Boost Robotic Vision: A Guide to Bandpass, IR-Cut & Longpass Filters

Boost Robotic Vision: A Guide to Bandpass, IR-Cut & Longpass Filters

In robotic sensing, clarity is everything. To perform tasks with precision, your robots need to "see" perfectly every time. But the real world is full of visual noise—ambient light, reflections from shiny parts, and mixed light sources can easily confuse a sensor.

The solution is simpler than you think: Robotic Sensing Optical Filters.

These specialized filters from KUPO Optics give your machine-vision cameras superhuman sight by controlling exactly what light they see. The result is better signal-to-noise, more repeatable results, and easier system integration.

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Why Do Filters Make Such a Big Difference?

Optical filters are the fastest way to solve common machine vision headaches. By isolating the right wavelengths of light and blocking the rest, they deliver immediate performance gains.

• Stable, High-Contrast Images: Maintain consistent image quality regardless of changing ambient light, exposure shifts, or production line variance.
• Fewer False Triggers: Eliminate errors caused by stray infrared (IR) light and distracting reflections.
• Higher Throughput: Achieve shorter exposure times, enabling faster inspection speeds and increasing line efficiency.
• Simplified Setup: Spend less time on trial-and-error adjustments to camera gain, shutter speed, and image processing software.

Ultimately, this means higher first-pass yield and fewer unnecessary stops on your production line.

Understanding the Three Main Filter Types

Choosing the right filter depends on your application and lighting. Here’s a simple breakdown of what each type does.

1. Bandpass Filters — For Pinpoint Precision

Use a bandpass filter when you control the illumination (like with a specific color LED or a laser) and want the camera to see only that light. It creates a powerful signal by ignoring all other visual noise.

• What it does: It isolates a narrow "band" of light around a specific wavelength (e.g., 660 nm for red light, or 850/940 nm for near-infrared) and blocks everything else.
• Key Performance:
  • Peak Transmission: Excellent light throughput, typically ~90–95% at the target wavelength.
  • Blocking Power (OD): Powerful blocking of unwanted light, with an Optical Density (OD) of 4 or higher (meaning it blocks over 99.99% of ambient light).
  • Edge Tolerance: Precise cutoffs, usually within ±5–10 nm of the specified wavelength.
• Best for: Presence/absence detection, barcode reading, print inspection, and inspecting shiny or metallic parts where glare is an issue.

2. IR-Cut (Visible-Pass) Filters — For True-to-Life Color

Use an IR-cut filter when you need accurate color and sharp images, especially with modern CMOS/CCD sensors that are sensitive to near-infrared (NIR) light.

• What it does: It allows visible light to pass through while completely blocking NIR light. This prevents the "color contamination" and focus shift that invisible IR light can cause.
• Key Performance:
  • Peak Transmission: High transmission of ~90% across the visible spectrum for bright, clear images.
  • Blocking Power (OD): Strong blocking of OD 3+ in the near-infrared range (typically from 750 nm to 1100 nm).
  • Edge Tolerance: A sharp cut-off around 700-750 nm (±10 nm).
• Best for: Robotic guidance cameras, bin picking, and any inspection task using white light where true color and sharp edges are critical.

3. Longpass Filters — For Superior Infrared Sensing

Use a longpass filter when you want to leverage the advantages of NIR light, such as seeing through haze or reducing visible glare.

• What it does: It blocks all visible light and only passes long-wavelength light (infrared) above a certain point (e.g., 780 nm, 830 nm, or 950 nm). This allows the sensor to focus only on NIR signals.
• Key Performance:
  • Peak Transmission: High transmission of ~90% in the desired NIR range.
  • Blocking Power (OD): Strong blocking of OD 3+ for all visible light below the cut-on wavelength.
  • Edge Tolerance: A clean cut-on point, typically within ±10 nm of the specified wavelength.
• Best for: Surveillance and monitoring, 3D/Time-of-Flight (ToF) sensing, and inspecting materials with high glare or marks that are only visible in NIR.

