Should I choose hard-coated or soft-coated bandpass filters?
Should I choose hard-coated or soft-coated bandpass filters? Let's break it down with a no-nonsense, practical comparison so you get the answer you need for your specific project.
Hard-coated vs. Soft-coated Bandpass Filters
Choose hard-coated (sputtered/IBS, dense films) for durability, spectral stability, steep edges, high blocking, higher laser power, and field use.
Choose soft-coated (evaporated, more porous) when you need very large sizes at lower cost or good enough performance for more gentle indoor environments.
What the terms actually mean
Hard-coated = ion-beam or magnetron sputtered (often with ion assistance). Layers are dense, hard, and non-porous.
Soft-coated = evaporated multilayers (typically without full densification). Layers are more porous/softer and can take in moisture.
| Factor | Hard-coated (Sputtered/IBS) | Soft-coated (Evaporated) |
|---|---|---|
| Durability & Environment | Excellent: resistant to humidity, cleaning, abrasion; good thermal cycling | Fair–good: can shift with humidity/cleaning; more delicate |
| Spectral Stability (humidity, temp, time) | Stable center wavelength & FWHM; low drift | More drift over time (moisture uptake); more sensitive |
| Edge Steepness & Ripple | Very steep edges, low ripple, high uniformity | Edges less steep for same cost; more ripple if pushed |
| Blocking OD | High blocking (OD 5–6+) over wide ranges | High OD possible but often needs more complex stacks |
| Surface Scatter/Stray Light | Lower scatter (dense films, fewer pinholes) | Higher stray light risk (porosity, pinholes) |
| Laser Damage Threshold | Higher (better for lasers/high irradiance) | Lower (avoid high fluence) |
| AOI & Angle Sensitivity | Dense films keep specs more consistent | Angle works, but more drift/variation lot-to-lot |
| Size Availability | Getting better; cost rises above 50–100 mm | Often cheaper for large diameters (100–300 mm+) |
| Lead Time/Cost | Generally higher cost per part; longer for tight tolerances | Lower cost for moderate specs; faster on standard builds |
| Best Use Cases | Machine vision, fluorescence/biomed, LIDAR, harsh field, laser, astronomy | Large formats, cost-sensitive builds, benign lab/indoor use |
Quick chooser: When to pick hard-coated or soft-coated?
- Pick Hard-coated if you need:
- Tight spectral control: steep edges (10–20 nm FWHM with OD 5–6 outside band), minimal ripple.
- Specs to hold under high humidity, cleaning, and temperature swings.
- Field or rental gear (photo/cine/stage), or outdoor sensors.
- High irradiance or laser exposure.
- Long service life and consistent re-orders.
- Pick Soft-coated if you need:
- Large diameters (150–300 mm+), tighter budget.
- Moderate performance is okay (OD 3–4 blocking, edges not ultra-steep).
- Benign environments (clean lab, controlled humidity), limited cleaning.
- Shorter lead time/cost matters more than ultimate robustness.
Common specs to check (for any filter):
- Center wavelength / FWHM at your AOI (e.g., 0° or 5°) and temperature (e.g., 23 °C).
- Blocking range and OD (OD ≥ 5 from 200–1100 nm outside the passband).
- Wavefront/flatness & scratch-dig (e.g., λ/4 P-V @ 632.8 nm, 40-20 for imaging; 60-40 can be fine for stage).
- Humidity/thermal drift (request ΔCWL and ΔT specs after high-RH soak and thermal cycle).
- Angle sensitivity (specify your AOI; even a few degrees may shift the passband).
- AR coatings on both sides to cut ghosting; ask for stray light data if important.
Pitfalls to avoid
- Treating soft coat like glass: aggressive wiping can micro-scratch or alter spectra over time.
- Ignoring humidity: soft coats can shift a few nm in the visible after humidity exposure.
- Under-spec'd blocking: OD 3 may look fine but can allow flare/ghosting in bright scenes.
- Assuming all bandpass filters are the same: edge steepness and out-of-band behavior vary a lot between methods.
Application-specific nudges
- Machine vision / factory lines: Hard-coated preferred (stable CWL, lower scatter, repeatable).
- Fluorescence/biomed: Hard-coated for high OD off-band and clean edges.
- LIDAR / NIR beacons: Hard-coated for stability and sun/temperature swings; better for high flux.
- Stage/film lighting: Hard-coated holds up better (if very large panels are needed on a budget, soft-coated can work with protective cover glass and looser specs).
- Large astronomy windows/shrouds: Soft-coated may be most economical for size, but mind moisture/temperature and verify blocking.
- Environment harsh? (humidity, cleaning, temperature, high flux) → Hard
- Need steep edges/high OD? → Hard
- Diameter >150 mm and budget tight? → Soft (with cover/protection)
- Laser or bright source? → Hard
- Short, low-risk pilot build? → Soft can be a cost-saving bridge
Spec wording you can drop into a PO (edit values):
Bandpass filter, CWL ___ nm, FWHM ___ nm, AOI 0° (±___°) at 23 °C; Blocking OD ≥ 5 from 200–1100 nm outside passband; peak T ≥ ___%. Preference: hard-coated (sputtered); provide ΔCWL after 24 h at 95% RH and thermal cycle −20 to +70 °C. Surface quality 40-20 (or 60-40 for non-imaging), AR both sides, substrate ___, diameter ___ mm, thickness ___ mm.Bottom line
- If performance, stability, and robustness matter—even a little—go hard-coated.
- Choose soft-coated mainly for very large, budget-sensitive parts in gentle environments where moderate drift and blocking are okay.
- If you want, tell me your target wavelength, bandwidth, diameter, AOI, and environment. I can help you turn it into a precise, vendor-ready spec and suggest realistic tolerances.
