Dichroic Filters for Multispectral Drones: Weight, Wavebands & Real-World Wins

From the workbench at China Star Optics—engineered for NDVI/NDRE, tuned for UAV angles, and built to survive sun, salt, and dust.

Why grams decide air time—and map quality

Every gram is a tax on flight time. Slimmer filters reduce heat load on the gimbal, ease motor torque, and extend the battery curve. Less mass also damps vibration, so micro-blur drops and mosaics stitch tighter. Across hundreds of sorties, those small wins compound into more hectares per charge and cleaner indices per flight.

Which wavebands matter (UV → NIR)

Most multispectral stacks focus on:

  • Blue (≈450 nm): water clarity, algal dynamics

  • Green (≈550 nm): canopy vigor

  • Red (≈660 nm): plant stress

  • Red-edge (≈705–740 nm): early-stage change detection

  • Near-IR (≈800–900 nm): biomass & moisture
    Add UV for coatings/minerals; move to SWIR with dedicated sensors when targets demand it. The goal isn’t “more bands,” it’s correct splits held across the field so index math is stable from center to corners.

How dichroic filters elevate NDVI & NDRE

Dichroic filters steer light with thin-film interference—not pigments—so they run cool, waste little light, and can be tailored with crisp cut-on/cut-off edges. NDVI needs a clean Red vs NIR separation; NDRE prefers a tight red-edge window. We match pass/reflect splits to popular camera bodies and index recipes, so your maps align with agronomy toolchains and UAV processing pipelines out of the box.

Angle-tuned dichroic filter mounted near the aperture stop of a multispectral drone camera, AR-coated fused-silica substrate, AOI optimized for F/2.8–F/8 to keep NDVI/NDRE bands stable; lightweight design by China Star Optics.

Angles, cones, and why F/# matters

Drone sensors don’t see a single angle; they see a cone of rays set by lens F-number.

  • F/2.8 → wider cone → larger blue-shift of the passband

  • F/8 → narrower cone → smaller shift
    We design coatings for your actual AOI range, not a lab-perfect beam. If you change lens or gimbal geometry, tell us—minute tweaks to the design AOI keep seasonal datasets comparable.

Practical ways to limit color shift

  • Favor a slightly slower lens when possible.

  • Place the filter close to the aperture stop to narrow the cone.

  • Use gentle pre-tilt if your sensor stack demands it.

  • Observe orientation marks—flipping parts in the field can nudge colors.

Materials that cut grams (and stay flat)

Substrate choice sets mass, stiffness, and thermal behavior.

Optical Substrates — quick reference
Substrate Where it shines Typical use cases
BK7 Cost–clarity balance in VIS General RGB/Red–NIR splits
Fused Silica UV transmission, low CTE, thermal shock Summer heat, high-altitude chill
Optical Polymers Feather-light, shatter-safe Small gimbals, weight-critical fleets (with hardcoats)

Wedge & bevel: A slight wedge diverts ghosts off-sensor; bevels protect edges and keep adhesive fillets tidy.

Weight snapshot (25.5 × 25.5 mm)

  • Fused silica 1.10 mm: ~1.6 g

  • Fused silica 0.55 mm: ~0.8 g

  • BK7 1.10 mm: ~1.8 g
    Small thickness trims across a wheel add up fast.

Build lighter without sacrificing sharpness

  • Mounts: Metal frames are rigid but can add thermal stress; composites flex slightly to ride out temperature cycles.

  • Adhesives: Low-shrink, soft-cure systems guard coatings and preserve flatness under vibration.

  • Flatness: You don’t need thick glass—smart support zones and even bond lines hold shape. Add AR + slight wedge and keep ghosts out.

Stray-light control for harsh sun

Sun hits UAVs from odd angles. Stack defenses:

  • Matte baffles + blackened edges

  • AR on every air–glass surface

  • Staggered gaps that disrupt straight light paths
    The payoff: cleaner histograms and steadier indices.

