4D in the NASA News

Please check out NASA's recent article on a joint effort between NASA’s Goddard Space Flight Center and 4D to develop a picometer-stability interferometer. 4D is very excited to be working with NASA on this ground-breaking project.

http://www.nasa.gov/feature/goddard/2016/nasa-seeks-picometer-accuracy

Pixelated Camera Highlights Polarization at Jupiter's Poles

We are highlighting an exciting application of PolarCam technology today in the Practical Tips blog.

In a recent astronomy application 4D Technology mounted a custom micropolarizer array to a thermoelectric cooled, 4 Megapixel Finger Lakes camera. Dmitry Vorobiev, an astrophysics PhD candidate at the Rochester Institute of Technology, installed the instrument in the 1-meter Cerro Tololo Inter-American Observatory telescope in Chile, shown here.

The camera is visible at the base of the telescope in the image below. 

"Polarization at the poles of Jupiter was first identified in the 1920s," says Mr. Vorobiev. "Since then, scientists have lead periodic attempts to determine the nature of the polar clouds. However, their efforts were hindered by the large distance to Jupiter and the practical difficulty associated with conventional polarimetric techniques. New snapshot polarimetric cameras allow, for the first time, routine polarimetric measurements."

Mr. Vorobiev trained the telescope and sensor on Jupiter and captured the impressive polarization images below. A strong Degree of Linear Polarization signal can be seen at the poles of the planet. Several theories have been proposed for such a strong polarization signal at the poles, including a different composition of atmospheric particles at this region, and the presence of methane ice crystals.

A strong Degree of Linear Polarization (DoLP) at Jupiter's poles as imaged by micropolarizer camera technology. All images courtesy of Dmitry Vorobiev, Rochester Institute of Technology.

The micropolarizer array has become an essential tool for polarization imaging. The technology, which is at the heart of 4D PolarCam snapshot micropolarizer cameras, enables image enhancement for applications ranging from astronomy to birefringence measurement, industrial monitoring, polarization microscopy, medical imaging and autonomous vehicle vision.

More information about Mr. Voribiev's research is available at https://people.rit.edu/dxv2686/research.html.

 

Measuring Surface Roughness on Challenging Large Optical Components

Measuring surface roughness on large optics has long been a challenge for manufacturers. Workstation based optical profilers do not have sufficient clearance to allow access to the central portions of larger components. Replication methods are messy and can potentially lead to damage of the optic.

4D's NanoCam Sq Dynamic Surface Profiler was developed to measure surface roughness on large optics and in other difficult applications. In addition to a standard workstation stand, several other measurement configuration have been developed.

The NanoCam tripod stand, shown below, enables the NanoCam to be placed directly on the optic for rapid sampling across the surface.

Measurements close to the edge of the optic can be accomplished by placing one leg of the tripod on a riser. The image below shows the NanoCam positioned with one leg off of the surface in order to measure the edge region. Roughness data from the surface is shown to the right.

The tripod below was developed for an application in which space constraints precluded the use of the riser method. The extended tripod was developed to image and measure the regions of a mirror blank that will eventually become the edges and vertices of a hexagon mirror, following trimming operations.

Some test configurations and optic geometries preclude on-optic measurement entirely. The images below show a custom gantry system which incorporates a NanoCam Sq to measure large optics with significant form. The NanoCam Sq is mounted on the overhead gantry and can be tilted to align to, and focus upon, the test surface. The test optic is mounted to the cart below for precision positioning below the measurement head. Dynamic Interferometry enables accurate measurement despite vibration, even though the metrology and test piece are not directly coupled.

In one more case, shown below, a NanoCam Sq was mounted on an industrial robot for freedom of movement in six axes. This flexibility enabled measurement of surface roughness over complex part geometries.

When Your Lab Extends to the Great Outdoors

PolarCam Snapshot Micropolarizer Cameras provide image enhanceme​​nt for scene discrimination, remote sensing, industrial monitoring, autonomous vehicle vision and other applications. Taking advantage of that capability requires effective means to get the camera on location, which may be in cramped spaces, on uneven terrain or out of reach of a power source.

A new series of accessories for the PolarCam makes it possible to use the camera in remote locations. ​With the ​extra power provided by a single ​Portable Battery Power Unit​ the PolarCam and laptop computer​ can be operated​ for up to ​10 hours. The mobile cart securely holds the laptop and battery, as well as providing cable management and storage for the camera. A telescoping tripod stand with individually locking legs lets you position the PolarCam in tight quarters or on uneven ground, while the macro zoom lens makes it possible to hone in on the important portions of a scene.

