Longpass vs. Shortpass vs. Notch Filters: How to Choose for Stray-Light Control

Stray light—unwanted illumination from sources outside your intended signal—can severely affect optical systems, causing noise, false signals, and costly errors. Whether it comes from ambient light, reflections, or laser scatter, controlling stray light is crucial for reliable results in imaging, spectroscopy, and many other applications. The right optical filter is your primary defense for reducing stray light while protecting the signals you care about.

Quick Decision Guide
Choose longpass if your signal is at longer wavelengths than the stray light (for example, NIR imaging affected by visible ambient), the stray is broadband at shorter wavelengths, and you can discard those shorter components. Choose shortpass if your signal is at shorter wavelengths and the problem is broadband stray at longer wavelengths (for instance, when you want UV/blue but block red/IR leakage). Choose notch if the interference is a specific, narrow line (such as a laser or sodium lamp) and you need to keep signal on both sides. If broadband stray exists both above and below, a bandpass filter is likely better.

What Each Filter Does
Longpass (LP): Passes everything longer than a set cutoff wavelength and blocks shorter. Used for capturing fluorescence emission (blocking excitation), NIR machine vision (blocking visible), and IR sensing.
Shortpass (SP): Passes everything shorter than a chosen cutoff and blocks longer. Useful for UV/blue imaging, diagnostic devices, and any system that wants to block IR or red background.
Notch (Band-stop): Blocks a narrow wavelength region but transmits below and above. Great for laser line suppression or rejecting narrow lamp lines while retaining the full rest of the spectrum.

Pros & Cons at a Glance

• Longpass: High passband throughput, simple, affordable, robust to source drift. However, you lose all shorter-wavelength signal, edge shifts with AOI, and sufficient margin is required between cutoff and unwanted light.
• Shortpass: High throughput in passband, easy to align, ideal for blue/UV only systems. Downside: you lose all longer-wavelength signal, AOI and cutoff shift, and not suitable for red/IR-weighted applications.
• Notch: Selective removal of narrow lines, preserves both sides, can reach high OD. But, they are more expensive, sensitive to angle/polarization, and require careful alignment to avoid reflections and artifacts.

Practical Selection Workflow (5 Steps)

• Define your spectra: Plot or estimate signal, stray-light, illumination, and sensor QE.
• Identify discrete lines: For narrow lines use a notch; for broadband stray above use shortpass; below, use longpass.
• Set required blocking and margin: Determine level of OD needed for your SNR, and ensure margin for AOI/edge shifts.
• Check system geometry: AOI and FOV affect cutoff/center, polarization can impact performance, use AR coatings and tilt filters as needed.
• Confirm practicalities: Consider throughput, substrate, handling, environment, and damage thresholds.

Rules of Thumb & Examples

• Fluorescence Imaging: Excitation is shorter than emission—longpass for emission; add a notch if leaks persist.
• UV/Blue Imaging: If you only want short wavelengths, use a shortpass.
• Raman Spectroscopy: For both Stokes and anti-Stokes, use a notch; Stokes only, a laser-line longpass works.
• NIR Vision: Use a longpass to suppress visible background.
• Laser projectors or streetlamp line: Use notch filters for specific wavelength suppression.

Spec Checklist for Vendors

• Filter type: longpass / shortpass / notch (with cutoff or stopband center/width)
• Signal band: ____–____ nm
• Stray to block: ____ nm
• Blocking (OD): ____ (at stray)
• Transmission (average): ____% over ____–____ nm
• Edge/stopband width: ≤ ____ nm (SP/LP), FWHM for notch
• AOI/field: ____°; f/# = ____
• Polarization: unpolarized / s / p
• Reflections: AR coating, tilt, wedge as needed
• Substrate/size: material, thickness, flatness
• Environment: temperature, humidity, cleaning, laser power

Common Pitfalls & Fixes

• Cutoff/center shifts: Add margin or specify AOI, stop down your lens when needed.
• Notch reflections (ghosts): Tilt filter, AR-coat passbands, use a beam dump.
• Notch too wide: Specify a narrower stopband or dual notches.
• Insufficient OD for lasers: Specify OD 5–6+ for target lines, and confirm with vendor test data.

If you only remember three things

• Broad short-wavelength stray with long-wavelength signal, use a longpass.
• Stray above your signal band, use a shortpass.
• Single known narrow stray inside your band, use a notch.
• Always consider AOI shifts, OD levels, and manage reflections in real-world systems.