Choosing the right optical filter is critical for any imaging, spectroscopy, or laser system. Selecting the wrong type can lead to poor signal-to-noise ratio, false readings, or complete system failure.
Three of the most common filter types are Long Pass Filters (LPF) , Short Pass Filters (SPF) , and Bandpass Filters (BPF) . While they may look similar, each serves a fundamentally different purpose.
This guide explains how they work, compares their spectral performance, and helps you decide which one you actually need.
A Long Pass Filter from the professional long pass filter supplier transmits wavelengths longer than a specific cut-on wavelength and blocks (reflects or absorbs) shorter wavelengths.
Common applications:
Fluorescence microscopy (blocking excitation light, passing emission light)
Infrared illumination systems (blocking visible light)
Raman spectroscopy (blocking Rayleigh scattering)
Example: A 550nm long pass filter blocks blue/green light (<550nm) and transmits yellow/orange/red light (>550nm).
A Short Pass Filter is the opposite of a long pass filter. It transmits wavelengths shorter than a specific cut-off wavelength and blocks longer wavelengths.
Common applications:
Removing infrared contamination from visible light cameras
UV spectroscopy (passing UV, blocking visible/IR)
Solar simulation (blocking thermal IR)
Example: A 650nm short pass filter transmits visible light (400-650nm) but blocks infrared (>650nm) that would otherwise heat the sensor.
A band pass optical filter transmits only a narrow range of wavelengths between a lower cut-on and an upper cut-off point. It blocks all wavelengths outside this passband.
Common applications:
Laser line clean-up (isolating a single laser wavelength)
Flow cytometry (detecting specific fluorophores)
Astronomical imaging (isolating hydrogen-alpha or oxygen-III lines)
Example: A 532nm bandpass filter with 10nm bandwidth only transmits light from 527nm to 537nm, blocking everything else.
This table summarizes the key differences. Use it as a quick reference during selection.
| Feature | Long Pass Filter (LPF) | Short Pass Filter (SPF) | Bandpass Filter (BPF) |
| Transmits | Wavelengths > cut-on | Wavelengths < cut-off | Only a specific band (λ center ± Δλ) |
| Blocks | Short wavelengths | Long wavelengths | All wavelengths outside the passband |
| Spectral Edge | One sharp edge (rising) | One sharp edge (falling) | Two edges (rising + falling) |
| Typical Bandwidth | Broadband (>100nm) | Broadband (>100nm) | Narrow (1nm – 50nm) |
| Key Parameter | Cut-on wavelength (λ cut-on) | Cut-off wavelength (λ cut-off) | Center wavelength (CWL) + FWHM |
| Typical Transmission | >90% in passband | >90% in passband | 70-95% at peak |
| Blocking Depth | OD4 – OD6 | OD4 – OD6 | OD4 – OD6 |
Follow this decision flowchart to avoid common mistakes.
Choose: Long Pass Filter
In fluorescence microscopy, you illuminate with blue light (e.g., 488nm) but want to capture green/red emission (>500nm). A 500nm long pass filter perfectly blocks the reflected excitation light while passing the signal.
Choose: Short Pass Filter
CMOS and CCD sensors are highly sensitive to near-infrared (700-1100nm), which causes thermal noise and color shifts. A 700nm short wave pass filter (also called an IR-cut filter) blocks IR while passing visible light.
Choose: Bandpass Filter
If your system uses a 1064nm laser and you want to block ambient light and other laser harmonics, a 1064nm bandpass filter with 3nm bandwidth is the only correct choice.
| Your Primary Goal | Best Filter Type |
| Block blue/UV, pass red/IR | Long Pass |
| Block IR, pass visible | Short Pass |
| Isolate one specific wavelength | Bandpass |
| Split excitation from emission | Long Pass |
| Clean up laser output | Bandpass |
| Remove ambient light interference | Bandpass |
Bandpass filters have limited transmission range. If your signal spans 100nm, a 10nm bandpass will block >90% of your signal.
A long pass filter will transmit the laser wavelength but also any longer-wavelength stray light (e.g., thermal emission). Use a bandpass filter instead.
For interference-based filters (all three types), increasing the incidence angle shifts the spectral curve toward shorter wavelengths. This is especially critical for long pass and short pass filters in converging beams.
Now that you understand the differences, you need a reliable supplier.
At Bena Optics, we manufacture custom and standard:
Long pass filters (UV, visible, NIR, SWIR)
Short pass filters (including IR-cut for machine vision)
Bandpass filters (narrowband, broadband, laser-line)
All filters feature:
Blocking depth: OD4 to OD6 (custom OD8 available)
Transmission: >90% in passband (anti-reflection coated)
Substrates: Fused silica, B270, N-BK7, colored glass
Customization: Any cut-on/cut-off wavelength, any diameter, AR coating on request