How to Choose the Right Filter Wavelength

Your system's performance hinges on matching the filter to your hardware and environment. Here’s a checklist:

• Match Your Light Source: Start with the primary wavelength of your LED or laser.
• Check the Bandwidth (FWHM): The filter's bandwidth should be wide enough to accommodate slight shifts from your LED but narrow enough to reject ambient light.
• Align with Your Sensor: Check your camera's Quantum Efficiency (QE) curve. For NIR, 850 nm or 940 nm are popular choices due to sensor sensitivity and eye safety.
• Consider Ambient Light: If working near sunlight or fluorescent lights, a filter with stronger blocking (higher OD) and a tighter bandwidth will provide more robust results.
• Factor in Motion: For high-speed lines, a filter with higher transmission (like a 660 nm red bandpass) can enable the shorter exposure times you need.

Tips for Easy Integration

A great filter performs best when it's installed correctly.

• Placement: Front-of-lens mounts are easy, but internal holders inside the lens or camera housing offer better protection from dust and stray light.
• Back-Focus: A filter adds thickness to the optical path. Be prepared for minor refocusing.
• Avoid Vignetting: Ensure the filter's clear aperture is larger than your lens opening to avoid dark corners in your image.
• Stacking: It's best to use a single interference filter with a protective window rather than stacking multiple filters, which can cause reflections and artifacts.
• Cleaning: Handle with care. Use lint-free swabs and appropriate solvents to clean coated surfaces.

Need a Custom Solution? We Can Help.

Every robotic cell is unique. If standard sizes don't fit, KUPO Optics provides custom solutions with practical lead times.

• Custom Shapes and Sizes: From small M12 circles to large rectangular plates.
• Tighter Tolerances: For applications requiring extremely precise wavelength control.
• Durable Coatings: Hard, vacuum-deposited coatings for harsh factory environments.
• Full Documentation: We provide transmission curves, inspection reports, and compliance documents upon request.

The Bottom Line

Optical filters are a simple, powerful tool for improving the performance and reliability of any robotic sensing system.

• Bandpass filters isolate your light source for maximum precision.
• IR-cut filters deliver true color and sharp focus for imaging cameras.
• Longpass filters unlock the power of infrared for challenging materials and environments.

Take the guesswork out of your vision system. Select the right filter, validate it on your line, and watch your performance improve.

Ready to see the difference?

CTA: Request a sample or custom size.

Frequently Asked Questions (FAQ)

1) Bandpass vs. IR-cut vs. Longpass—how do I choose? Use bandpass when you control the light source and need the highest possible signal-to-noise. Use IR-cut with color cameras to ensure accurate colors and sharp focus. Use longpass when you want to use infrared light to reduce glare, see through materials, or work in low light.

2) What does OD 4 blocking actually mean for my image? OD 4 means the filter blocks 99.99% of unwanted light (a 10,000x reduction). In practice, this prevents bright ambient light from washing out your image or reducing contrast, which is crucial for reliable performance.

3) Will a filter make my exposure times longer? Not usually. Bandpass and longpass filters have very high transmission (~90-95%) in their target range, so you often reduce exposure times thanks to the improved contrast. IR-cut filters also have high transmission in the visible range, so changes are minimal.

4) Can I stack filters, like a bandpass and a protective window? Yes. Stacking an interference filter (like a bandpass) with a simple protective window is a great way to protect the coating. Avoid stacking two interference filters together, as this can create unwanted reflections and alter performance.

5) What sizes and threads are available? We stock common machine vision sizes like M25.5, M27, and 1-inch mounts. We also support M12 lenses and can create custom sizes and shapes for nearly any application.

6) How do anti-reflection (AR) coatings help? AR coatings minimize ghosting and glare caused by light reflecting off the filter's surface. This increases contrast and image clarity. Be sure to use an AR coating optimized for your wavelength (Visible or NIR).

7) What are the lead times for custom filters? Lead times depend on the complexity of the size, coating, and testing requirements. We provide a clear estimate during the quoting process and can often expedite small batches.