Quick answer

A transmission witness sample is a polished optical flat placed alongside your production optics during coating runs. After deposition, you measure its transmission spectrum to verify coating performance without handling or risking your actual components.

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

Testing the real optic can introduce scratches, contamination, or mounting damage. A witness sample serves as a sacrificial proxy that preserves your production part. It also allows you to measure transmission at a consistent thickness and surface quality, removing variables that could skew results.

Does the witness sample material need to match the production optic?

Ideally yes, but not always. For broadband coatings, matching the substrate refractive index is important because the coating design is optimized for that index. For narrowband or single-wavelength coatings, a common substrate like fused silica may still give useful data. Always check with your coater or engineer.

Frequently asked questions about transmission witness samples

How do I use a transmission witness sample during coating?

Place the witness sample in the coating chamber adjacent to your production optics. After the coating run, remove the witness sample and measure its transmission spectrum with a spectrophotometer. Compare the measured curve to the coating design specification. Always record the melt lot ID and chamber position for traceability.

Can I reuse a transmission witness sample?

No. Once coated, the witness sample carries that specific run's coating. Cleaning or recoating would alter the measured data. Each run requires a fresh, uncoated witness sample. Order witness samples in quantities matching your expected coating runs.

What does the refractive index option affect?

The refractive index of the witness sample substrate must match the production optic's substrate to obtain accurate transmission data. If your production optic is N-BK7, for example, use a witness sample with the same index. Mismatch can shift the measured transmission curve. Confirm the substrate material of your production optic before selecting a witness sample.

Product facts

  • Witness Sample, Transmission, 1.800-2.000: This witness sample is polished on both sides to a scratch-dig of 60-40 with flatness < 1 wave @ 633 nm at 80% CA and parallelism < 1 arc min. These tolerances ensure the measured transmission is dominated by the coating, not by surface defects or wedge, giving you reliable data for coating qualification.

    Witness Sample, Transmission, 1.800-2.000

    Witness Sample, Transmission, 1.800-2.000

  • Witness Sample, Transmission, 1.800-2.000: All witness samples are melt lot controlled and individual melt data is available on request. Melt lot control means you can correlate coating results with a specific glass batch, which is critical for repeatable production and troubleshooting.