Broadband Notch Filters
Broadband Notch Filters in Machine Vision: A Practical Guide
What Is a Broadband Notch Filter?
A broadband notch filter (also called a band-stop filter or band-rejection filter) is an optical filter that blocks a specific range of wavelengths while allowing everything else – both shorter and longer wavelengths – to pass through. Think of it as the opposite of a bandpass filter: instead of passing a narrow window, it removes a narrow window from the spectrum.
Broadband notch filters are defined by two key specifications:
- Center wavelength – The midpoint of the blocked wavelength range. For example, a notch filter centered at 530nm is designed to block green light around that wavelength.
- Notch width (bandwidth) – The range of wavelengths being blocked, often specified as the full width at half maximum (FWHM) or the width at a certain optical density. A "broadband" notch typically blocks a wider range (40nm–100nm or more) compared to narrow notch filters.
The result is a transmission curve with a "dip" or "notch" cut out of it – light on either side of the notch passes freely, while light within the notch is rejected. A well-designed broadband notch filter has steep transition edges, meaning it cuts cleanly into and out of the blocked region without gradual roll-off.
How Is a Broadband Notch Filter Different from Other Filters?
Understanding where notch filters fit in the optical filter family helps you choose the right tool:
- Notch filter (band-stop) – Blocks a specific wavelength range, passes everything else on both sides.
- Bandpass filter – Passes a specific wavelength range, blocks everything else on both sides. The exact opposite of a notch filter.
- Longpass filter – Passes everything above a certain wavelength, blocks everything below.
- Shortpass filter – Passes everything below a certain wavelength, blocks everything above.
The key distinction is that a notch filter removes a "slice" from the middle of the spectrum while preserving the rest. This makes it uniquely useful when you need to eliminate a specific problematic wavelength without sacrificing your broader spectral response.
Why Use a Broadband Notch Filter in Machine Vision?
Broadband notch filters solve a specific problem: removing unwanted light at a particular wavelength range while maintaining sensitivity across the rest of the spectrum. Here's why this capability is valuable:
Eliminate specific interference sources
Some environments contain problematic light at specific wavelengths – sodium vapor lighting (around 589nm), mercury vapor lamps, certain colored indicator lights, or specific LED wavelengths. A notch filter lets you surgically remove that interference while preserving your camera's response to everything else.
Remove unwanted fluorescence or reflection peaks
Some materials produce strong fluorescence or reflection at specific wavelengths that can overwhelm the features you're trying to inspect. A notch filter can suppress these peaks while preserving the rest of your spectral information.
Maintain broad spectral sensitivity
Unlike a bandpass filter (which limits you to a narrow window) or a longpass/shortpass filter (which cuts off an entire end of the spectrum), a notch filter preserves sensitivity across most of the spectrum. This is valuable when you need general-purpose imaging but have one specific wavelength causing problems.
Suppress problematic ambient lighting
Industrial facilities often have mixed lighting – daylight, fluorescent, sodium vapor, mercury vapor, and various indicator lights. If one specific type is creating problems for your imaging, a notch filter removes just that source while letting everything else through.
Enable multi-spectral imaging with selective exclusion
In advanced imaging setups where you're capturing information across a wide spectral range, a notch filter lets you exclude a specific band that's causing interference without sacrificing your broader spectral coverage.
Improve color accuracy by removing dominant wavelengths
Some lighting environments are dominated by a narrow wavelength band that skews color perception. A notch filter removes that dominant peak, allowing more balanced color imaging across the remaining spectrum.
Common Applications for Broadband Notch Filters
Blocking sodium vapor lighting
Sodium vapor lamps (common in warehouses, parking areas, and some industrial facilities) emit intense light concentrated around 589nm. This narrow-band orange light can create color accuracy problems or overwhelm other spectral information. A notch filter around 589nm removes the sodium light while preserving response to other wavelengths.
Suppressing mercury vapor lamp interference
Mercury vapor lamps produce strong emission peaks at specific wavelengths. In facilities with this type of lighting, a notch filter can suppress these peaks to improve imaging consistency and color accuracy.