Used individually or together, the new PolarCam accessories let you use the PolarCam in more applications that ever.

PolarCam mobile rig showing laptop computer, battery, tripod and cart.

Measuring Plane-Parallel Optics

Plane-parallel optics are transparent components or systems with two or more parallel surfaces. Measuring them with an interferometer can be challenging, as all of the parallel surfaces can contribute interference fringes, making measurement extremely difficult or impossible. 

A new method makes it possible to exclude all but the surface of interest so that measurements can be made of both surfaces, and transmitted wavefront error, optical thickness and homogeneity can be calculated as well. 

4D will be hosting a webinar on measure plane-parallel surfaces using the Surface Isolation Source for the AccuFiz Fizeau Interferometer. The webinar will take place January 12 at 1pm Eastern Standard Time. Save your seat for the event today!

Fringes on a thin glass disk with a standard interferometer (left) and with Surface Isolation (right). 

Register for the "Measuring Plane-Parallel Optics" Webinar.

Wedge Factor in Autocollimation Configuration

We've received some questions on our recent Setting the Wedge Factor post regarding measurement in autocollimation.  

The first question concerns the relative quality of the optics M1 versus M2. The mirror that will serve as the reference must be of significantly higher quality than the mirror to be tested; if the two mirrors are of similar quality then a measurement of either mirror will lose fidelity, as it is impossible to distinguish which mirror is contributing to the departure.

The second question regards measuring the surface height of M1 (which, in our Wedge Factor chart is shown as "N/A, not applicable"). It is not an uncommon practice to measure M1 in the arrangement shown, using a Wedge Factor of 1/4 (A=1/2, B=1/2 and C=1) when M1 has a larger F/#. However, in this arrangement, the angle of incidence varies across the parabolic surface, and this variation cannot be accounted for with a single Wedge Factor value. Instead, the system would need to be modeled and the resulting function applied to accurately determine surface height.

Determining surface height is typically only necessary for applications such as providing feedback in computer controlled polishing. The reflected wavefront measurement adequately characterizes the optical performance of the mirror, and thus, should be sufficient for determining if an optic is within a specified tolerance (e.g. < 1/10th wave).

What is the "Bits to Show" Setting (and How Does it Affect My Measurements)?

The dynamic range of a camera is the difference in intensity from pure black to saturation (the brightest intensity that the camera can distinguish).

Bit Depth is the number of bins into which the camera’s full dynamic range is divided. For example, a 12-bit camera has 2¹² bins, or 4096 bits. A higher bit depth provides smaller increments and therefore higher resolution of the detected light level. Calculations for data analysis are always performed at maximum bit depth in order to see the smallest changes in the signal.

With some 4D systems, the included 4Sight Analysis Software has a Bits to Show setting in the Camera Settings dialog box (this is the window in which the operator selects the camera Gain and Exposure for a measurement). This setting alters the Bit Depth that is displayed on-screen, to make the data more easily discernible. It does not, however, affect the calculations.

Consider an example in which a 4D interferometer is being used in a high-vibration environment. An extremely short Exposure time will mitigate the effects of the vibration but will also decrease the integrated signal of the incident light. Consequently, a higher gain value would normally be selected in order to decrease the exposure time yet still maintain enough image brightness to provide sufficient signal to make the measurement.

Sometimes, however, even with the Gain set to maximum it may still be impossible to obtain sufficient image brightness to align the system or acquire good quality data. In this case, changing the Bits to Show value from 12 to 10 will truncate the top 2 bits (which, in this case, would be all black anyway) and promotes the remaining 10 bits to display full scale. Doing so will effectively increase the displayed light level by a factor of 4, making it easier to see the fringe pattern and align the test configuration. Changing the bit level from 12 to 8 can also be done and would increase the displayed signal level by a factor of 8. Neither selection, however, would affect the measurement analysis, which is always completed at the maximum available bit depth.

PV, PVq, PVr

For every measurement 4D's 4Sight Analysis Software calculates the peak-to-valley height (PV), which is the difference between the highest point and lowest point in a measurement. PV gives a very general, though limited sense of surface roughness. Since it relies upon only two points in the entire dataset it is highly susceptible to spikes, noise, diffraction, edge effects and missing data. 

