How much blocking OD do I need outside the passband?

When selecting optical filters, one of the most important performance questions is: How much blocking OD do I need outside the passband? The right answer depends on your application and system details—but a smart choice improves your imaging or detection performance and saves unnecessary cost.

• General machine-vision imaging: OD 3–4 outside the passband
• Laser/fluorescence (life-science, Raman): OD 6+ at rejection bands
• Spectroscopy/stray-light control: OD 4–6
• Daylight NIR imaging (e.g., 850–940 nm + visible rejection): OD 3–4 across the visible

What does OD mean?
OD, or optical density, is a measure of how much light is blocked by the filter at a given wavelength. It is defined as:
OD = -log₁₀(T)   where T = transmission (as a decimal). Higher OD = lower leakage.

• OD 3 → 0.1% transmission
• OD 4 → 0.01% transmission
• OD 5 → 0.001% transmission
• OD 6 → 0.0001% transmission

1. Translate OD to Leakage
Use the above definition to understand how much unwanted light gets through.

2. Base OD on Signal-to-Background at the Sensor
Decide the minimum signal-to-background ratio (SBR) you need at the camera (usually ≥10:1 for clean imaging). If out-of-band light is R times stronger than your in-band signal, you want blocking OD so that:
Required OD ≥ log₁₀(R/SBR)

• Example: If out-of-band light is 1000x your signal and you require SBR ≥ 10, then you need OD ≥ 2 (100x suppression). In practice, add margin for lens flare, AOI shifts, and sensor limits → choose OD 3–4.

3. Check Camera Dynamic Range / Noise
A 12-bit camera (4096:1 range) can handle less stray light than a 10-bit camera; scientific CMOS with low noise often needs more blocking to keep background under a few electrons.
Sanity check: To keep background < 1/1000 of full scale, target OD ≥ 3 compared to the dominant stray-light source.

• Machine vision (broadband LEDs or room lights): OD 3–4 outside the passband
• Daylight NIR imaging: OD 3–4 across 400–700 nm (visible region)
• Laser/LIDAR: OD 4–6 at unwanted lines and visible if needed
• Fluorescence imaging: OD 6+ in the excitation stopband (combine dichroic + emission filter if needed)
• Raman spectroscopy: OD 6–8 at the laser wavelength and blocking regions
• Spectrometers/stray-light control: OD 4–6 out of passbands

• Blocking range: Specify blocking over your camera's sensitive range—not just next to the passband.
• Angle of incidence (AOI): Fast lenses (f/1.4–f/2.8) mean rays up to 10–20° AOI. Interference filters can blue-shift at higher AOI, eroding blocking. Always check for blocking over your AOI, not just 0°.
• Temperature: Filter spectra can shift; spec your blocking over the expected working range.
• Measurement resolution: Standard 5–10 nm spectral scans may miss leaks. For lasers, request high-res scans.
• Stacking filters: OD values approximately add at a given wavelength. Typical in fluorescence setups.
• Coating vs. substrate: True blocking should be absorptive or highly reflective, not just from substrate absorption.
• Stray-light sources: Lens scatter, cover glass, and mounts also contribute—mechanical baffling is as crucial as OD.

• If you are unsure and it is machine vision: OD 3 outside the passband, move up to OD 4 if glare persists.
• If suppressing a bright laser or excitation source: OD 6 at that wavelength is a safe default.
• For outdoor NIR plus visible blocking: OD 3–4 in the visible is usually sufficient with proper baffling.

• IR tracking under bright stage lighting: OD 4–5 visible; OD 5 if LED walls or strobes are present in FOV
• Protect camera / reduce laser streaks: At each laser line: OD ≥ 6 (OD 4–5 may still saturate)
  Request high-resolution scans around laser lines for safety.
• Blacklight scenes (UV rejection): OD ≥ 6 at 365–405 nm; use dichroic LP with absorptive UV-cut for robust results.
• NIR depth/ToF/structured light on stage: OD 4–5 visible; OD 3–4 other NIR bands to reduce cross-talk.
• Concert video/color isolation: OD 3–4 out-of-band; OD 4–5 for strong spike regions (e.g., 450 nm LED pump).

• Choose your out-of-band OD to match your application, background strength, and signal needs. For general imaging, start with OD 3–4; for laser/fluorescence, OD 6+ is typical.
• Always consider system factors (AOI, temperature, camera noise, stray light) and consult your vendor for full-range, high-resolution blocking specs.