Advantages of Stitching

SSI Metrology: Advantages 

The Benefits of Subaperture Stitching Interferometry (SSI Metrology)

By automatically combining multiple subaperture measurements traceable to form a full-aperture measurement, many formerly difficult surfaces can be measured quickly and affordably.  Subaperture stitching enables larger apertures to be analyzed precisely and aspheres, including high-departure aspheres, can be measured without a dedicated null optic. As an automated, easy-to-use process, subaperture stitching significantly enhances precision optics manufacturing.

SSI Metrology has four major advantages over standard interferometry:

1. Measure a larger field of view – you can see the entire surface

There are two critical advantages to full aperture measurements. The first is that advanced deterministic finishing techniques, such as MRF, are optimized by the use of full aperture metrology. The lack of full aperture metrology prevents leveraging such processes to cost-effectively manufacture surfaces to high precision.

The second is that subaperture quality is a necessary but insufficient condition for full aperture quality. When one visits optics manufacturing companies, it is not unusual to see a variety of interferometers (typically with 4” or 6” aperture sizes) being used in production or final quality control. Interestingly, and no matter how many interferometers are purchased, most optics shops can only see a portion of the optics they manufacture. From these subaperture measurements they gauge the quality of the entire surface. This, however, can be quite misleading, and surfaces that look good in the subaperture will often fail over the full surface specification. (See Figure 1).

2. Measure higher lateral spatial frequencies – you can see a better picture of the surface
One of the most powerful attributes of subaperture stitching is that the full resolution of the interferometer is concentrated over a sub-aperture instead of spread over the full aperture. Stitching has an inherent ability to provide a measurement with much higher lateral spatial resolution, thereby allowing much finer “features” to be seen on a part (Figure 2).This is becoming increasingly important in many applications where higher spatial frequency surface errors increase scatter, thereby reducing scatter to noise.

SSI Metrology effectively bridges the gap between standard Fizeau interferometers and interference microscopes, giving access to a greater part of the power spectral density (PSD) spectrum increasingly used to specify high end optics.

3.  Measure with improved accuracy – you can feel confident in the quality of your results
An interferometric measurement relies on a reference optic or transmission element (TE) to gauge the quality of the surface. Although for the sake of the measurement process, the transmission elements are assumed to be “perfect,” the reality is that they are often manufactured with less than perfect results. To compensate for the imperfections in the reference optic, it is necessary to calibrate the interferometer. However, these techniques tend to be time-consuming and difficult to execute, requiring highly trained personnel to spend many hours in front of an interferometer. The reality is that the processes are prohibitively difficult, slow and are rarely–if ever–used.

The stitching process automatically compensates for the quality of transmission elements by using the overlapping areas to compute the errors. TE error is automatically subtracted and reported, thereby allowing the user to see the errors and track them over time.  The stitching process also calibrates for systematic errors, such as reference wave and distortion, thus, achieving better accuracy than a standard full aperture measurement. The end result is truly a picture of the surface quality, not a reflection of TE imperfections.


4.  Measure aspherical elements without the use of null lenses
Another benefit of subaperture stitching is that it can dramatically improve the testable aspheric departure of an interferometer. A standard interferometer can only acquire wavefronts that are very close to the shape of the interferometer’s reference surface. In practical terms, this limits non-null test capability to only a few micrometers of aspheric departure. Stitching can expand this capability by dividing a wavefront that is beyond the interferometer’s capability into sections that are measurable individually. Stitching allows the part’s asphericity to be effectively “spread” among the subapertures, so that no single measurement must handle the entire aspheric departure. These capabilities enable a breakthrough improvement to an interferometer’s lateral and longitudinal dynamic range capability.

Measurement example
The process is straightforward and rapid. Each sub-aperture typically takes between 20 and 40 seconds (depending on settings). A typical measurement might involve a few dozen sub-apertures, resulting in total measurement time of 15 minutes or less. Besides the full aperture map of the measured surface, also an estimate of the measurement’s quality is given, calculated from the differences in overlapped areas, as well as the measurement of the TE’s quality during the measurement (figure 3).

In the example depicted in figure 4, a 42.6mm diameter convex sphere with a radius of curvature of 21.6mm (i.e., almost a perfect hemisphere) was first measured on the SSI, then polished on an MRF machine. The part was re-measured on the SSI and showed an improvement from lambda/20 PV to lambda/30 PV. It is important to note that this level of quality typically exceeds the quality of transmission elements (a 4” f/0.75 in this case) that are commercially available.

The bottom line... QED’s subaperture stitching technology produces better measurements on a broader range of parts than any other measurement system. QED’s line of metrology products now includes the SSI-A for optical surfaces up to 200mm, including aspheres, the MFA-400 which allows customers to capture mid-spatial frequency surface roughness on parts up to 400 mm and the ASI, which can measure 100s of waves of aspheric departure. These comprehensive metrology solutions will improve the efficiency, flexibility, reliability, and capability of every optical shop.