Revolutionizing Precision Optics Measurement: Sydor Optics Harnesses Äpre Spectrally Controlled Interferometry (SCI)

Apre interferometer at Sydor Optics

Sydor Optics is widely regarded as a world leader in processing and delivering precision flat-surfaced, parallel, and wedged glass optical components. Patrick Drury, Director, Quality & Operational Excellence, Tom Grambo, QA Technician, and John Mandelaro, Precision Optics Manufacturing Technician Apprentice, share how they are using Äpre Spectrally Controlled Interferometry (SCI) to automate and speed the measurement process of the entire part.

When and why did you start working with Äpre?

We’ve been working with Äpre for around seven years. We use interferometers so often in our process, and we have so many pieces of equipment they approached us to beta-test an early version of their SCI interferometer. Apre leads the charge on scanning coherence. Other systems followed suit, but the competing technologies just aren’t the same.

Their service is better than the service of any other system that we have.  Äpre has a small group we can collaborate directly with – they’re responsive and eager to try new things. Their team is really good at getting back to us if we have questions on anything software related. Bob visits us to talk about the systems – he’s an owner. We don’t get that level of service from others.

What volumes do you work in?

On a given month, we check thousands of parts. Annually, I’d estimate it at 100,000 parts or more. We have three Apre systems that streamline our processes. On some programs, we test EVERY part, not just a sample, and this type of interferometer saves us 30 minutes per part in some cases. You can imagine how that adds up over time in these volumes.

 


On some programs, we test EVERY part, not just a sample, and this type of interferometer saves us 30 minutes per part in some cases. You can imagine how that adds up over time in these volumes.


How else does the interferometer save time?

A big time-saver is having the ability to set up and save part-specific parameters using the software. Say you’re measuring a wedge you might forget to change the refractive index. If missed our customers will demand a retest. The software saves us that step because it automatically loads all set up parameters when that specific analysis configuration is loaded. Plus scripting makes the multi-step process 5X faster by taking 4 separate measurements and doing them in one automatic sequence.

 


Scripting makes the multi-step process 5X faster by taking 4 separate measurements and doing them in one automatic sequence.


Tell us more about the scripting.

We hired John out of MCC’s Optical Systems Technology program. As an apprentice, he moves through every single department. He faced a big learning curve when he landed in the inspection department.

The apprentice program allowed him the time he needed to learn how to use the interferometer. And he just went further down the rabbit hole – he learned it and went the extra mile. He’s been working with Artur at Äpre on writing and implementing code to automate the measurement process so any one of the operators can load the part and run the script.

It’s been a huge benefit to the whole company and has also added to his skillset.

 


Apre interferometer at Sydor Optics

John Mandelaro, an apprentice at Sydor, works on automating their metrology for faster inspection.


How has it changed your experience in optics?

The problem-solving makes it exciting. Just seeing it work, understanding why, and empowering other people to keep projects moving. When you combine all these different skill sets: optics, manufacturing, quality assurance, and even sprinkling in some programming and scripting knowledge, it’s not just a job anymore. It’s a career. You’re changing the space that you’re in.

 


When you combine all these different skill sets: optics, manufacturing, quality assurance, and even sprinkling in some programming and scripting knowledge, it’s not just a job anymore. It’s a career.”

Tom Grambo, Sydor Optics


 

At Äpre we’re keeping our eyes on the horizon. Let us provide you with state-of-the-art systems to advance your process. Contact us today to get started.

Game-Changing Interferometry

Spectrally Controlled Interferometry (SCI) revolutionizes traditional Fizeau interferometry, and empowers optical manufacturers with enhanced process improvement and manufacturing control. But how?

  • Spectral temporal coherence control gives you the best of both worlds – both high and low temporal in one source.
  • Spectral cavity selection gives you a live view of the fringes on the surface of interest, vs. the traditional Fourier methods.
  • Spectral synchronized phase shifting minimizes environmental influences and measures mid-spatial frequencies.

Spectrally controlled interferometryToday’s optics industry demands this level of optimal performance and precision. Read our feature in Laser Focus World to see how you can use SCI to your advantage.

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Optical Surface Radius of Curvature

Optical surface radius is a primary control parameter. It must be measured to confirm the optical element meets specification. Spectrally Controlled Interferometry promises a faster and more accurate method to measure surface radius.

Multiple methods are presently used to measure radius.

