Articles and Application Notes

The following technical articles and application notes provide additional information about 4D products and technologies. You can also visit our Practical Tips blog for helpful information about working with 4D Technology interferometers, profilers, cameras and gauges.

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Dynamic Interferometry: Measurement of Space Optics and Structures

Over the last two decades the use of single-frame interferometric techniques, known as Dynamic Interferometry, have become widely available in commercial interferometer systems and they have been used extensively in the production of state-of-the-art space-based optical systems. This paper presents an overview of the techniques and configurations used to build dynamic interferometers and measurement results for a variety of space-based optical components as well as the structures that hold them under simulated space-flight conditions. These techniques and configurations have applicability for many non-space applications as well.

Keywords: laser interferometry, space optics, measure optics, optical metrology

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Optimizing the Light Level

The optimal light level for interferometric measurements depends upon the characteristics of the test part, test system and test environment. Techniques to optimize the light level will be described.

Keywords: laser interferometry, optimizing gain and exposure

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Artifact Free Calibration of Spatial Carrier Interferometry

An automated calibration technique is introduced for spatial carrier interferometry, allowing nanometer-level measurement of precision optics even in difficult environments.

Keywords: interferometer, optical testing, spatial carrier, interferometer calibration

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Setting the Wedge Factor

This technical note explains the purpose of the Wedge Factor and its correct values for measuring wavefront error and surface height in common testing configurations.

Keywords: laser interferometer, optical testing, wedge factor, vertical scaling

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Snapshot Imaging Polarimeter Performance and Applications

A camera is introduced capable of obtaining single snapshot, quantitative, polarimetric measurements. The camera employs a micropolarizer array, enabling instantaneous measurement of the linear Stokes parameters.

Keywords: polarimetry, micropolarizer array, pixelated polarizer, snap shot imaging polarimeter, Stokes parameters

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Nano Surface Standards Gain Traction

Understanding the accuracy, resolution and stability of surface texture measurement results requires exacting nanoscale standards.

Keywords: surface texture measurement, surface roughness measurement, standards

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On-Screen Statistics in 4Sight Software

A new feature in 4Sight software provides highly readable, real-time feedback for easy alignment and production measurement.

Keywords: interferometry software, 4Sight

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Correction of Errors in Polarization Based Dynamic Phase Shifting Interferometers

A general calculation of phase calculation error is presented, as is an algorithm that is capable of mitigating phase-dependent measurement error on-the-fly.

Keywords: laser interferometry, phase shifting, dynamic interferometry

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Dynamic Phase Imaging Microscope Enables Measurement of Motion of Living Cells

Images and movies created with this microscope let researchers track the movement of cells, visualize cell interactions, and measure small motions within cells, tissues and structures.

Keywords: dynamic phase imaging microscope, cell imaging, biological imaging

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Measuring Precision Telescope Optics (with a Streetcar Outside Your Door)

When Tucson's Modern Streetcar was installed, NOAO's Optics Shop turned to 4D's Dynamic Interferometry to measure despite the vibration.

Keywords: dynamic interferometry, laser interferometer, vibration immunity, optics measurement

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Dynamic Phase Imaging and Processing of Moving Biological Organisms

A novel interference Linnik microscope system enables instantaneous measurement of live biological samples, producing videos and images of cell motion and interactions.

Keywords: dynamic phase imaging microscope, cell imaging, biological imaging

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Dynamic Surface Roughness Profiler

A dynamic profiler is presented that is capable of precision measurement of surface roughness in the presence of significant vibration or motion. Utilizing a special CCD camera incorporating a micro-polarizer array and a proprietary LED source, quantitative measurements were obtained with exposure times of

Keywords: interferometer, optical testing, surface roughness, optical profilometer.

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Multilayer Thin-film Inspection Through Measurements of Reflection Coefficients

A vibration immune interferometer is described to measure the thickness, refraction index and surface profile of a thin film stack.

Keywords: laser interferometer, dynamic interferometer, thin film thickness, thin film metrology

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Dynamic Phase Imaging Utilizing a 4-Dimensional Microscope System

This label-free, vibration insensitive imaging system enables measurement of biological objects using harmless light levels, with fields of view up to 1mm.

Keywords: dynamic phase imaging microscope, cell imaging, biological imaging

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A Pixelated Polarizer Based Camera for Instantaneous Interferometric Measurements

A pixel level micropolarizer array bonded to a scientific camera has been developed for use in commercial dynamic interferometers.

Keywords: micropolarizer array, pixelated polarizer, snapshot imaging polarimeter, spatial phase shifting, wiregrid

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Extended Vertical Range Roughness Measurements in Non-ideal Environments

A dynamic interferometry technique is being developed in which the range is extended vertically to enhance surface roughness measurements made in non-ideal environments.

