Laser Protection 101: Choosing Notch Width and Specifying the Right Outside-the-Notch Transmission
When specifying laser protection filters for military, aviation, or imaging use, getting the notch filter width correct—and maintaining high transmission outside the notch—can be the difference between safety and compromised performance. This post explains how to choose the right notch width for your field of view (FOV), angle of incidence (AOI), and laser threat, and how much out-of-notch transmission is 'right' for your application.
What Is Notch Width, and Why Does It Matter?A notch filter blocks a specific laser wavelength (the 'notch') while passing the rest of the band with minimal loss. The notch width is the full-width-at-half-maximum (FWHM) of the stopband, typically specified at 0° AOI. Narrow notches let in more 'useful' light and preserve color, but risk missing off-axis threats. Wider notches improve safety for wide angles, but can dim or distort the scene.
How Wide to Make the Notch? Threat Bands, AOI, and Field-of-ViewNotch width depends on your threat laser wavelength and how far off-axis your filter must work. As AOI increases, the blocked wavelength blue-shifts, so systems with wide FOV or large AOI (e.g., visors, multi-sensor windows) require a wider notch to ensure continuous laser blocking. Use these guidelines:
| Threat Wavelength | Typical Use | Target Notch (0° AOI) | If Wide FOV/AOI (~±15–25°) |
|---|---|---|---|
| 532 nm | Laser pointers, dazzlers, DPSS | 12–17 nm FWHM | 20–30 nm FWHM |
| 808 nm | IR pointers, illuminators | 18–22 nm FWHM | 25–35 nm FWHM |
| 980 nm | IR diodes, NVG cutoff | 22–30 nm FWHM | 30–45 nm FWHM |
| 1064 nm | Nd:YAG, LRF | 30–40 nm FWHM | 40–80 nm FWHM |
| 1550 nm | SWIR, eye-safe LRF | 40–60 nm FWHM | 60–100 nm FWHM |
AOI blue shift: If your window or eyewear can see rays from up to ±20°, expand the notch by about 1–2% of the center wavelength per side. For 532 nm, that means an extra 5–10 nm; for 1064 nm, 10–20 nm; for 1550 nm, 15–30 nm. Add margin for manufacturing and polarization effects. If uncertain, always verify with your supplier.
What OD and Transmission to Specify?Inside the notch (laser-blocking band), aim for OD ≥ 6 for sensors/windows (OD6 = 1 part per million through), OD ≥ 4 for eyewear (military often prefer OD6 for critical ops). Outside the notch, you want as much light as possible: target average transmission (Tavg) ≥ 90–93% across your mission band (e.g., 400–700 nm for VIS, 1000–1500 nm for SWIR). Top vendors routinely deliver these levels.
If you have multiple laser threats (such as 532 and 1064 nm), specify dual- or triple-notch designs instead of asking for low outside-notch leakage.
Sample Procurement Specs- 'Notch at 532 nm, OD≥6, FWHM 20 nm (0° AOI), Tavg ≥93% from 400–700 nm; maintain spec to ±15° AOI.'
- 'Dual-notch at 532 & 1064 nm, both OD≥6; FWHM 20 nm @532, 40 nm @1064; Tavg ≥90% 550–950 nm; validated to cone half-angle ≥15°.'
- 'Notch at 1550 nm, OD≥6, FWHM 60 nm, Tavg ≥90% 1000–1500 nm excluding notch; AOI spec to ±15°.'
- Map your window or sensor's AOI or FOV (how far off-axis rays can enter).
- Pick base notch width from the 0° AOI column above.
- Add extra margin for blue shift and tolerance if FOV/AOI is wide.
- Specify OD ≥ 6 (or as required), confirm eyewear tint/NVG compatibility if necessary.
- Ensure Tavg ≥ 90–93% outside the notch over your mission-relevant spectrum.
- Work with your filter supplier to confirm catalog or custom designs meet your requirements.
- Notch width must cover all potential angles of incidence in your system, not just 0°.
- OD6 at the laser line is standard for sensors; OD4-6 for eyewear. Specify high average transmission outside the notch (93%+ when possible).
- Overly wide notches can degrade vision or imaging—strike the right balance for your field application.
- Plan carefully and ask suppliers for real spectral test data before finalizing your spec.
