Choosing the Right Attenuation Filter for 4-Inch Interferometry
By Zygo | Published: 2026-10-23
Category: Buying Guides
Learn how to select the right 4 inch attenuation filter for interferometry, including key specs and decision rules for optimizing measurements of highly reflective parts.
Quick answer
For 4-inch interferometry, choose a 4 inch attenuation filter with a transmittance around 20% and a transmitted wavefront quality of ≤ λ/10 to ensure good fringe contrast without over-attenuating the signal.
Why attenuation matters for reflective parts
Highly reflective surfaces can produce excessive light return, washing out interference fringes. An attenuation filter reduces the light intensity to a level where fringes become visible and measurable. Without proper attenuation, you risk saturating the detector and losing measurement accuracy.
Key specification: Transmittance
The transmittance value determines how much light passes through the filter. A 20% transmittance is a common sweet spot for most reflective parts—it cuts enough light to prevent saturation while retaining enough signal for good fringe contrast. If your parts are extremely reflective, you may need a lower transmittance, but 20% works for the majority of cases.
Trade-off: Wavefront quality vs. cost
Filters with higher transmitted wavefront quality (lower λ/10 or better) preserve the interferometer's measurement accuracy but cost more. For routine quality control, a λ/10 filter is sufficient. For high-precision metrology, consider a better wavefront specification if available.
Decision rules for choosing your attenuation filter
- Check your part reflectivity: If your test parts are highly reflective (e.g., polished metal or coated optics), you need an attenuation filter. If they are diffuse or low-reflectivity, you likely do not.
- Match filter size to interferometer: Use a 4 inch attenuation filter only with a 4-inch interferometer beam. A larger or smaller filter will not fit or will vignette the beam.
- Verify transmittance and wavefront: Select a filter with a transmittance near 20% for standard use. Ensure the transmitted wavefront quality meets your measurement tolerance (≤ λ/10 is typical). Confirm your interferometer's detector dynamic range to avoid under- or over-attenuation.
Product facts
- 4 inch Attenuation Filter: The 4 inch Attenuation Filter has a transmitted wavefront quality of ≤ λ/10 (PVr) and a transmittance of 20 ± 4% at 10° angle of incidence at 632.8 nm through the pelicle. This means it reduces light enough for most reflective parts while maintaining excellent wavefront fidelity, making it a reliable choice for general interferometric measurements.
4 inch Attenuation Filter
Who should buy this filter?
Best for
- Operators measuring highly reflective parts on a 4-inch interferometer
- Users needing a reliable, off-the-shelf attenuation solution with known performance specs
Not for
- Interferometers with beam diameters other than 4 inches
- Applications requiring variable attenuation (consider a variable filter instead)