Fluorescence imaging with background suppression
In some fluorescence applications, unwanted background fluorescence or scattered excitation light occurs at specific wavelengths that interfere with your target signal. A notch filter can suppress these specific interference bands while passing both your excitation wavelength (on one side) and your emission wavelength (on the other side).
Selective color suppression
In some inspection tasks, a particular color or wavelength range creates confusion or masks the features you're trying to detect. A notch filter removes that specific color while maintaining full sensitivity to others – more surgical than switching to monochrome or using a bandpass filter.
Display and screen inspection
When inspecting displays, screens, or illuminated panels, certain wavelength peaks from LEDs or backlights may overwhelm defect visibility. A notch filter can suppress these peaks while maintaining overall color assessment capability.
Outdoor and mixed-lighting environments
Light pollution from artificial sources (particularly sodium and mercury vapor lamps) can interfere with imaging in outdoor or semi-outdoor settings. Notch filters targeting these specific emission lines improve contrast and image quality by removing the pollution while preserving natural light.
Process monitoring in industrial environments
Manufacturing floors often have complex lighting environments with multiple sources. When a specific wavelength band creates glare, color shifts, or interference, a notch filter provides targeted suppression without requiring a complete lighting overhaul.
Food and agricultural inspection
Some inspection environments use specific lighting that, while beneficial for human workers, creates problems for machine vision. A notch filter can remove the problematic wavelength while allowing accurate imaging of the product.
Printing and packaging inspection
Certain inks, coatings, or substrates may reflect or fluoresce strongly at specific wavelengths, masking defects or creating false readings. A notch filter suppresses these peaks for more accurate inspection.
How to Choose the Right Broadband Notch Filter
Step 1: Identify the wavelength you need to block
Start by determining exactly which wavelength is causing problems. Is it a specific lighting source with a known emission peak? Measure or look up the spectral characteristics of your interference source. Your notch filter's center wavelength should match this.
Step 2: Determine the required notch width
How wide is the problematic spectral region? A narrow emission line might need only a 20–30nm notch, while a broader interference source might require 50nm or more. Broadband notch filters offer wider rejection bands, which provide more tolerance for wavelength variation but also block more of the spectrum.
Step 3: Consider what you need to pass
Make sure the notch doesn't cut into wavelengths you need for your imaging. If your interference is at 589nm but your application relies heavily on orange-yellow color accuracy, blocking that region will affect your results. Map out your requirements on both sides of the notch.
Step 4: Check the blocking depth (optical density)
How completely does the filter need to block the unwanted wavelength? Optical density (OD) indicates blocking strength – OD 2.0 means 1% transmission, OD 3.0 means 0.1%. For general interference suppression in machine vision, OD 2.0–3.0 is typically sufficient.
Step 5: Evaluate transmission outside the notch
A quality notch filter should have high transmission (90%+) at wavelengths outside the blocked region. Low transmission means lost signal and reduced image quality. Review the filter's full transmission curve to ensure it's efficient where you need it.
Step 6: Check the transition edge steepness
Steep edges mean the filter transitions quickly from passing to blocking (and back). This is important when the wavelengths you need to pass are close to the wavelengths you need to block. Look for specifications indicating edge width or steepness.
Step 7: Consider environmental factors
Will the filter be exposed to high temperatures, humidity, or intense light? Some notch filters (particularly thin-film interference types) can shift their center wavelength with temperature or degrade under harsh conditions. Choose filters rated for your environment.
Step 8: Select the right size and mount
Broadband notch filters come in standard threaded sizes (M25.5, M27, M30.5, M35.5, etc.), drop-in formats, and unmounted glass for custom integration. Match the filter to your lens thread or filter holder.
A Few Practical Tips
- Know your interference source precisely. A notch filter only works if it's centered on the right wavelength. Verify the exact wavelength of your problem source – don't guess based on appearance.
- Consider wavelength drift. Some light sources can drift slightly in wavelength with temperature or age. A slightly wider notch provides margin for this drift, though it also blocks more of the spectrum.
