Using Wedge Witness Samples for Coating Thickness Checks
By Esco Optics, Inc. | Published: 2026-10-19
Category: How-to Guides
Learn how to use a wedge witness sample to verify optical coating thickness during deposition. Step-by-step process with common mistakes to avoid.
Quick answer
Place a wedge witness sample next to your coated optics during deposition, then measure its transmission spectrum to verify coating thickness without damaging production parts.
Why a wedge witness sample instead of a flat one?
A wedge witness sample has a small angle (typically 8 degrees) between its two polished faces. This eliminates interference fringes that would otherwise appear in transmission measurements on a parallel flat. Without fringes, the measured spectrum directly reflects the coating's performance, making thickness verification faster and more reliable.
The trade-off is that you need to account for the wedge angle when interpreting the data. Most spectrophotometers and software handle this automatically, but if you are using a manual setup, you must enter the angle correctly.
Choosing the right material for your witness sample
The witness sample should match the substrate material of your production optics as closely as possible. Differences in refractive index and thermal expansion can shift the coating's optical performance and stress. Esco Optics offers wedge witness samples in various refractive index and material options, each melt lot controlled with individual melt data for traceability.
If you cannot match the exact material, use a witness sample with a known and stable refractive index. Document the difference so you can correct the thickness reading later.
Step-by-step: Coating thickness check with a wedge witness sample
- 1. Load the witness sample into the chamber: Place the wedge witness sample on a holder next to your production optics. Ensure the polished faces are clean and the wedge face points toward the coating source. Record the sample's melt lot number for traceability. Common mistake: Loading the sample backwards (wedge face away from source) can shift the coating thickness gradient.
- 2. Run the coating deposition cycle: Deposit the coating as planned. The wedge witness sample will receive the same coating thickness as nearby optics if positioned correctly. Monitor the process using a quartz crystal monitor for real-time thickness feedback. Checkpoint: Verify that the witness sample is at the same radial distance from the source as your production parts to ensure equal deposition rate.
- 3. Measure the transmission spectrum: After deposition, remove the witness sample and place it in a spectrophotometer. Scan the transmission spectrum over the wavelength range of interest. The wedge design prevents interference fringes, so the spectrum is clean and ready for analysis. Common mistake: Forgetting to enter the wedge angle (8°) into the software. This can cause a systematic shift in the calculated thickness.
- 4. Compare the measured spectrum to the design target: Use coating design software to compare the measured transmission curve to the theoretical curve for your target thickness. Adjust the modeled thickness until the curves match. The difference tells you the actual coating thickness deviation. Checkpoint: If the deviation exceeds your tolerance, adjust the deposition time or rate for the next run. Document the correction for process control.
Recommended product
- Transmission Wedge Witness Sample: Precision polished on both sides to a scratch dig of 60-40 with flatness < 1 wave @ 633nm at 80% CA and parallelism < 1 arc min. High surface quality ensures that the measured transmission spectrum is dominated by the coating, not by scatter or surface defects, giving you a reliable thickness reading.
Transmission Wedge Witness Sample
