In interferometric testing, the transmission sphere generates the reference wavefront used to evaluate lenses, mirrors, and optical systems. Because all measurements are compared against this reference, transmission sphere design and reference surface quality have a direct impact on measurement accuracy.
Selecting the optimal transmission sphere requires more than matching a test RoC (Radius of Curvature) with an F/# within its range. The required clear aperture, part geometry, available clearance, and amount of overfill or underfill of total aperture size all determine whether a transmission sphere is best suited for the measurement.
Kreischer, now part of the Torrent Photonics group, began developing transmission spheres in the 1990s to support its own metrology requirements. What started as an in-house solution evolved into a commercial range that continues to expand today, with new designs developed to provide broader measurement coverage, additional F-number options, and features intended to reduce common sources of measurement error.
That first transmission sphere was built for in-house use rather than as the start of a product line. This allowed the design to be optimized around the manufacturing processes and measurement requirements. As customers began asking about the technology, Kreischer then produced additional spheres and eventually developed a commercial range.
Although cost prompted the original project, Kreischer's focus shifted as the product line developed. Increasing radius coverage, reducing sensitivity to retrace error, and controlling unwanted reflections became the main design priorities.
A transmission sphere converts the collimated beam from a Fizeau interferometer into a converging spherical wavefront. During a spherical surface test, the optic is positioned so that the center of curvature of the test surface coincides with that of the wavefront produced by the transmission sphere.
The transmission sphere’s F-number determines the angle of the converging beam. The F-number required for a particular test therefore depends on the radius of curvature and the diameter of the area to be measured.
A transmission sphere that is too slow may not illuminate the required aperture. F-number alone, however, does not establish whether a measurement is possible. The reference radius and the physical space available for positioning the part must also be considered.
Convex surfaces can be particularly restrictive because the optic is positioned between the transmission sphere and its focal point. The available range may be limited by contact between the test part, its mounting arrangement, and the transmission sphere cell.
The current Torrent Photonics transmission sphere range uses proprietary extended-range optical designs to provide a longer reference radius than would normally be expected for a transmission sphere of the same F-number. The housing design also minimizes mechanical interference, allowing many models to be used with the test optic positioned extremely close to the reference surface. By addressing both optical and mechanical limitations, these designs extend the practical convex radius range while maintaining the beam angle required to cover the measurement aperture.
The 4-inch F/1.0 transmission sphere shows why additional F-number options can be useful.
Torrent’s current technical data specifies a reference radius of 50 mm for the 4-inch F/0.75 model, 90 mm for the F/1.0 model, and 142.7 mm for the F/1.5 model. The F/1.0 sits between two commonly used configurations.
For some smaller optics, the F/1.0 can provide greater convex radius coverage than the F/0.75 while covering a larger measurement aperture than the F/1.5 at the same test radius. It is particularly useful for optics around 1 inch in diameter and is Torrent’s most frequently requested nonstandard F-number.
Additional F-number options allow a metrology laboratory to match its transmission spheres more closely to the parts it expects to test. Depending on the combination of diameters and radii involved, this may reduce the number of spheres needed to provide the required coverage.
Kreischer also addressed retrace error during development. In a Fizeau interferometer, the reference and test beams travel through much of the same optical path. If the test optic is displaced or tilted, the returning test beam may follow a slightly different path through the transmission sphere. The difference can introduce an error that appears to be part of the measured surface.
A transmission sphere's optical design affects its sensitivity to this error. The current Torrent Photonics designs are optimized to reduce the effects of lateral retrace error.
Unintended reflections were another consideration. Light reflected from internal surfaces can return to the interferometer detector and produce ghost fringes. These additional fringes may obscure the required interferogram or make it more difficult to interpret. The current Torrent Photonics transmission sphere range combines AR coatings with an optical layout intended to direct unwanted reflections away from the detector.
The Torrent Photonics transmission sphere range uses spherical and plano optical elements. These elements can be pitch polished to control surface smoothness and slope error without introducing the manufacturing and alignment complexity associated with aspheric internal elements.
Reference surface grades include λ/10 and λ/20, with λ/40 available for applications requiring higher accuracy. The stated grade is the minimum specified performance rather than the exact result expected from every unit.
Each transmission sphere is individually measured and supplied with a Certificate of Conformance, an interferogram of its as-built system error, and the corresponding data file. The same transmission sphere designs are used within Torrent’s own fabrication and inspection operations.
The current range includes 4-inch and 6-inch models, fast and slow F-numbers, and converger and diverger configurations. Custom F-numbers and wavelengths are also available for test requirements that fall outside the standard range.
The development of the Kreischer transmission sphere range was driven by practical metrology requirements. Extended radius coverage, additional F-number options, reduced retrace error sensitivity, and documented system performance were introduced to address real-world measurement challenges. The result is a transmission sphere range that gives users greater flexibility when testing optical components while maintaining the accuracy required for precision interferometric measurement.
Please contact our technical sales team to discuss your transmission sphere requirements: sales@torrentphotonics.com.