Built for heat, moisture, and movement

We deposit dense, low-absorption thin-film stacks that resist humidity creep and keep color stable across hot noons and cold climbs. Hardcoats + sealed edges tame salt fog and dust. Cleaning playbook: air first, then approved swabs—no shirt sleeves.

What to send in your RFQ (we’ll tune to it)

  • Lens F-number and chief ray/AOI range

  • Camera body/sensor cover glass details

  • Target pass/stop bands & slopes

  • Weight limit and mounting geometry
    We’ll return a drawing with thickness, size, wedge, AOI tuning, and AR stacks. Parts then run through optical scans, environmental stress, and a class-controlled clean—ready to drop in and fly.

Specs that actually move the needle

  • Cut-on / Cut-off: keep bands from smearing

  • Slope: faster edge = cleaner separation

  • OD (blocked zones): suppress sun glint & bright water

  • Ripple: smooth passbands = stable sensor response

  • Wavefront & flatness: sharp images across the frame

  • Scatter: low haze at key wavelengths

How we qualify what you’ll see in the air

  • Incoming: substrate flatness, thickness, edge finish

  • In-process: witness parts track passband & OD

  • Final: full-aperture scans at your AOI range, AR on both sides, scatter checks, and a post-vibration inspection

Lab ≈ Flight: telecentric AOI mapping

A telecentric bench reproduces your F/# cone, so spectral scans reflect real-world angles. That’s how we keep index math steady from center to corners.

Price–performance for fleet builders

Costs fall fast with standard sizes and shared passbands. Common thicknesses speed coating slots. For custom parts, batch orders to align with run cycles. Tiny dimensional nudges (e.g., trimming 1 mm) can save days without touching image quality.

Case snapshots

  • Farms: NDVI/NDRE on tight batteries → thinner fused-silica plate, sharp red-edge split, AR both sides. Result: longer flights, lower map noise, earlier stress flags.

  • Mines: Noon, dusty roads → higher OD in blocked bands, blackened edges, slight wedge to kick ghosts. Higher contrast, fewer artifacts.

  • Coasts: Algal bloom watch → green/blue split with low scatter and salt-hard hardcoat. Clarity holds across multi-week runs.

Ready-to-use spec templates

  • NDVI splitter: 650–680 nm block, 700–900 nm pass, slope ≤ 15 nm, OD ≥ 4 blocked, Tavg ≥ 95% pass, AOI tuned to 0–10°.

  • Red-edge focus: 705–740 nm pass, 660–690 nm block, OD ≥ 4, ripple ≤ 1%, AR both sides Ravg ≤ 0.5% (450–900 nm).

  • Tri-band wheel (B/G/R): 450 / 550 / 660 nm windows with narrow splits for single-shot mosaics. Thickness matched to your hub torque.

Pitfalls to avoid at scale

  • Swapping lenses mid-project without sharing F/# & chief-ray—your passbands will move.

  • Adding uncoated protective windows—glare spikes.

  • Storing spares unsealed—humidity creeps into edges, especially near coasts.

  • Ignoring orientation marks—color drift creeps in.

FAQs

Q1: Do I need dichroic filters for simple RGB?
If you’re only shooting color, an IR-cut may be enough. Once you chase NDVI/NDRE, the precise splits of dichroic filters lift map quality.

Q2: How do angles change the passband on a drone?
Rays arrive as a cone set by lens F/#. Wider cones nudge band edges toward shorter wavelengths. We tune coatings for that cone so edges stay put.

Q3: Can one filter serve multiple cameras?
Sometimes. If AOI range and cover glass are similar, a single design can work. Share specs—we’ll confirm.

Q4: What protects filters in salt air?
Dense coatings, hardcoats, sealed edges, and a rinse/clean routine. Store with desiccant after marine flights.

Q5: How much weight can thickness save?
A 25.5 × 25.5 × 1.10 mm fused-silica plate is ~1.6 g; at 0.55 mm it’s ~0.8 g. Multiply by the number of filters in your wheel for total savings.