Longpass Filters

Longpass Filters in Machine Vision: A Practical Guide

Longpass Filters

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What Is a Longpass Filter?

A longpass filter is an optical filter that blocks shorter wavelengths and allows longer wavelengths to pass through. Think of it as a "cutoff point" – everything below a certain wavelength gets blocked, while everything above that point transmits freely to your camera sensor.

Longpass filters are defined by their cut-on wavelength – the point at which transmission begins. Common examples include:

• 500nm longpass – Blocks UV and blue light, passes green, yellow, orange, red, and infrared.
• 600nm longpass – Blocks UV, blue, and green, passes yellow, orange, red, and infrared.
• 700nm longpass – Blocks all visible light, passes only deep red and infrared.
• 850nm longpass – Blocks all visible light and near-IR, passes only longer-wave infrared.

The transition from "blocking" to "passing" happens over a slope. A well-designed longpass filter has a steep cut-on edge, meaning it transitions sharply from blocking to transmitting rather than gradually rolling over. This gives you precise control over exactly which wavelengths reach your sensor.

How Is a Longpass Filter Different from a Bandpass Filter?

It's worth clarifying the distinction, since both filter types are common in machine vision:

• Bandpass filter – Passes a specific narrow range of wavelengths (e.g., 530nm ± 20nm) and blocks everything outside that window – both shorter and longer wavelengths.
• Longpass filter – Passes everything above a certain wavelength and blocks everything below. There's no upper limit to what it transmits.

In practice, bandpass filters give you tighter spectral control, while longpass filters give you broader flexibility. The right choice depends on your application, lighting, and what you're trying to see (or not see).

Why Use a Longpass Filter in Machine Vision?

Longpass filters offer unique advantages when you need to eliminate specific wavelengths while preserving a wide range of others. Here's why they're valuable in industrial imaging:

Block unwanted short wavelengths while preserving signal

Sometimes your problem isn't that you need a specific wavelength – it's that you need to get rid of certain wavelengths that are causing trouble. UV and blue light can create haze, scatter, or unwanted fluorescence. A longpass filter removes these troublemakers while allowing the rest of your illumination through.

Enable infrared imaging by blocking visible light

One of the most powerful uses of longpass filters is in IR imaging applications. A 700nm or 850nm longpass filter blocks all visible light, allowing your camera to see only infrared. This is essential when you want to image using IR illumination without interference from ambient visible light.

Isolate fluorescence emission from excitation light

In fluorescence imaging, you excite a material with one wavelength (e.g., UV or blue) and it emits light at a longer wavelength (e.g., green or red). A longpass filter placed in front of your camera blocks the excitation wavelength while passing the emitted fluorescence. This is often simpler and more cost-effective than using a narrow bandpass filter for the emission.

Reduce chromatic aberration effects

Shorter wavelengths (blue, violet, UV) are more prone to chromatic aberration – they focus at slightly different points than longer wavelengths, causing color fringing and softness. By filtering out the short end of the spectrum, a longpass filter can improve overall image sharpness, especially with lower-cost lenses.

Enhance contrast for specific materials

Different materials interact with light differently across the spectrum. Some defects, coatings, or markings may be more visible when you eliminate blue or green wavelengths and image only in the red or IR range. A longpass filter gives you a simple way to shift your spectral response and find the contrast you need.

Work with broadband illumination more selectively

If you're using white light or broadband illumination but want to capture only a portion of the spectrum, a longpass filter offers a simple solution. Rather than changing your entire lighting setup, you add a filter and instantly limit your camera's response to the wavelengths you care about.

Common Applications for Longpass Filters

Infrared imaging and inspection

For applications requiring IR illumination – such as covert inspection, night vision, seeing through certain materials, or detecting moisture and contamination – a longpass filter (typically 700nm, 780nm, or 850nm) blocks all visible light and allows only IR to reach the sensor. This is essential when ambient visible light would otherwise overwhelm your IR signal.

Fluorescence detection and inspection

Many industrial materials fluoresce when exposed to UV or blue light – adhesives, lubricants, optical brighteners, security inks, and biological contaminants all exhibit this property. A longpass filter matched above your excitation wavelength captures the fluorescent emission while rejecting the excitation light. For example, if you're exciting with 365nm UV, a 400nm or 420nm longpass filter passes the visible fluorescence while blocking the UV.

Laser line imaging without narrow filtering

In some laser profiling applications, you may want to capture your laser line (say, a 660nm red laser) while blocking ambient light but don't need the ultra-tight selectivity of a narrow bandpass. A 600nm longpass filter blocks blue, green, and most ambient light while passing your laser line plus some red and IR. This can be a simpler, more cost-effective solution when extreme precision isn't required.

Solar and outdoor imaging

When imaging outdoors or in environments with significant sunlight, short wavelengths (UV and blue) create haze and scatter. A longpass filter cutting at 450nm or 500nm reduces atmospheric haze and improves contrast for distant objects – a principle borrowed from traditional photography but equally valuable in industrial outdoor inspection.

Reducing UV damage and sensor noise

Some sensors exhibit increased noise or degradation when exposed to UV light over time. A longpass filter that cuts UV (e.g., 400nm or 420nm cut-on) protects your sensor while passing the full visible spectrum. This is particularly relevant for cameras operating in environments with UV exposure from sunlight or certain industrial lighting.

