A bandpass filter is an optical filter that allows a selected wavelength range to pass through while blocking unwanted wavelengths.
These optical filters are commonly used in imaging systems, machine vision, fluorescence systems, laser systems, sensors, optical instruments, and custom optical assemblies.
What Is a Bandpass Filter?
A bandpass filter is an optical filter designed to transmit a specific range of wavelengths and block other unwanted wavelengths.
In simple words, a bandpass filter works like a “wavelength selector.”
It allows only the required light band to pass through, while reducing or blocking light outside that range.
For example, if a system only needs light around 850 nm, a spectral filter can be designed to transmit light near 850 nm and block other wavelengths.
Why Are Bandpass Filters Used?
Bandpass filters are used when an optical system needs to separate or select a specific wavelength range.
They help improve image contrast, reduce background light, block unwanted signals, and allow the system to detect only the required wavelength.
Common applications include:
- Machine vision
- Fluorescence imaging
- Biomedical instruments
- Laser systems
- Optical sensors
- Spectroscopy systems
- Camera systems
- Measurement equipment
- Custom optical assemblies
Common Types of Bandpass Filters
Bandpass filters can be designed in different types depending on the wavelength range and application.
Common types include:
- Narrow bandpass filters
- Broad bandpass filters
- Multi-bandpass filters
- Custom bandpass filters
Narrow Bandpass Filter
A narrow bandpass filter allows only a small wavelength range to pass through.
It is used when the system needs to detect or isolate a very specific wavelength.
For example, these optical filter may be used around a laser wavelength, LED wavelength, or fluorescence emission wavelength.
Common applications:
- Laser systems
- Fluorescence imaging
- Biomedical instruments
- Optical sensors
- Scientific measurement systems
- Spectroscopy equipment
Simple explanation:
A narrow bandpass filter is used when the system needs a very specific wavelength and wants to block most other light.
Broad Bandpass Filter
A broad bandpass filter allows a wider wavelength range to pass through.
It is used when the system needs more light or when the application does not require a very narrow wavelength selection.
For example, these optical filter may be used in imaging systems where a wider color or spectral range is useful.
Common applications:
- Imaging systems
- Machine vision
- Camera systems
- Optical inspection
- General optical instruments
- Custom optical systems
Simple explanation:
A broad optical filter is used when the system needs a wider transmission range instead of a very narrow wavelength band.
Multi-Bandpass Filter
A multi-bandpass filter is designed to transmit two or more separate wavelength bands.
This means one filter can allow multiple selected wavelength ranges to pass through while blocking unwanted wavelengths between and outside those bands.
Common applications:
- Multi-wavelength imaging
- Fluorescence systems
- Optical detection systems
- Biomedical instruments
- Laser or LED systems with multiple wavelengths
- Custom optical measurement systems
Simple explanation:
A multi-bandpass filter is useful when the system needs to detect more than one wavelength range using one filter.
Simple Comparison Table
| Filter Type | Main Function | Simple Explanation | Common Use |
|---|---|---|---|
| Narrow Bandpass Filter | Transmits a small wavelength range | Selects a very specific wavelength | Laser, fluorescence, sensors |
| Broad Bandpass Filter | Transmits a wider wavelength range | Allows more light within a broader range | Imaging, machine vision, cameras |
| Multi-Bandpass Filter | Transmits two or more wavelength bands | Allows multiple selected bands through one filter | Multi-wavelength imaging, biomedical systems |
| Custom Bandpass Filter | Designed based on project requirements | Customized wavelength, size, and coating | Special optical systems |
Bandpass Filter vs Other Optical Filters
A bandpass filter is different from shortpass and longpass filters.
A bandpass filter allows a selected middle wavelength range to pass through.
A shortpass filter allows shorter wavelengths to pass and blocks longer wavelengths.
A longpass filter allows longer wavelengths to pass and blocks shorter wavelengths.
The right filter depends on what wavelength range the optical system needs to transmit or block.
What Information Should You Provide Before Requesting a Quote?
To request a bandpass filter, it is helpful to provide:
- Center wavelength
- Bandwidth or transmission range
- Blocking wavelength range
- Transmission requirement
- Filter size or diameter
- Thickness requirement
- Coating
- Substrate material, if required
- Surface quality
- Quantity
- Drawing or specification, if available
- Application, such as laser, fluorescence, sensor, or imaging system
If you are not sure which bandpass filter is suitable, you can provide your application and target wavelength. Our team can help review the requirement and suggest a suitable filter design.
Example Specification for Bandpass Filter
| Item | Example Specification |
|---|---|
| Filter Type | Narrow bandpass filter |
| Center Wavelength | 850 nm |
| Bandwidth | 10 nm / 20 nm / custom |
| Transmission Band | 845–855 nm, or based on requirement |
| Blocking Range | 400–1100 nm, except transmission band |
| Peak Transmission | >85%, or based on requirement |
| Substrate | Optical glass / Fused silica / Custom |
| Shape | Round / Square / Rectangular |
| Size | Ø25.0 mm / 50 × 50 mm / Custom |
| Thickness | 1.0 mm / 2.0 mm / Custom |
| Surface Quality | 60/40 scratch-dig |
| Coating | Bandpass coating |
| Quantity | 10 pcs / batch production |
| Application | Imaging system / laser system / optical sensor |
Conclusion
A bandpass filter is used to select a specific wavelength range and block unwanted light.
Narrow bandpass filters are suitable for precise wavelength selection. Broad bandpass filters are useful when a wider wavelength range is needed. Multi-bandpass filters are suitable for systems that need more than one transmission band.
Shape Optics Technologies supplies custom optical filters, including narrow bandpass filters, broad bandpass filters, multi-bandpass filters, shortpass filters, longpass filters, coated filters, and custom optical filter components based on customer requirements.
Contact us to discuss your custom bandpass filter requirements.