How to Choose a Transmission Sphere: Aperture, f/#, and Wavefront

By Zygo | Published: 2026-10-15

Category: Buying Guides

A practical guide to transmission sphere selection: match aperture and f/# to the part under test, confirm wavefront quality, and choose the right mounting accessory.

Quick answer

Match the transmission sphere's clear aperture to the largest optic you need to test, then choose the fastest f-number that still covers that aperture. Confirm the wavefront specification is tight enough for your required measurement uncertainty. Finally, check the mounting and any converter accessories you will need to hold the optic safely in the test beam.

Why aperture is the first decision, not the last

The clear aperture of a transmission sphere sets the maximum diameter of the test beam it can deliver. If you buy a sphere that is too small, you cannot test larger optics at all. If you buy one that is much larger than needed, you pay for glass and alignment complexity you may never use. Start by listing the largest and smallest parts you expect to test over the next few years, then pick an aperture that covers the largest one with a small margin for alignment.

Aperture also affects how much of the test beam is usable. A larger sphere may produce a beam that overfills a small part, wasting light and potentially adding stray reflections. In practice, most labs settle on one or two aperture sizes that cover the bulk of their work and use adapters or converters for the rest.

How f-number changes what you can measure

The f-number (f/#) describes how fast the cone of light converges or diverges from the sphere. A lower f-number means a steeper cone, which allows you to test faster (more strongly curved) surfaces. A higher f-number produces a slower cone, which is better for nearly flat or gently curved surfaces. The transmission sphere f number must be compatible with the surface under test; if the cone is too fast, you may not be able to align the return beam, and if it is too slow, you may lose sensitivity or waste aperture.

For a given aperture, a faster sphere is generally harder to manufacture to a given wavefront quality, so there is a trade-off between speed and achievable accuracy. If your work is mostly slow optics, you can often use a slower sphere with better wavefront performance for the same budget.

Wavefront specification: what the number really means

Wavefront specifications for transmission spheres are typically given in waves PV (peak-to-valley) at a specified wavelength. A 1/20 wave PV sphere is a common high-quality reference, but you should confirm the wavelength and the test conditions. The wavefront error of the sphere adds directly to the error of your measurement, so it should be a small fraction of your total error budget.

Do not assume that a tighter wavefront number is always better for your application. If your test part or environment introduces larger errors, a premium sphere may not improve the final result. Match the sphere's wavefront to the uncertainty you actually need, and consider whether the measurement is for production screening or for final qualification.

Mounting and accessories: the parts that make it usable

A transmission sphere is only as good as its mount. The mount must hold the sphere securely without introducing stress that distorts the wavefront. It also needs to position the sphere accurately relative to the interferometer and the test part. For some setups, an aperture converter is used to adapt a smaller sphere to a larger test beam; that converter then needs its own stable stand to keep it horizontal and aligned.

Check whether the mount is compatible with your existing interferometer and whether you need additional adapters. If you plan to use a converter, confirm that the stand holds it in the correct orientation and that the combined assembly does not sag or shift during measurement.

Decision rules for transmission sphere selection

Aperture first: Choose the smallest aperture that still covers your largest test part with a small margin. A 4-inch sphere handles many common optics; move to a 6-inch sphere only if you regularly test larger parts. If you are unsure, start with a 4-inch sphere and add a converter later if needed.
Match f/# to surface curvature: Use a fast sphere (low f/#) for strongly curved surfaces and a slower sphere for flatter surfaces. The cone must match the test surface to avoid alignment problems. List the f/# of your most curved test parts before you buy.
Wavefront budget: Pick a wavefront specification that is a small fraction of your total error budget. A 1/20 wave PV sphere is a good default for high-accuracy work, but you may not need it for production screening. If your test environment is noisy, a premium sphere may not improve results.
Mounting and accessories: Confirm the mount fits your interferometer and holds the sphere without stress. If you use a converter, plan for a stable stand to keep it horizontal. Budget for the mount and any converter stand as part of the total cost.

Relevant Zygo accessories

4" Transmission Sphere F/.75: The ZYGO 4" Transmission Sphere F/0.75 is specified at 1/20 wave PV and is designed for use with compatible ZYGO interferometer systems. A 4-inch aperture with a fast f/0.75 cone suits testing small to medium optics with significant curvature, and the 1/20 wave PV figure indicates high reference quality for demanding measurements.
6" . f/0.8 Transmission Sphere , 1/20 W , (6024-0458-13): The ZYGO 6" f/0.8 Transmission Sphere, part number 6024-0458-13, is a 6-inch transmission sphere with an f/0.8 designation and a 1/20 wave PVr specification. The larger 6-inch aperture covers bigger parts than a 4-inch sphere, while the f/0.8 cone remains relatively fast; the 1/20 wave PVr spec helps you budget measurement uncertainty.
4" -to- 6" Aperture Converter Stand: The ZYGO 4"-to-6" Aperture Converter Stand is an accessory that holds the 4"-to-6" Aperture Converter in a horizontal configuration. If you use a converter to adapt a 4-inch sphere to a 6-inch beam, this stand keeps the converter stable and horizontal, which is essential for repeatable alignment.
88nm Step Standard with ZYGO Certification: The 88nm Step Standard with ZYGO Certification is a ZYGO-branded step standard supplied as SKU 6300-0917-01 in the Standard configuration, presented with ZYGO certification and associated with an ISO 17025 certified laboratory. A certified step standard lets you verify the performance of your interferometer and transmission sphere setup against a known reference, which supports traceable measurements.

Who should choose which

Best for

Labs that test a mix of small and medium optics and need a versatile 4-inch sphere with high wavefront quality.
Applications requiring a larger clear aperture for bigger parts, where a 6-inch f/0.8 sphere covers the beam.
Users who already own a 4-inch sphere but occasionally need 6-inch coverage, using a converter and a stable stand.

Not for

Testing very large optics beyond 6 inches; you will need a larger transmission sphere or a different interferometer configuration.
Measuring surfaces with extremely fast f-numbers that are faster than the sphere's cone; you will need a faster sphere.
Low-precision production screening where a lower-cost sphere with a looser wavefront specification would be sufficient.