- Beware of multiple interference sources. If you have problems at multiple wavelengths, you may need multiple notch filters (which can be stacked) or a different filtering approach entirely. Each additional filter adds complexity and potential light loss.
- Watch for angle sensitivity. Like most interference filters, notch filters can shift their center wavelength when light hits them at an angle. If you're using wide-angle lenses or placing filters far from the lens, this could affect performance. Check specifications for angle-of-incidence effects.
- Consider dichroic vs. absorptive types. Dichroic notch filters reject unwanted wavelengths by reflecting them, while absorptive types absorb them. Dichroic filters are more common for precision applications, but be aware that reflected light could cause issues if it bounces back into your optical path.
- Verify with testing. Specifications tell you what the filter should do – testing tells you what it actually does in your system. Always verify performance under real conditions.
- Check for out-of-band effects. Some notch filters have secondary features (additional smaller notches or ripples) elsewhere in the spectrum. Review the full transmission curve, not just the notch region.
Broadband Notch Filter vs. Other Filter Types: A Quick Comparison
Broadband notch filter vs. bandpass filter
These are essentially opposites. A bandpass filter passes only a narrow wavelength range and blocks everything else. A notch filter blocks a narrow range and passes everything else. Choose a bandpass when you want to isolate a specific wavelength for imaging. Choose a notch when you want to eliminate a specific wavelength while maintaining broad spectral response.
Broadband notch filter vs. UV/IR cut-off filter
A UV/IR cut-off filter blocks both ends of the spectrum (UV and IR) while passing the visible range. A notch filter blocks a specific band in the middle while passing wavelengths on both sides. Choose UV/IR cut-off for general color imaging free from invisible wavelength contamination. Choose a notch filter when your problem is a specific visible wavelength, not UV or IR.
Broadband notch filter vs. longpass or shortpass filter
Longpass and shortpass filters divide the spectrum at a single cut point – one side passes, the other blocks. A notch filter creates a "hole" in the middle while passing both sides. Choose longpass or shortpass when you want to eliminate an entire region of the spectrum above or below a certain point. Choose a notch filter when you need to remove a specific band while preserving wavelengths on both sides of it.
When to Use a Broadband Notch Filter vs. Other Options
Choose a broadband notch filter when:
- You need to block a specific problematic wavelength while maintaining broad spectral sensitivity
- Specific lighting sources (sodium vapor, mercury vapor, certain LEDs) create narrow-band interference
- You want to suppress a specific fluorescence or reflection peak without losing other spectral information
- Your environment has mixed lighting and only one source is problematic
- You need to improve color accuracy by removing a dominant wavelength peak
Choose a bandpass filter when:
- You want to isolate a specific wavelength for imaging, not eliminate it
- You're matching a single-wavelength LED for illumination
- Maximum rejection of everything outside your target band is needed
Choose a UV/IR cut-off filter when:
- Your problem is UV and/or IR contamination, not a specific visible wavelength
- You need general-purpose color imaging matching human vision
- You want to eliminate invisible wavelengths without affecting the visible spectrum
Choose a longpass or shortpass filter when:
- You want to block an entire end of the spectrum, not just a narrow band in the middle
- Your application requires passing everything above (longpass) or below (shortpass) a certain wavelength
- The problematic wavelengths are all grouped at one end of your spectral range
Bringing It Together
Broadband notch filters are specialized tools for a specific job: surgically removing a problematic wavelength range while preserving the rest of your spectral response. They're not the right choice for every application, but when you need to eliminate sodium lighting interference, suppress a specific reflection peak, or remove a dominant wavelength that's skewing your color imaging, a notch filter is exactly the right solution.
The key is precisely identifying the wavelength you need to block and selecting a filter that matches. With the right notch filter in place, your camera can image freely across most of the spectrum while remaining blind to the specific wavelength causing problems.
Need help selecting the right notch filter for your application? [Explore our optical filter range →https://www.kupooptics.com/en/collections/broadband-notch-filter] or contact us for application support.