  • Test plates: A test plate has a pre-measured radius to which the test surface is compared, the difference is power fringes is reported.
  • Mechanical probe: The sagitta of the lens surface is measured over a fixed diameter, from which radius is calculated. Small changes in sagitta measurement can lead to large errors in reported radii.
  • Interferometer “Cateye to Confocal”: An interferometer with a transmission sphere (TS) is set up at the catseye position off the surface of interest, and position noted. Then the lens is moved to the confocal position and the position noted. The difference in positions is the radius. PMI interferometers also compensate for any residual power in the measurements due to imperfect positioning. The positions are shown in figure 1.

Interferometer radius measurement is the “gold standard” method today. To achieve this measurement the following additional equipment is needed:

  • Lens mount that allows for free motion along the axis of the lens (Z axis) to micrometer position accuracy
  • Free Z motion over up to a meter, with micro-radian angular alignment to minimize cosine errors.
  • A separate Z position metrology tool. Either a glass scale or a displacement measuring interferometer (DMI) The DMI is the most accurate as it minimizes Abbe offset errors, though it adds environmental uncertainty if not calibrated.
Figure1: The distance between catseye and confocal is the surface radius. Note extra mechanical fixturing and metrology instruments are needed to measure the positions accurately

Spectrally Controlled Interferometry (SCI) Radius Measurement

SCI has a unique combination of four properties:

  • Easy alignment in laser mode
  • Electronically switch to white light mode to isolate fringes to the surface of interest
  • Measures the cavity phase electronically (no mechanical motions)
  • Measures the absolute distance of the measured cavity

SCI Absolute Position Measurement

The SCI source parameters that determine the absolute distance measurement are:

λ0 = The nominal wavelength of the SCI source
Δλ = A tunable source parameter

The absolute distance from the transmission sphere to the optical surface is:

lc = λ20 / 2Δλ   (1)

This combination means SCI can provide all the measurements required for surface radius with neither an additional DMI, nor the precision mechanics to move from Catseye to Confocal position.

SCI Radius Measurement

Since SCI can measure absolute position there is no requirement to measure at two positions. For the first time surface irregularity and radius can be measured at the same time. This also improves measurement accuracy by the elimination of several error sources as discussed below.

SCI Radius is measured by first calibrating the radius of the TS = RoCTS. Then directly measuring the cavity length lc with SCI and calculating the part RoCx from:

RoCx = RoCTS – lc   (2)

Figure 2: Measuring surface radius of SCI requires calibrating the TS radius and then directly measuring the cavity length and calculating the Test Optic radius.

Measurement Accuracy

Repeatability

The foundation of accuracy is repeatability. If a measurement process is repeatable it can then be calibrated to yield accurate results. Repeatability of measurement has been shown to be <0.5 µm for radii in the 20 mm to 175 mm range. This repeatability is better the best measurements with DMI’s.

Accuracy NIST Study

In 2001 NIST ran an experiment to determine the error sources and limits of accuracy for surface radius1. The results of that experiment and error sources are shown in figure 3, along with estimates from Typical DMI and Typical Scale (Glass) radius benches and SCI. 

A key observation is most of the error sources are due to measuring the distance between catseye and confocal. In the NIST study fully 90% of the errors are in this part of the measurement. The same is true for the DMI and Glass Scale. The only difference is the magnitude of the errors that are 16X and nearly 500X greater, respectively, than the NIST study. 

These values are a precaution to users of a DMI or glass scale radius bench to not underestimate the errors in the measurements.

Figure 3: Surface radius error budget for several methods of measurement.

SCI Eliminates the Z Axis Motion Error Sources

No Z motion eliminates the first six error sources (green). Leaving the balance of the error sources that are equal to the Typical DMI set up. SCI can potentially improve the uncertainty to better than the NIST study.

An additional error source is the calibration of the TS radius that also must be added. Since it can be bootstrapp calibrated to the same level a simple doubling of the errors to ~50 nm is expected. 

As of October 2020 SCI can measure surface radius up to 250 nm cavity lengths, and potentially out to 500 mm cavity lengths. 

Summary

SCI promises to replace traditional catseye-confocal lens surface radius measurement by saving time and greatly improving accuracy. It is the go to method for high-performance lens radius metrology.


1 T. Schmitz, A. Davies, C. Evans; Uncertainties in interferometric measurements of radius of curvature, Appeared in Optical Manufacturing and Testing IV, H. Philip Stahl, Editor, Proceedings of SPIE Vol. 4451, 432-447, 2001