4 Inch Transmission Sphere F/7.1 1/20 Wave Buyers Guide
By Zygo | Published: 2026-10-27
Category: Buying Guides
Learn how to choose a 4 inch transmission sphere with F/7.1 and 1/20 wave accuracy. This guide covers key specs, decision rules, and fit guidance.
Quick Answer
For most general interferometry applications, a 4 inch F/7.1 transmission sphere with 1/20 wave accuracy offers a solid balance of aperture, speed, and precision. Confirm your test setup's f-number and required wavefront error before purchasing.
Why Aperture and f/# Matter
The 4 inch aperture determines the maximum beam size you can test. If your optics are larger, you'll need a beam expander or a bigger sphere. The F/7.1 focal ratio affects the divergence of the test beam; it must match your interferometer's output f-number to avoid vignetting or spherical aberration.
A 1/20 wave PV specification indicates the sphere's transmitted wavefront accuracy. This is a high-quality level, but ensure it aligns with your measurement tolerance. If you need even tighter accuracy, consider a sphere with a lower PV value, though cost and availability may vary.
Trade-offs of a 1/20 Wave Sphere
A 1/20 wave sphere offers excellent precision, but it may be overkill for routine shop-floor testing where 1/4 or 1/10 wave is sufficient. Higher accuracy often comes with a higher price and more delicate handling requirements.
Also, the F/7.1 speed is relatively slow. This is fine for collimated or moderate-divergence beams, but if you need a fast converging beam for small optics, a lower f-number sphere might be more appropriate.
Decision Rules for Choosing a 4 Inch F/7.1 1/20 Wave Sphere
- Match the f-number: Ensure the F/7.1 sphere matches your interferometer's output f-number. If not, you may need a beam expander or different sphere to avoid beam mismatch. Check your interferometer's f-number before purchase.
- Verify aperture size: The 4 inch aperture should cover the diameter of the optic you plan to test. If your test part is larger, consider a larger sphere or a beam expander. Measure your largest test optic to confirm the aperture is sufficient.
- Assess wavefront accuracy needs: A 1/20 wave PV sphere is suitable for high-precision testing. If your tolerance allows, a lower accuracy sphere may save cost without compromising results. Define your required wavefront error tolerance before selecting.
Product Facts
- 4" Transmission Sphere , F/7.1, 1/20 Wave: The ZYGO 4" Transmission Sphere is an F/7.1 optic specified at 1/20 wave, PV. This specification tells you the sphere's aperture, focal ratio, and wavefront accuracy, which are the three key parameters to match with your interferometer and test requirements.
4" Transmission Sphere , F/7.1, 1/20 Wave
Who Should Choose This Sphere?
Best for
- Optical shops testing moderate-aperture optics with a matching F/7.1 interferometer
- Laboratories that need high wavefront accuracy (1/20 wave) for precision measurements
Not for
- Users with interferometers having a different f-number unless they use a beam expander
- Applications testing very large optics (larger than 4 inches) without additional beam expansion
