SPONSORED: WAVEFRONT SENSING A new wave of
quality control Keely Portway takes a look at the Shack-Hartmann wavefront sensing technique and how it can be used to simply and quickly assess the quality of optical systems
O
ptical systems can be highly complex, with extensive knowledge and
experience required in their design and characterisation. Due to the number of components involved, the skills required must extend to both the system and individual component levels. This includes the evaluation of optical components for quality. Interferometry has
traditionally been used to evaluate optical components and systems, comparing interference fringes of a reference optic to that when the test optic is present. A variety of interferometers and test procedures have been developed over the years, leading to the latest generation of instrumentation and software. But this technique is not without its challenges, and its reliance on the ‘perfect’ reference beam is something that can be tricky to achieve in practice due to vibrations. The Shack-Hartmann
wavefront sensing technique offers an alternative solution. A Shack-Hartmann wavefront sensor consists of a microlens array placed in front of a 2D image sensor. If the sensor is placed at the geometric focal plane of the lens and uniformly illuminated, the integrated gradient of the wavefront across the lens is proportional to the displacement of the centroid. Consequently, any phase aberration can be approximated by a set of discrete tilts.
30 Electro Optics May 2022
James Webb: A technician inspects the primary mirror of the James Webb Space Telescope. The team’s improvements involving a Shack-Hartmann sensor have opened up applications for eye doctors examining human eyesight
Proven technology The technique itself is not new. In fact, the sensor’s design improves upon an array of holes in a mask that had originally been developed in 1904 by German physicist, Johannes Franz Hartmann, as a way of tracing individual rays of light through the optical system of a large telescope. The late 1960s saw American physicist Roland Shack and research student Ben Platt modify the ‘Hartmann screen’ by replacing the apertures in an opaque screen by an array of microlenses. The Shack-Hartmann
technique has become more widely adopted in recent times, thanks to its high accuracy, rapidity and reduced sensitivity to vibrations than interferometers. The technique
is even becoming a driving factor in the overall global wavefront sensor market, which a research report by Transparency Market Research (TMR) estimates will grow by about $4bn by 2031. In optical metrology, said the report, Shack-Hartmann wavefront sensors are being used for their versatility and improved performance. This means that vendors operating in the global wavefront sensor market are strengthening their production capabilities in Shack-Hartmann wavefront sensors.
“The technique is becoming a driving factor in the overall global wavefront sensor market”
A recent and significant
development in the field came from the launch of NASA’s James Webb Space Telescope. To accurately measure the shape of Webb’s mirrors during manufacturing, the team made new improvements to wavefront sensing technology, with a measurement device known as the Scanning Shack-Hartmann Sensor. The improvements have opened up further applications, allowing, for example, eye doctors to get more detailed information about the shape of an eye in seconds rather than hours.
Gaining momentum Preetesh Mistry, sales manager – photonics at Pro-Lite Technology, has seen for himself the increase
@electrooptics |
www.electrooptics.com
NASA/Chris Gunn
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
