
.
Shack–Hartmann sensors are used to characterize eyes for corneal treatment of complex refractive errors.
Recently, Pamplona et al. developed an inverse of the Shack–Hartmann system to measure one's eye lens aberrations. While Shack–Hartmann sensors measure localized slope of the wavefront error using spot displacement in sensor plane, Pamplona et al. make the user shift the spots till they are aligned. The knowledge of this shift provides data to estimate the first-order parameters such as radius of curvature and hence error due to defocus and spherical aberration.
Wavefront sensor
A wavefront sensor is a device for measuring the aberrations of an optical wavefront. Although an amplitude splitting interferometer such as the Michelson interferometer could be called a wavefront sensor, the term is normally applied to instruments that do not require an unaberrated reference...
commonly used in adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
systems. It consists of an array of lenses (called lenslets) of the same focal length. Each is focused onto a photon sensor (typically a CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
array or quad-cell). The local tilt of the wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
across each lens can then be calculated from the position of the focal spot on the sensor. Any phase aberration
Aberration
An aberration is something that deviates from the normal way.Aberration may refer to:In optics and physics:*Optical aberration, an imperfection in image formation by an optical system...
can be approximated to a set of discrete tilts. By sampling an array of lenslets all of these tilts can be measured and the whole wavefront approximated.
Since only tilts are measured the Shack–Hartmann can not detect discontinuous steps in the wavefront.
The design of this sensor was based on an aperture array
Hartmann mask
Hartmann mask is a tool to help focusing telescopes, mainly used by amateur astronomers.- Theory and practice :...
that had been developed in 1900 by Johannes Franz Hartmann
Johannes Franz Hartmann
Johannes Franz Hartmann was a German physicist and astronomer. In 1904, while studying the spectroscopy of Delta Orionis he noticed that most of the spectrum had a shift, except the calcium lines, which he interpreted as indicating the presence of interstellar medium-External links:*...
as a means to trace individual rays of light through the optical system of a large telescope, thereby testing the quality of the image. In the late 1960s, Roland Shack and Ben Platt modified the Hartmann screen by replacing the apertures in an opaque screen by an array of lenslets. The terminology as proposed by Shack and Platt was Hartmann screen. The fundamental principle seems to be documented even before Huygens
Huygens
Huygens is a Dutch patronymic surname, meaning "son of Hugo". People with the name Huygens include:People* Constantijn Huygens , Dutch poet and composer...
by the Jesuit philosopher, Christopher Scheiner, in Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
.
Shack–Hartmann sensors are used to characterize eyes for corneal treatment of complex refractive errors.
Recently, Pamplona et al. developed an inverse of the Shack–Hartmann system to measure one's eye lens aberrations. While Shack–Hartmann sensors measure localized slope of the wavefront error using spot displacement in sensor plane, Pamplona et al. make the user shift the spots till they are aligned. The knowledge of this shift provides data to estimate the first-order parameters such as radius of curvature and hence error due to defocus and spherical aberration.