4Sight also includes two additional statistics, PVq and PVr.  These parameters are computed using nearly all of the measured data points and will give much more reliable and repeatable peak-to-valley results. You can choose whether or not to calculate these statistics in the Analysis Options > Statistics tab.

Choose to calculate PVq and PVr in the 4Sight analysis software Statistics options tab.

Q Peak-Valley (PVq) disregards the highest and lowest pixels and only considers the remaining Q Percent of pixels. To calculate PVq, 4Sight generates a histogram of surface heights, then determines the narrowest band of the histogram that contains the Q Percent of pixels (Q must be >50). The result will be displayed in the Dataset Statistics table found on many 4Sight screens.

Robust Peak-Valley (PVr), originally proposed by Dr. Chris Evans of the University of North Carolina Charlotte (and formerly of NIST), provides a more robust measure of surface shape. It is calculated from the peak-to-valley of a 35-term Zernike fit to the measured data plus 3*RMS of the residual (the residual is the dataset minus the 35-term fit). The result will again be displayed in the Dataset Statistics table found throughout 4Sight.


Click here for more information on 4Sight software. 

Choosing the Best Method for Measuring Various Roughness Ranges

The NanoCam Sq dynamic surface profiler can measure surface roughness on smooth to super-smooth surfaces. To reliably measure nanometer-scale surface texture, the height resolution of the measurement method must be less than 1⁄10 of the Sq (the Root Mean Square Roughness) of the surface. Three techniques are suggested, depending on the range of roughness to be measured.

A. RMS roughness greater than ~1nm 

Averaging multiple measurements improves the height resolution; the more measurements that you average, the finer the roughness that can be measured.

The figure below shows that a single measurement can be used to measure RMS roughness greater than ~5nm. As the roughness decreases, increase the number of averages to maintain reliable measurement. For example, to measure RMS roughness of ~1nm an average of 10 measurements or more should be used.

Note that surfaces with very low reflectivity may require a greater number of averaged measurements.

Number of measurements to average based on RMS roughness of surface. 

B. RMS roughness is between ~ 0.2nm and 1nm

When the RMS roughness is below 1nm averaging alone may not provide sufficient resolution. In this range is it recommended to subtract a reference measurement from the data to remove the residual noise due to the system and optics.

One way to do this is to measure the residual system errors using a precision optic with a surface roughness better than the desired accuracy. The precision optic must be free of defects such as scratches and digs. 4D Technology can supply a calibration mirror with a surface roughness less than 0.1nm. The reference measurement must be created using the interference objective, reflectivity setting, and aperture stop setting and resolution that will be used to measure the actual test piece.

More information on reference subtraction can be found in the 4D Technology 4Sight Software User Manual.

C. RMS roughness is less than ~ 0.2nm 

To measure samples with an Sq less than 0.2nm it is necessary to eliminate all effects of the reference path. In this case the Absolute Sq technique can be used. Two averaged measurements are made of the sample at positions laterally separated by a distance greater than the sample surface correlation length. The difference between the measurements is calculated, and the effects of the reference path are thereby removed from the resulting “difference.” See the figure above to determine the required number of averaged measurements.

The Sq of the sample is then estimated as:

Sq = [Difference]/√2 

This technique does not produce a measurement map as in methods A and B; instead, the Sq of the sample surface is reported as a single number. The two averaged measurements should be acquired as rapidly as possible to prevent environmental changes from influencing the results.

More information on surface roughness measurement can be found in the ASME B46.1 2009 standard on "Surface Texture (Surface Roughness, Waviness, and Lay)."

Upgrading an Older Laser Interferometer

We are often asked whether options are available to update older fringe analysis and phase-shifting interferometers to a more modern state of operation. The good news is that there are, in fact, several upgrade options for improving, or even reviving, older instruments to stretch their useful life.

Visual fringe analysis (or inspection) systems are mechanically simple, but they rely on an operator, or external software, to analyze the interferograms and estimate the shape errors in the test part.  Upgrading the analysis software is an excellent ways to add power to these systems. In many cases converting a system to temporal phase shifting is an option; adding a piezo driven phase-shifter can significantly improving measurement quality and enabling a wider array of data analysis options.

Older phase-shifting laser interferometers greatly benefit from modern software and a computer upgrade. In some cases, a newer, lower noise camera can be added to improve resolution and repeatability.

Depending on the model and vintage, upgrade kits are available to address some or all of these improvements. Contact 4D to learn which upgrade options are available for your laser interferometer system.

Instruments from Buccini, ADE Phase Shift and Kugler with upgrades from 4D Technology.