Shack-Hartmann Wavefront Sensors, 1.3 Megapixel Resolution
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Schematic of a Mircrolens Array Focusing a Distorted Wavefront
The Shack-Hartmann sensor consists of a lenslet array and a camera. When a wavefront enters the lenslet array, a spotfield is created on the camera; each spot is then analyzed for intensity and location. Using this method, Shack-Hartmann wavefront sensors can dynamically measure the wavefronts of laser sources or characterize the wavefront distortion caused by optical components. In addition, they can provide real-time feedback for adaptive optics systems, such as Thorlabs' Adaptive Optics Kits. For more details on the theory of Shack-Hartmann wavefront sensing, see the SH Tutorial tab above.
- CCD Camera Provides 1.3 Megapixel Resolution
- Real-Time Wavefront and Intensity Distribution Measurements
- Includes Interchangeable High-Quality Photolithographic Microlens Array
- Nearly Diffraction-Limited Spot Size
- Use with CW or Pulsed Light Sources
- USB Connection to PC
- Live Data Readout via TCP/IP
- Compact Housing: 48.5 mm x 32.0 mm x 40.4 mm with Baseplate
- Flexible Software Options
- GUI Software
- Instrument Driver Package for C Compilers
Thorlabs' High-Resolution Shack-Hartmann Wavefront Sensors, which incorporate CCD cameras with 1.3 meagpixel resolution, provide accurate measurements of the wavefront shape and intensity distribution of beams. These wavefront sensors are available with either a chrome-masked microlens array for use in the 300 - 1100 nm range or an AR-coated microlens array for use in the 400 - 900 nm range. The former has a lenslet pitch of 150 µm whereas the latter is available with a lenslet pitch of either 150 or 300 µm. These three offerings allow the end user to select a system that offers high spatial resolution, enhanced contrast, or high wavefront accuracy. Please note that calibration of the microlens-camera pair is required; to purchase a new lenslet array for a previously purchased Shack-Hartmann Wavefront Sensor, please contact Technical Support for a quotation on the microlens array and calibration service.
If your application would benefit from a fast wavefront sensor, please see our line of Shack-Hartmann wavefront sensors with frame rates up to 450 Hz. For more information about choosing the appropriate Shack-Hartmann wavefront sensor for a particular application, see the Selection Guide tab above.
Shack-Hartmann Kits with Two Microlens Arrays
Thorlabs also offers wavefront sensor kits (Item # WFS-K1 and WFS-K2) that include two microlens arrays and the base CCD camera unit loaded with the appropriate calibration data for the two lenslet arrays. Switching lenses is easy using the provided pick-up tool; the patented magnetic holder (US Patent No: 8,289,504) precicely postions the array correctly every time. These kits are ideal for situations where more than one light source or optical setup needs to be analyzed.
Each Shack-Hartmann Wavefront Sensor and Kit comes in a convenient storage and carrying case. Mounting accessories include an SM1A9 C-Mount to internal SM1 (1.035"-40) thread adapter for mounting Ø1" lens tubes and mounted optics, such as Neutral Density Filters, and a base plate for attaching Ø1/2" posts.
The included software package offers a user-friendly grapical interface with tools for choosing camera setting, calibration, analysis, and display options. All sensors require a USB2.0 port to operate. The software also includes drivers for C compilers, LabVIEWTM, LabWindows/CVITM, and DotNet for integration into custom system control and data collection software. For more information on the included software or to download the latest version, see the Software tab above.
|Detector Array Type||CCD|
|Camera Resolution (Max)||1280 x 1024 Pixels, Selectable|
|Pixel Sixe||4.65 µm x 4.65 µm|
|Aperture Size (Max)||5.95 mm x 4.76 mm|
|Frame Rate (Max)||15 Hz|
|Exposure Range||79 µs - 65 ms|
|Image Digitization||8 Bit|
*A global shutter exposes the entire detector at one time.
