Transmission Witness Samples for Coating Verification

Quick answer

Transmission witness samples let you verify coating performance without testing your actual optics. Choose a sample with the same refractive index as your coated part for accurate results.

Why use a witness sample instead of testing the actual optic?

Testing the actual coated optic can be impractical—it may be too large, oddly shaped, or already mounted in an assembly. A witness sample is a small, standardized substrate that goes through the coating run alongside your parts. You can measure its transmission without disturbing the production workflow.

Since witness samples are polished to tight flatness and parallelism specs, they give you a clean baseline for coating performance. That said, the sample’s surface quality (scratch-dig 60-40) may differ from your final optic, so treat the measurement as a process check, not a certification of every individual part.

How do I pick the right refractive index range?

The witness sample’s material should match the substrate of your coated optic as closely as possible. A mismatch in refractive index shifts the coating’s spectral performance, especially for narrowband or angle-sensitive designs.

Esco offers witness samples in three broad index ranges: 1.600–1.799, 1.800–2.000, and an IR range for materials like silicon or germanium. If your substrate index falls near a boundary, choose the range that brackets it. For IR coatings, the IR witness sample is the natural pick because it’s fabricated from single-crystal materials that transmit beyond the visible.

What do the flatness and parallelism specs mean for my test?

A witness sample with <1 wave flatness at 633 nm and <1 arc min parallelism ensures that measured transmission losses come from the coating, not from surface figure errors. This is especially important for anti-reflection or high-reflection coatings where even small wavefront distortions can skew results.

If your coating process is highly sensitive to substrate geometry, you may need a witness sample with tighter specs. However, for most production monitoring, these standard tolerances are sufficient.

Frequently asked questions

Can I reuse a witness sample after it’s been coated?

Yes, but only if you strip the coating completely and repolish the surface. Most users treat each sample as single-use to avoid cross-contamination or residual coating effects.

How many witness samples should I include per coating run?

At least one per run, but two is safer—one for transmission measurement and one as a backup or for destructive testing like adhesion or humidity soak. Order extra samples for critical runs to avoid delays if a sample is damaged.

Does the witness sample need to be the same thickness as my optic?

No, thickness does not affect the coating performance measurement. Esco witness samples are 1.50 mm thick, which is a standard that works for most coating fixtures.

Product facts

• Witness Sample, Transmission, 1.600-1.799: Precision polished on both sides to scratch-dig 60-40; flatness <1 wave @ 633 nm at 80% CA; parallelism <1 arc min; melt lot controlled with individual data available. These specs give you a reliable, traceable baseline for coating verification without needing to test your actual optics.

  

• Witness Sample, Transmission, IR: Fabricated from single-crystal materials; same surface quality and geometry as the visible-range samples but optimized for IR transmission. Single-crystal construction minimizes scatter and absorption in the infrared, making this sample the right choice for IR coating runs.