Bandpass optical filters are used to selectively transmit light within a defined wavelength range and block light outside that range. These lens filters are essential in applications requiring high precision, such as fluorescence microscopy, laser-based systems, and spectroscopy.
What Is a Bandpass Filter?
A bandpass filter is an optical device that permits light of certain wavelengths to pass through while blocking others. The range of light allowed is referred to as the passband, while the range that is blocked is known as the stopband.
Bandpass filters are designed using various materials and techniques, such as interference coatings, absorptive glass, or dielectric stacks, depending on the desired performance. These filters are used in systems that require precise wavelength control, including:
- Fluorescence and biomedical imaging
- Remote sensing
- Astronomy
- Machine vision systems
How do optical filters function?
Depending on the method of filtering, optical filters are categorized as absorptive optical filters and dichroic optical filters (also called thin-film filters or interference filters).
- Absorptive optical filters block the light using the absorption properties of the glass substrate. They retain their transmission and absorption characteristics regardless of the light’s angle of incidence.
- Dichroic optical filters, on the other hand, reflect unwanted wavelengths and transmit the desired ones using a thin dielectric coating on the glass. These filters demonstrate sharp transitions between transmitted and reflected wavelengths and are angle-sensitive.
Popular types of optical filters
- Monochromatic filters: Transmit only a single, narrow wavelength—ideal for laser-based or fluorescence imaging.
- Ultraviolet (UV) filters: Block UV light while transmitting visible wavelengths—common in cameras or biological systems.
- Infrared (IR) filters: Transmit or block IR radiation depending on the application—widely used in night vision, thermal imaging, and remote sensing.
- Neutral density filters: Reduce intensity across the entire spectrum without altering color—often used in photography or scientific setups.
- Longpass & Shortpass filters: Used to isolate wavelength ranges either above or below a certain threshold—essential in multi-spectral imaging setups.
- Bandpass filters: Allow a specific “band” of wavelengths to pass while blocking the rest. These are used in microscopy, laser systems, spectroscopy, and embedded sensing applications.
All About Bandpass Filters
Bandpass filters isolate a precise wavelength range while blocking out-of-band light. They’re fully customizable and often deployed in imaging and detection systems where signal-to-noise ratio is key.
Industries that rely on them include:
- Life sciences
- Fluorescence imaging
- Spectral analysis
- Astronomy
- Environmental monitoring
- Industrial automation
Real-World Uses:
In 3D cinema, red-blue filters separate images for stereoscopic perception. In astronomy, custom filters isolate emission lines from gases and stars. In embedded sensors, narrowband filters improve detection accuracy by isolating target wavelengths while rejecting ambient light.
Bandpass filters are typically categorized as:
- Narrowband: Passing <1% of light; ideal for very specific wavelength selection.
- Wideband: Passing 10% or more of light; better for systems needing broader illumination or detection.
How Bandpass Filters Are Built
Most bandpass filters are created by stacking longpass and shortpass coatings or combining multiple dielectric layers into an interference structure.
Each filter usually includes:
- Two reflective stacks separated by carefully tuned spacer layers
- A substrate, such as optical glass or fused silica
- Up to eight cavities for higher transmission accuracy and steeper cutoffs
Types of Filter Design:
- All-Dielectric: Uses alternating layers of materials to control transmission. Offers high blocking with good environmental durability.
- Metal-Dielectric: Adds a thin metal layer to boost performance in rugged or high-temperature environments.
Design Considerations for Engineers & OEMs
When designing an optical system that relies on a bandpass filter, engineers must consider:
- Sensor compatibility (CWL vs. sensor sensitivity)
- FWHM bandwidth
- Blocking range requirements
- Environmental exposure (heat, vibration, moisture)
- Mechanical integration with the lens or imaging path
At Universe Kogaku, we collaborate with engineers to create complete optical assemblies. From rugged coatings and sealed housings to miniature form factors and thermal tolerance, we provide lens filters & accessories that meet real-world challenges.