Microlens Array Specifications
|Wavelength Range||300 nm - 1100 nm||400 nm - 900 nm||400 nm - 900 nm|
|Effective Focal Length (When Mounted in WFS)||3.7 mm||5.2 mm||14.2 mm|
|Nominal Focal Length||5.2 mm||6.7 mm||18.6 mm|
|Number of Active Lenslets||Selectable by Software, Depending on Microlens Array|
|Number of Active Lenslets (Max)||39 x 31||19 x 15|
|Substrate Material||Fused Silica (Quartz)|
|Free Aperture||Ø9 mm|
|Lenslet Grid Type||Square Grid|
|Lenslet Pitch||150 µm||300 µm|
|Lens Shapea||Round, Plano-Convex Spherical||Square, Plano-Convex Parabolic|
|Fill Factor (Approximate)b||74.5%||100%|
|Lens Size||Ø146 µm||300 µm x 300 µm|
|Array Size||10 mm x 10 mm x 1.2 mm|
|Wavefront Accuracy a||λ/15 rms @ 633 nm||λ/50 rms @ 633 nm|
|Wavefront Sensitivity b||λ/50 rms @ 633 nm||λ/150 rms @ 633 nm|
|Wavefront Dynamic Range c||>100λ @ 633 nm||>50λ @ 633 nm|
|Local Wavefront Curvature d||>7.4 mm||>10 mm||>40 mm|
|Optical Input Connector||C-Mount|
|Physical Size (H x W x D)||34 mm x 32 mm x 45.5 mm|
|Power Supply||<1.5 W via USB|
|External Trigger Input Specifications|
|Trigger Slope||Software Selectable: Low-High or High-Low|
|Maximum Low Level||2 V|
|Minimum High Level||5 V|
|Input Current (Max)||10 mA|
a Absolute accuracy using internal reference. Measured for spherical wavefronts with a known radius of curvature.
b Typical relative accuracy with respect to a reference wavefront (user calibration). Reference and each measurement values are averaged over 10 frames.
c Over entire aperture of wavefront sensor.
d Radius of wavefront curvature over single lenslet aperture.
How a Shack-Hartmann Wavefront Sensor Works
A Shack-Hartmann wavefront sensor uses a lenslet array to divide an incoming wavefront into an array of smaller beams. Each beam is focused onto a CMOS camera that is placed at the focal plane of the lenslet array, as shown in the figure to the left. If a uniform, planar wavefront is incident on the Shack-Hartmann sensor, each lenslet forms a spot along the optical axis of the lenslet. This yields a regularly spaced grid of spots on the detector.
A distorted wavefront, however, will cause some lenslets to focus with the spots displaced from the optical axis. Therefore, the light imaged on the sensor will consist of some regularly spaced spots mixed with displaced spots and missing spots. This information can be used to calculate the shape of the wavefront that was incident on the microlens array. Shack- Hartmann type wavefront sensors can be used to characterize the performance of optical systems. In addition, they are increasingly used in applications where real-time monitoring of the wavefront is used to control an adaptive optic with the intent of removing the wavefront distortion before creating an image.
Wavefront Distortion and Spot Displacement
As discussed above, each microlens of the lenslet array collects the light falling onto its aperture and generates a single spot at the detector plane. The figure below is a detail of a wavefront incident on a single microlens. The spot positions will be directly behind the lenses (shown in green) only if the incident wavefront is flat and parallel to the plane of the lenslets. For a wavefront which is distorted in the region of the microlens, the spot positions will be deviated in the X and Y direction (as shown by the red dot) so that every spot lies away from the optical axis z of its associated microlens by an angle θ. This angle θ is the same as the angle between the distorted wavefront and the planar wavefront, as shown in the figure.
Parameters Affecting Shack-Hartmann Performance
Four parameters that influence the performance of a Shack-Hartmann wavefront sensor are the number of lenslets that cover the detector active area, the dynamic range, the measurement sensitivity, and the lenslet focal length. The number of lenslets restricts the maximum number of Zernike coefficients that a reconstruction algorithm can reliably calculate. When selecting the number of lenslets required, consider the amount of distortion being modeled (i.e., how many Zernike coefficients are needed to effectively represent the true wave abberation).
Sensitivity (αmin) is a function of the minimum detectable spot displacement (δymin), as described by the equation:
αmin = δymin / f
where f is the focal length of the microlens. Dynamic range, θmax, is a measure of the maximum extent of phase that can be measured:
αmax = δymax / f = (d / 2) / f
where d is the diameter of the microlens. Both of these equations were derived using the small angle approximation. αmin is the minimum detectable wavefront slope that can be measured by the wavefront sensor. The minimum detectable spot displacement δymin depends on the pixel size of the detector, the accuracy of the centroid algorithm, and the signal to noise ratio of the sensor. αmax is the maximum wavefront slope that can be measured by the wavefront sensor and corresponds to a spot displacement of δymax, which is equal to the lenslet radius.
