Industrial optical inspection often requires high speed and high throughput of materials. Engineers use a variety of techniques to handle these inspection needs. Some examples include line scan cameras, high speed multi-spectral and laser-based systems. High-volume manufacturing presents different challenges for inspection engineers. For example, manufacturers produce some components in quantities of millions per month, per week or even per day. Quality control of so many parts requires creativity to achieve the measurement needs. At times, traditional vision systems lack the contrast to provide the data required. In this paper, we show how dynamic polarization imaging captures high contrast images. These images are useful for engineers to perform inspection tasks in some cases where optical contrast is low. We will cover basic theory of polarization. We show how to exploit polarization as a contrast enhancement technique. We also show results of modeling for a polarization inspection application. Specifically, we explore polarization techniques for inspection of adhesives on glass.
3D microscopes based on white light interference (WLI) provide precise measurement for the topography of engineering surfaces. However, the display of an object in its true colors as observed under white illumination is often desired; this traditionally has presented a challenge for WLI-based microscopes. Such 3D color display is appealing to the eye and great for presentations, and also provides fast evaluation of certain characteristics like defects, delamination, or deposition of different materials. Determination of color as observed by interferometric objectives is not straightforward; we will present how color imaging capabilities similar to an ordinary microscope can be obtained in interference microscopes based on WLI and we will give measurement and imaging examples of a few industrial samples.
3D microscopes based on white light interferometry (WLI) with vertical scanning have been widely used in many areas of surface measurements and characterizations for decades. This technology provides fast, non-contact, and full-field surface 3D measurements with vertical resolution as low as the sub-nanometer range. Its applications include measurements of step height, surface roughness, film thickness, narrow trench and via depths as well as other geometric and texture parameters. In order to assure the highest accuracy of the measurement, scanner linearity needs to be maintained or monitored so that the nonlinearity can be accounted for during the measurement. This paper describes a method that accounts for nonlinearities in real time without the need to store frame data; in addition this method is shown to be less sensitive to vibrations than previous methods described. The method uses an additional interferometer, a distance measuring interferometer to measure the actual scanner position at each scan step.
A 4-mirror prime focus corrector is under development to provide seeing-limited images for the 10-m aperture Hobby-
Eberly Telescope (HET) over a 22 arcminute wide field of view. The HET uses an 11-m fixed elevation segmented
spherical primary mirror, with pointing and tracking performed by moving the prime focus instrument package (PFIP)
such that it rotates about the virtual center of curvature of the spherical primary mirror. The images created by the
spherical primary mirror are aberrated with 13 arcmin diameter point spread function. The University of Arizona is
developing the 4-mirror wide field corrector to compensate the aberrations from the primary mirror and present seeing
limited imaged to the pickoffs for the fiber-fed spectrographs. The requirements for this system pose several challenges,
including optical fabrication of the aspheric mirrors, system alignment, and operational mechanical stability.
The 8-meter mirror production capacity at the University of Arizona is well known. As the Arizona Stadium facility is
occupied with giant mirrors, we have developed capability for grinding, polishing, and testing 4-m mirrors in the large
optics shop in the College of Optical Sciences. Several outstanding capabilities for optics up to 4.3 meters in diameter
are in place:
A 4.3-m computer controlled grinding and polishing machine allows efficient figuring of steeply aspheric and nonaxisymmetric
Interferometry (IR and visible wavelengths) and surface profilometry making novel use of a laser tracker allows quick,
accurate in-process measurements from a movable platform on a 30-m vertical tower.
A 2-meter class flat measured with a 1-m vibration insensitive Fizeau interferometer and scanning pentaprism system;
stitching of 1-m sub-apertures provides complete surface data with the technology ready for extension to the 4 m level.
These methods were proven successful by completion of several optics including the 4.3-m Discovery Channel
Telescope primary mirror. The 10 cm thick ULE substrate was ground and polished to 16 nm rms accuracy,
corresponding to 80% encircled energy in 0.073 arc-second, after removing low order bending modes. The successful
completion of the DCT mirror demonstrates the engineering and performance of the support system, ability to finish
large aspheric surfaces using computer controlled polishing, and accuracy verification of surface measurements. In
addition to the DCT mirror, a 2-meter class flat was produced to an unprecedented accuracy of <10 nm-rms,
demonstrating the combined 1-m Fizeau interferometer and scanning pentaprism measurement techniques.
New developments in fabrication and testing techniques at the College of Optical Sciences, University of Arizona have
allowed successful completion of 1.4-m diameter convex off-axis aspherics. The optics with up to 300 μm aspheric
departure were finished using a new method of computer controlled polishing and measured with two new optical tests:
the Swingarm Optical CMM (SOC) and a Fizeau interferometer using a spherical reference surface and CGH correction.
