| Shack-Hartmann wavefront sensors are based upon the principals proposed late in the 19th century by a German physicist named Hartmann. At that time, the instruments needed to bring that idea to life had not yet been conceived. In 1971, when technology had matured to a point where putting Hartmann’s idea into action became possible, two American scientists named Shack and Platt improved on Hartmann’s idea and built the first wavefront sensor of this type, calling it a Hartmann-screen. Since then, it has become commonly known as Shack-Hartmann and is considered around the world to be the technique of choice for precision wavefront metrology. Imagine Optic has further improved on this technique by incorporating several proprietary technologies, including Dynamic Spot Tracking™ and Auto Spot Finder™, to increase the HASO™ family of wavefront sensors’ dynamic range.
Hartmann’s idea was to measure the aberrations (imperfections) present in a luminous source by passing it through a screen of apertures that would spatially decompose it into the equivalent number of points, each point being relative to a specific zone of the incoming source. To do this, he would have to measure the light at a given moment in time and, today, we call this the source’s wavefront.
Because he knew that the light coming through the screen from a perfect source would intersect the measurement grid at a specific point, he knew that the aberrations could be measured by calculating the difference between where a plane (perfect) wavefront would appear on the grid versus that of the one to be measured. This calculation would also provide the local slopes, or first derivative (angle), at which the wavefront intersected the surface behind each aperture of the grid.
Shack and Pratt improved on Hartmann’s idea by using microlenses to focus the incident wavefront onto the grid of a CCD camera. These subdivisions of the original source created what are called “multiple elementary beams.” These two improvements on Hartmann’s original idea gave birth to what have come to be known as Shack-Hartmann wavefront sensors. Devises based on this technology, including Imagine Optic’s HASO family of products, are the industry standard for precision wavefront metrology.
 The preceding figure illustrates how HASO Shack-Hartmann wavefront sensors function. In this diagram, d is the distance between the microlens array (principal image plane) and the CCD detector grid, i is the number of the microlens in the array, and ∆x is the intersection point of the elementary beam on the CCD detector grid. By using the using the simple formula to the right, the wavefront's local slopes are calculated and the phase map is displayed.
The screen captures below illustrate how HASO uses the information to construct both 2D and 3D images of the wavefront. Whereas some wavefront sensor providers’ software interface may present a wavefront image with smooth edges, this is not a true representation of your wavefront because, to do this, they must perform what’s call circular truncation of the wavefront data. This entails either the removal of data from the outside edge or the supposition of that data to smooth the edge. Imagine Optic provides you with a true image of your wavefront from each and every measured point.
Key HASO technology features
- Design optimization using wave propagation and signal detection modeling
- High-quality microlens array
- Ultra-precise coupling between camera and microlens array
- Robust bulk mechanical assembly
- Wide-range spot detection algorithms
- Rigorous calibration and test procedure
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