A Shack-Hartmann sensor's measurement accuracy (i.e., the minimum wavefront slope that can be measured reliably) depends on its ability to precisely measure the displacement of a focused spot with respect to a reference position. A conventional algorithm will fail to determine the correct centroid of a spot if it partially overlaps another spot or if the focal spot of a lenslet falls outside of the area of the sensor assigned to detect it (spot crossover). Special algorithms can be implemented to overcome these problems, but the limit the dynamic range of the sensor. The dynamic range of a system can be increased by using a lenslet with either a larger diameter or a shorter focal length. Increasing the dynamic range by increasing the lenslet diameter decreases the number of Zernike coefficients available to represent the wavefront. Conversely, increasing the dynamic range by shortening the focal length decreases the sensor's sensitivity. Ideally, a lenslet with the longest focal length that meets both the dynamic range and measurement sensitivity requirements should be used.
The Shack-Hartmann wavefront sensor is capable of providing information about the intensity profile as well as the calculated wavefront.
Selecting the Proper Shack-Hartmann Wavefront Sensor
Thorlabs offers two different cameras for a variety of wavefront sensing applications. The wavefront sensors on this page feature a CCD camera with a resolution of 1.3 Megapixels. Thorlabs also offers a line of Shack-Hartmann wavefront sensors with a CMOS camera capable of measuring frame rates up to 450 Hz. Each camera type is available with one of three microlens arrays offering flexibility in wavelength range, spatial resolution, spot contrast, and wavefront accuracy.
Choosing a Camera
The high resolution of the 1.3 Megapixel CCD camera can make wavefront measurements of the spot field with high accuracy and sensitivity. This makes the wavefront sensors built with these cameras ideal for accurate analysis of wavefront distortions of light sources and optical components.
The high frame rate of the CMOS detector enables more wavefront measurements per second and thus can detect faster wavefront fluctuations. This is ideal as a sensor for a high-speed adaptive optics system.
Camera Responsivity Comparison
Choosing a Microlens Array
|Features of WFS Microlens Arrays|
|Item #||High Spot Contrast||High Wavefront Accuracy||High Spatial Resolution||Low Back Reflection|
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Each Shack-Hartmann Wavefront Sensor is available with 3 different microlens arrays. The table to the right details the features of the microlens included with each item.
WFS150-5C and WFS10-5C Microlens Features
These sensors include a chrome-masked microlens array, which prevents light from passing between the microlenses. This leads to a higher contrast in the spot field but will considerably increase the amount of back reflections. This microlens can be used over an extended wavelength range of 300 nm to 1100 nm. This microlens array features a 150 µm lens pitch, which offers a larger number of spots and thus a higher spatial resolution of the wavefront, and a wider wavefront dynamic range because of their shorter focal length.
WFS150-7AR, WFS10-7AR, WFS300-14AR and WFS10-14AR Microlens Features
These sensors include a microlens array that is AR coated for the 400 nm to 900 nm wavelength range, making them ideal for applications that are sensitive to back reflections. The WFS150-7AR and WFS10-7AR includes a microlens array with a 150 µm lens pitch, which offers a larger number of spots and thus a higher spatial resolution of the wavefront, and a wider wavefront dynamic range because of their shorter focal length. The WFS300-14AR and WFS10-14AR includes a microlens array with a 300 µm lens pitch, which offers higher wavefront accuracy and sensitivity at the expense of dynamic range and spatial resolution.
Software and Graphical User Interface
GUI Display of Measured Wavefront
For screen images of the GUI display options, please click on the links:
- Beam Centroid and Diameter
- Modal and Zonal Reconstructed Wavefront
- Max Variance of Wavefront, Peak-to-Valley (PV), and RMS of Wavefront
- Zernike Representations of Tilt, Defocus, Astigmatism,
Coma, Spherical, and Higher Order Aberrations
- Fourier and Optometric Parameters
The software includes a driver package for constructing custom applications with the following software packages:
- C Compilers
Click here for the latest version of the Shack-Hartmann Wavefront Sensor Software Package. The software requires WindowsTM XP, Vista, 7, or later (32 or 64 bit). A USB 2.0 port is also required.