Sorting and grading by color range

Rather than isolating a single color (which would call for a bandpass filter), sometimes you want to see "warm" colors (red, orange, yellow) while blocking "cool" colors (blue, green). A longpass filter makes this distinction cleanly – for example, a 550nm longpass passes yellow through red while blocking green and blue, useful for certain agricultural or food sorting tasks.

Seeing through materials

Some materials that are opaque to visible light are transparent or translucent to infrared. Plastics, inks, paper, and certain coatings can be "seen through" using IR imaging with a longpass filter. This enables inspection of contents inside packaging, reading codes beneath coatings, or detecting subsurface features.

Welding and high-temperature process monitoring

Monitoring molten metal, welding arcs, or other high-temperature processes requires blocking the intense visible light while capturing the IR signature. Longpass filters in the 850nm–1000nm range allow thermal imaging cameras or IR-sensitive sensors to operate without being overwhelmed by visible glare.

How to Choose the Right Longpass Filter

Step 1: Define what you want to block

Start by identifying which wavelengths are causing problems or are simply unnecessary for your application. Do you need to eliminate UV? Cut out blue? Block all visible light? Your cut-on wavelength should be just above the highest wavelength you want to reject.

Step 2: Consider what you need to pass

Make sure your chosen cut-on wavelength doesn't accidentally block signal you need. If your illumination or feature of interest includes green light, don't choose a 600nm longpass that would block it. Map out your illumination spectrum and your target's spectral characteristics before selecting.

Step 3: Match to your illumination

If you're using a specific LED or laser wavelength, ensure your longpass filter passes that wavelength efficiently. For example, if you're imaging with 850nm IR LEDs, an 850nm longpass filter would cut right at your illumination wavelength – you'd want something lower, like a 780nm or 800nm cut-on, to ensure full transmission of your light source.

Step 4: Check the cut-on slope

A steep cut-on slope means the filter transitions sharply from blocking to passing. This is important when you need clean separation – for instance, in fluorescence applications where your excitation and emission wavelengths may be relatively close together. Look for filters with clearly specified edge steepness (often given as the wavelength range from 10% to 90% transmission).

Step 5: Evaluate blocking performance (optical density)

How well does the filter block the wavelengths you want to reject? For demanding applications like fluorescence, you need high optical density (OD 3.0 or greater) in the blocking region to ensure no excitation light leaks through. For simpler applications, moderate blocking may be sufficient.

Step 6: Check transmission in the pass region

A good longpass filter should have high transmission (90%+) in the wavelengths it's designed to pass. Low transmission means lost signal, requiring longer exposures or brighter lighting. Review the filter's transmission curve to ensure it's efficient where you need it.

Step 7: Select the right size and mount

Longpass 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. If you're building a custom optical assembly, unmounted filters give you flexibility but require careful handling.

A Few Practical Tips

• Combine with other filters when needed. A longpass filter can be stacked with a shortpass filter to create a bandpass effect – for example, a 500nm longpass combined with a 600nm shortpass creates a 500–600nm bandpass. This gives you flexibility when off-the-shelf bandpass filters don't match your exact needs.
• Watch for IR contamination. Remember that longpass filters pass everything above the cut-on wavelength, including near-IR. If you don't want IR in your image, you may need to add an IR cut-off filter or choose a combined filter solution.
• Consider dichroic vs. absorptive types. Longpass filters come in two main types: dichroic (reflective) filters reject unwanted wavelengths by reflecting them, while absorptive filters soak them up. Dichroic filters are generally more durable and stable, but absorptive filters can be simpler and cheaper. Choose based on your performance needs and budget.
• Beware of filter fluorescence. Some lower-quality glass filters can themselves fluoresce when exposed to UV, adding unwanted signal. For fluorescence applications, choose filters specifically rated as non-fluorescing.
• Test empirically. Filter specifications are helpful, but real-world testing is essential. Image your actual parts under actual lighting conditions with and without the filter to confirm you're getting the results you expect.

When to Use a Longpass vs. a Bandpass Filter

The choice between longpass and bandpass depends on your application:

Choose a longpass filter when:

• You need to block a range of short wavelengths but don't need to limit the long end
• You're doing fluorescence imaging and want to pass all emission wavelengths above the excitation
• You want to capture IR while blocking visible light
• You're working with broadband illumination and want to use only a portion of the spectrum
• Simplicity and cost are priorities, and you don't need narrow spectral control

Choose a bandpass filter when:

• You need to isolate a specific narrow wavelength range
• You're pairing with a single-wavelength LED or laser and want maximum rejection of everything else
• Color accuracy requires excluding both shorter and longer wavelengths outside your target band
• You need precise spectral control for quantitative measurements

In many systems, longpass and bandpass filters are used together – for example, a longpass filter blocking UV and blue on the illumination side, paired with a narrow bandpass filter on the camera for specific wavelength detection.

Bringing It Together

Longpass filters are versatile tools in the machine vision toolkit. Whether you're blocking UV haze, enabling infrared imaging, capturing fluorescence, or simply tailoring your spectral response to match your application, a well-chosen longpass filter gives you control over what your camera sees.

The key is understanding what you want to eliminate and what you need to preserve. Start with your lighting, consider your target's spectral characteristics, and select a cut-on wavelength that cleanly separates the two. From there, a longpass filter is a simple, cost-effective way to optimize your imaging system.

Need help selecting the right longpass filter for your application? [Explore our optical filter range →https://www.kupooptics.com/en/collections/longpass-filters] or contact us for application support.