This paper shows the methods and equipment used for manufacturing these surfaces.
We describe methods to correct both symmetric and asymmetric distortion mapping errors induced
by null testing elements such as holograms or null lenses. We show experimental results for direct
measurement and correction of symmetric mapping distortion, as well as an example result for
analytical mapping performed using an orthogonal set of vector polynomials for asymmetric
correction. The empirical determination of symmetric distortion is made via calculation from
predicted and measured changes to aberrations induced via known changes to the testing point.
Flat mirrors of around 1 meter are efficiently manufactured with large plano polishers and
measured with Fizeau interferometry. We have developed technologies and hardware that allow
fabrication and testing of flat mirrors that are much larger. The grinding and polishing of the
large surfaces uses conventional laps driven under computer control for accurate and systematic
control of the surface figure. The measurements are provided by a combination of a scanning
pentaprism test, capable of measuring power and low order irregularity over diameters up to 8
meters, and subaperture Fizeau interferometry. We have developed a vibration insensitive Fizeau
interferometer with 1 meter aperture and software to optimally combine the data from the
subaperture tests. These methods were proven on a 1.6 m flat mirror that was finished to 6 nm
rms irregularity and 11 nm rms power.
The largest limitation of phase-shifting interferometry for optical testing is the sensitivity to the environment, both vibration and air turbulence. In many situations the measurement accuracy is limited by the environment and sometimes the environment is sufficiently bad that the measurement cannot be performed. Recently there have been several advances in dynamic interferometry techniques for reducing effects of vibration. This talk will describe and compare two dynamic interferometry techniques; simultaneous phase-shifting interferometry and a special form of spatial carrier interferometry utilizing a micropolarizer phase-shifting array.
The measurement accuracy of an interferometric optical test is generally limited by the environment. 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. In both techniques averaging can be used to reduce the effects of turbulence and the normal double frequency errors generally associated with phase-shifting interferometry.
We demonstrate a new type of spatial phase-shifting, dynamic interferometer that can acquire phase-shifted interferograms in a single camera frame. The interferometer is constructed with a pixelated phase-mask aligned to a detector array. The phase-mask encodes a high-frequency spatial interference pattern on two collinear and orthogonally polarized reference and test beams. The phase-difference between the two beams can be calculated using conventional N-bucket algorithms or by spatial convolution. The wide spectral response of the mask and true common-path design permits operation with a wide variety of interferometer front ends, and with virtually any light source including white-light.
Mountain-top to mountain-top optical link experiments have been initiated at JPL, in order to perform a systems level evaluation of optical communications. Progress made so far is reported. The NASA, JPL developed optical communications demonstrator (OCD) is used to transmit a laser signal from Strawberry Peak (SP), located in the San Bernadino mountains of California. This laser beam is received by a 0.6 m aperture telescope at JPL's Table Mountain Facility (TMF), located in Wrightwood, California. The optical link is bi-directional with the TMF telescope transmitting a continuous 4-wave (cw) 780 nm beacon and the OCD sending back an 840 nm, 100 - 500 Mbps pseudo noise (PN) modulated, laser beam. The optical link path is at an average altitude of 2 Km above sea level, covers a range of 46.8 Km and provides an atmospheric channel equivalent to approximately 4 air masses. Average received power measured at either end fall well within the uncertainties predicted by link analysis. The reduction in normalized intensity variance ((sigma) I2) for the 4- beam beacon, compared to each individual beam, at SP, was from approximately 0.68 to 0.22. With some allowance for intra-beam mis-alignment, this is consistent with incoherent averaging. The (sigma) I2 measured at TMF approximately 0.43 plus or minus 0.22 exceeded the expected aperture averaged value of less than 0.1, probably because of beam wander. The focused spot sizes of approximately 162 plus or minus 6 micrometer at the TMF Coude and approximately 64 plus or minus 3 micrometer on the OCD compare to the predicted size range of 52 - 172 micrometer and 57 - 93 micrometer, respectively. This is consistent with 4 - 5 arcsec of atmospheric 'seeing.' The preliminary evaluation of OCD's fine tracking indicates that the uncompensated tracking error is approximately 3.3 (mu) rad compared to approximately 1.7 (mu) rad observed in the laboratory. Fine tracking performance was intermittent, primarily due to beacon fades on the OCD tracking sensor. The best bit error rates observed while tracking worked were 1E - 5 to 1E - 6.