Keywords: laser interferometry, dynamic interferometry, vertical range

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Spatial Frequency Response and Resolution Limitations of Pixelated Mask Spatial Carrier Based Phase Shifting Interferometry

Using the small phase step approximation, it is shown that the instrument transfer function can be approximated as the product of the system optical transfer function and the spatial carrier
processing filter transfer function.

Keywords: interferometry, optical testing, spatial carrier

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Dynamic Interferometry for On-Machine Metrology Summary

A compact, vibration insensitive interferometer that is well suited for measuring optics while mounted in situ on polishing equipment is presented.

Keywords: dynamic interferometry, optical testing, in situ measurement, polishing metrology

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Noise Reduction in Dynamic Interferometry Measurements

A method for reducing the coherent noise in dynamic interferometry measurements is presented.The method is particularly important in "on-machine"metrology applications where residual noise can be polished into the surface under test.

Keywords: dynamic interferometer, vibration insensitivity, coherent noise reduction

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Pixelated Mask Extends Spatial Response

The spatial resolution of a laser interferometer with a pixelated phase mask is found to extend virtually to the limit of the detector.

Keywords: pixelated phase mask, laser interferometer, spatial resolution

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High Throughput Measurement Speeds Production of Large Mirrors

Dynamic Interferometry provides early process feedback for faster, more aggressive polishing of large mirrors.

Keywords: pixelated phase mask, laser interferometer

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Measuring Wedge with the FizCam 2000

A new technique provides a faster, more repeatable method for vibration-insensitive measurement of wedge.

Keywords: wedge measurement, measuring wedge, Fizeau interferometer

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Dual Mode Interferometer for Measuring Dynamic Displacement of Specular and Diffuse Components

We present a dual mode interferometer system based on a single-frame acquisition sensor the measures the dynamic displacement of both specular and diffuse components.

Keywords: speckle interferometer, ESPI, specular surface, diffuse surface

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Measuring Homogeneity with the FizCam 2000

A new technique simplifies measurement of homogeneity to decrease measurement time and improve repeatability.

Keywords: measure homogeneity, homogeneity measurement

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Measuring Transmitted Wavefront Error with the FizCam 2000

A new technique for vibration-insensitive measurement of transmitted wavefront error (TWE) improves repeatability by more than 7X.

Keywords: measure transmitted wavefront error, TWE measurement, Fizeau interferometer

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Path Matching with the FizCam 2000

The ability to isolate and measure plane-parallel surfaces expands the capabilities of laser interferometry.

Keywords: laser interferometry, path matching, plane-parallel optics, Fizeau interferometer

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Measuring Large Mirrors With Secondary And Tertiary Support Structures

4Sight analysis software provides an extensive toolset for measuring mirrors partially obscured by support structures.

Keywords: primary mirror, secondary mirror, tertiary mirror, laser interferometry

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Best Practices for Long Path Measurements

Tips to ensure accurate, vibration-insensitive, long-standoff measurements.

Keywords: laser interferometry, long path measurement, long standoff measurement

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Measuring High NA Optical Systems Without Retrace Error

An on-axis, vibration-immune Fizeau interferometer enables accurate measurement of fast spheres.

Keywords: laser interometer, spherical optics, measure sphere

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Instantaneous Phase-shift Fizeau Interferometer Utilizing a Synchronous Frequency Shift Mechanism

An on-axis, vibration insensitive, polarization Fizeau interferometer is realized through the use of a pixelated polarization mask, spatial carrier phase shifting technique, in conjunction with a high coherence source and a polarization frequency shift device.

Keywords: dynamic Fizeau interferometer, vibration insensitivity, pixelated mask, short coherence

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Novel Interferometer Enables Challenging Measurements

The FizCam 2000 dynamic interferometer enables vibration insensitive measurement of plane-parallel optics, remote cavities and large convex mirrors.

Keywords: dynamic interferometer, remote cavity testing, plane parallel optics

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Optical Metrology for Large Telescope Optics

Measuring large optics requires metrology systems that function despite vibration, turbulence and other challenges.

Keywords: dynamic interferometer, air turbulence, telescope optics, large optics

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Improved Interferometric Optical Testing

While the basic principles of interferometry have been well known for more than 100 years, new developments are increasing its metrology applications for industry and research.

Keywords: dynamic interferometer, optical testing, optical metrology

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Pixelated Mask Spatial Carrier Phase Shifting Interferometry Algorithms and Associated Errors

A pixelated spatial carrier, phase shifting technique has been developed which allows for the grouping of phase shifted pixels around a single point in two dimensions, minimizing the phase shift change due to the spatial variation in the test wavefront.

