A long pass filter (LPF), also known as a long-wave pass filter or high-pass optical filter, is an optical component that transmits light with longer wavelengths while blocking shorter wavelengths.
In simple terms:
  • Shorter wavelengths (UV, blue, violet) → reflected or absorbed
  • Longer wavelengths (green, yellow, red, NIR) → transmitted efficiently
Long pass filters are widely manufactured as hard coated interference filters for high performance, durability, and sharp cut-on edges.

How Does a Long Pass Filter Work?

Most industrial-grade long pass filters use multi-layer thin-film coating technology (interference filters).

The coating structure is designed to:

  • Allow light above a certain wavelength to pass through
  • Reflect or block light below that threshold
  • Provide high transmission in the passband
  • Offer strong blocking in the stopband
Common specifications include:
  • Cut-on wavelength (e.g., 550 nm, 650 nm, 780 nm, 850 nm)
  • Transmission rate: >85%–95% in the passband
  • Optical density (OD): OD3 to OD6 for blocking
  • Hard dielectric coating for high stability

Key Features of Long Pass Filters

  • Sharp cut-on edge
  • High transmittance in the passband
  • High blocking of unwanted short wavelengths
  • High laser-induced damage threshold (LIDT)
  • Stable performance under different temperatures
  • Custom sizes and wavelengths available

Where Are Long Pass Filters Used?

1. Fluorescence Microscopy & Medical Devices

Long pass filters separate excitation light from emitted fluorescence signals.

They are widely used in:

  • PCR instruments
  • Fluorescence microscopes
  • Medical diagnostic equipment
  • Bioimaging systems

2. Laser Systems & Laser Protection

Used to block pump light or stray short wavelengths while letting laser beams pass.

Common in:

  • Laser pointers
  • Industrial laser equipment
  • Laser measurement tools
  • Laser safety systems

3. Machine Vision & Industrial Imaging

Improve image contrast by filtering out blue noise, UV interference, and ambient light.

Applied in:

  • Industrial cameras
  • Inspection systems
  • Barcode scanners
  • LED lighting systems

4. Environmental & Optical Sensing

Used in spectrometers, optical analyzers, and environmental monitoring devices.

5. Security & Surveillance Cameras

Especially 850nm and 940nm long pass filters for IR night vision.

6. Automotive & Consumer Electronics

Used in:
  • LiDAR systems
  • Optical sensors
  • Camera modules
  • Display panels

How to Choose a Long Pass Filter

When selecting a long pass filter, consider these key factors:
  1. Cut-on wavelength required by your system
  2. Transmission rate in the passband
  3. Blocking depth (OD value)
  4. Operating angle and environment
  5. Dimensions, thickness, and shape
  6. Coating type (hard or soft)
Most industrial applications prefer hard coated interference long pass filters for long service life and high stability.

Custom Long Pass Filters Available

We provide custom long pass filters with:
  • Custom cut-on wavelengths
  • Custom sizes and shapes
  • Various glass substrates (BK7, quartz, borosilicate)
  • High-transmission hard coatings
  • Small orders and sample orders welcome
If you need help selecting the right long pass filter for your project,

please feel free to contact us for professional support.