Keywords: spatial carrier phase shifting, interferometry

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Low Coherence Vibration Insensitive Fizeau Interferometer

An on-axis, vibration insensitive, polarization Fizeau interferometer is realized through the use of a novel pixelated mask spatial carrier phase shifting technique in conjunction with a low coherence source and a polarization delay-line.

Keywords: Fizeau interferometer, low coherence, short coherence

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Getting Rid of the Jitters

Mechanical vibration, moving parts and turbulent airflow present significant problems for most interferometric test methods. New techniques help solve the problem.

Keywords: Laser interferometer, vibration, turbulent airflow, interferometric test methods

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Going Through A Phase

Dynamic phase-shifting interferometry beats vibration.

Keywords: dynamic laser interferometry

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Analysis of a Micropolarizer Array-based Simultaneous Phase-shifting Interferometer

We present theoretical and experimental results for an interferometer based on a pixelated phase-shifting, dynamic (vibration-insensitive) interferometer using CCD pixel-level phase-shifting.

Keywords: dynamic interferometer, pixelated phase shifting, spatial phase shifting

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Dynamic Phase-Shifting Electronic Speckle Pattern Interferometer

We present a new electronic speckle pattern interferometer that takes advantage of a single-frame spatial phase-shifting technique to significantly reduce sensitivity to vibration and enable complete data acquisition in a single laser pulse. The interferometer was specifically designed to measure the stability of the James Webb Space Telescope (JWST) backplane.

Keywords: ESPI, electronic speckle pattern interferometry, James Webb Space Telescope (JWST)

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Modern Approaches In Phase Measuring Interferometry

This paper discusses two single-shot interferometric techniques for reducing the sensitivity of an optical test to vibration; simultaneous phase-shifting interferometry and a special form of spatial carrier interferometry utilizing a micropolarizer phase-shifting array.

Keywords: phase-shifting interferometry, spatial carrier interferometry, micropolarizer phase-shifting array

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Pixelated Phase-mask Dynamic Interferometer

We demonstrate a new type of spatial phase-shifting, dynamic interferometer that can acquire phase-shifted interferograms in a single camera frame.

Keywords: phase-shifting interferometry, micropolarizer array, dynamic interferometry

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Phase-shifting Multi-wavelength Dynamic Interferometer

We present a new multi-wavelength interferometer that uses two successive, single frame measurements obtained rapidly in time to significantly reduce sensitivity to vibration.

Keywords: multi-wave interferometry, multi-wavelength laser interferometer

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Interferometric Measurement of the Vibrational Characteristics of Light-weight Mirrors

We present a technique to characterize and quantitatively measure the vibrational mode shapes and amplitudes of mirrors concurrently with surface figure testing.

Keywords: vibration testing, lightweight mirrors, surface shape measurement

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Vibration Insensitive, Interferometric Measurements of Mirror Surface Under Cryogenic Conditions

We report on a technique to measure the surface figure of mirrors under extreme vibrational conditions. Measurements are presented of the surface figure changes of Zerodur primary mirrors with both spherical and parabolic shapes, manufactured for the NASA Deep Impact program. Conditions ranged from room temperature to 130K.

Keywords: cryogenic testing, primary mirror, Deep Impact

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Instantaneous Phase Shift Point Diffraction Interferometer

We present a method that accomplishes high resolution phase-shift interferometry with a self-referencing point diffraction plate, where all the phase-shifted data is acquired simultaneously.

keywords: phase shift point diffraction interferometer, phase shifting

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Overcoming the Fringe Jitters

High-speed testing can provide the necessary isolation for accurate nanometer-scale measurements.

Keywords: high speed testing, vibration insensitivity, laser interferometry

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Figure Testing of 300 mm Zerodur Mirrors at Cryogenic Temperatures

Figure testing for the 300mm, Zerodur primary mirror for the Deep Impact space mission was performed over the entire range of operational temperatures, from ambient through 130K.

Keywords: cryo-vac testing, primary mirror, Deep Impact

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Dynamic interferometry Handles Vibration

Given the advantages of PSI measurements, a number of vibration-tolerant solutions have been developed. Here we briefly review a range of the solutions that have been successfully implemented.

Keywords: dynamic interferometry, vibration tolerance

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Advances in Interferometric Metrology

Modern electronics, computers, and software have made interferometry an extremely powerful tool in many fields including the testing of optical components and optical systems. This paper will discuss some of the recent advances in reducing the sensitivity of phase-shifting interferometers to vibration.

Keywords: interferometry, metrology, optical testing, interference, optical measurements

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