UV/IR Cut-Off Filters
UV/IR Cut-Off Filters in Machine Vision: A Practical Guide
What Is a UV/IR Cut-Off Filter?
A UV/IR cut-off filter (also called a daylight filter or hot mirror) is an optical filter that blocks both ultraviolet and infrared wavelengths while allowing only visible light to pass through. Think of it as a "window" that lets your camera see exactly what the human eye sees – nothing more, nothing less.
UV/IR cut-off filters are defined by two key specifications:
- UV cut-on wavelength – The point where transmission begins, typically around 400–420nm. Everything below this (UV) is blocked.
- IR cut-off wavelength – The point where transmission ends, typically around 650–700nm. Everything above this (IR) is blocked.
The result is a clean passband covering the visible spectrum – roughly 400nm to 700nm – with both ends of the spectrum sharply rejected. A well-designed UV/IR cut-off filter has steep transition edges on both sides, meaning it cuts cleanly from blocking to passing (and back to blocking) without gradual roll-off or spectral leakage.
How Is a UV/IR Cut-Off Filter Different from an IR Cut-Off Filter?
The terms are sometimes used interchangeably, but there's an important distinction:
- IR cut-off filter – Blocks infrared wavelengths but may allow some UV through, depending on the glass type and coating design.
- UV/IR cut-off filter – Blocks both ultraviolet and infrared, passing only the visible spectrum. This provides complete spectral isolation matching human vision.
For most machine vision applications, a UV/IR cut-off filter is the more complete solution. It eliminates potential interference from both ends of the spectrum, ensuring your camera responds only to visible light.
Why Use a UV/IR Cut-Off Filter in Machine Vision?
Machine vision cameras are sensitive to a much broader range of wavelengths than the human eye – typically from around 350nm (UV) to 1000nm or beyond (near-IR). This extended sensitivity can cause unexpected problems in your imaging. A UV/IR cut-off filter brings your camera's response back in line with human vision.
Here's why this matters:
Restore true-to-life color accuracy
Without proper filtering, UV and IR light contaminate your color channels. Infrared sneaks into the red channel, causing washed-out reds and magenta shifts. UV can affect the blue channel, creating subtle color casts. A UV/IR cut-off filter blocks both contaminants, allowing your color camera to render accurate, natural colors that match human perception.
Eliminate focus shift and image softness
UV, visible, and IR light all focus at slightly different points due to chromatic aberration in your lens. When out-of-band wavelengths reach the sensor, the result is subtle blur, halo effects, or reduced sharpness. Blocking both UV and IR ensures all light reaching your sensor focuses on the same plane, producing crisp, sharp images.
Reduce interference from environmental sources
Industrial environments contain many sources of UV and IR radiation: sunlight streaming through windows, incandescent lighting, hot machinery, welding equipment, and warm products on the line. A UV/IR cut-off filter isolates your camera from all of these variables, giving you consistent imaging regardless of environmental conditions.
Match camera response to human visual inspection
If your machine vision system performs quality checks that will later be verified by human inspectors, you want the camera to "see" what humans see. A UV/IR cut-off filter aligns your camera's spectral response with the human eye, reducing discrepancies between automated and manual inspection results.
Improve contrast and feature definition
Materials that look distinct under visible light may appear similar in UV or IR (or vice versa). If you're relying on visible-light contrast for your inspection, out-of-band contamination can muddy those differences. A UV/IR cut-off filter ensures your camera responds only to the visible characteristics you're actually trying to measure.
Simplify your optical system
Rather than managing UV and IR separately with multiple filters, a single UV/IR cut-off filter handles both in one element. This reduces complexity, minimizes potential reflection issues from stacked filters, and simplifies system design.
Common Applications for UV/IR Cut-Off Filters
Color inspection and sorting
Any application where accurate color measurement matters – food grading, cosmetics inspection, textile quality control, print verification, paint matching – benefits from UV/IR cut-off filtering. Without it, your color data is contaminated by invisible wavelengths, leading to inconsistent or incorrect results. With a proper UV/IR cut-off filter, your camera's color response aligns with human perception and industry color standards.
Surface inspection under mixed lighting
Factory floors are rarely optically pristine. Sunlight through skylights, fluorescent overheads, incandescent indicators, warm machinery – all contribute UV and IR that can affect your imaging. A UV/IR cut-off filter isolates your camera from these variables, ensuring consistent results shift after shift, season after season.
Optical character recognition (OCR) and print inspection
Reading printed text, verifying labels, or checking package graphics requires crisp edges and accurate color. UV and IR contamination softens edges and shifts colors, making reliable OCR more difficult. A UV/IR cut-off filter keeps your text sharp and your colors true.
Medical and pharmaceutical inspection
Color accuracy is critical when inspecting pills, capsules, liquids, or biological samples. A shifted color balance could cause a system to misidentify a product or miss a contamination event. UV/IR cut-off filters ensure the camera sees what a trained human inspector would see.
Food and beverage quality control
From checking the ripeness of fruit to verifying the color of baked goods or beverages, food inspection relies heavily on accurate color. UV from fluorescent lighting and IR from warm production environments or products can interfere. A UV/IR cut-off filter removes both variables.
Cosmetics and personal care
Inspecting lipstick shades, foundation colors, or packaging requires precise color matching. Even subtle shifts caused by UV or IR contamination can result in products being incorrectly graded or rejected. UV/IR filtering ensures your vision system meets the same standards as your human quality team.
Semiconductor and electronics inspection
Accurate imaging is essential for detecting defects, verifying component placement, and reading markings on PCBs and components. A UV/IR cut-off filter ensures stable, accurate imaging regardless of environmental light sources.
Automotive and transportation
Outdoor imaging for traffic monitoring, license plate recognition, or vehicle inspection faces constantly changing lighting conditions. A UV/IR cut-off filter helps maintain consistent color and contrast despite variations in sunlight, weather, and time of day.
Packaging and label verification
Verifying that packaging colors, graphics, and text match specifications requires accurate color rendition. UV/IR cut-off filters ensure your inspection system responds to the same visible wavelengths that consumers will perceive on the shelf.
General-purpose machine vision with color cameras
Even if your application isn't explicitly about color, using a UV/IR cut-off filter with a color camera is good practice. It ensures predictable behavior, reduces variables, and gives you images that accurately represent the scene as humans perceive it.
How to Choose the Right UV/IR Cut-Off Filter
Step 1: Confirm your camera's spectral sensitivity
Check your camera's datasheet for its spectral response curve. Most machine vision sensors are sensitive from around 350nm (UV) to 900nm or beyond (near-IR). If your camera has built-in filtering, verify what it covers – many industrial cameras ship without filters or with only partial filtering, expecting you to add appropriate optical elements.
Step 2: Assess your lighting environment
Consider all sources of light in your imaging area – not just your designed illumination. Is there sunlight? Fluorescent fixtures (which emit some UV)? Incandescent bulbs (which emit significant IR)? Hot machinery nearby? The more out-of-band sources present, the more important a quality UV/IR cut-off filter becomes.
Step 3: Check the cut-on and cut-off wavelengths
A typical UV/IR cut-off filter passes roughly 400–700nm. Look at the specific cut-on (UV side) and cut-off (IR side) wavelengths to ensure they match your needs. If you're working with deep blue illumination around 400nm, make sure the filter doesn't attenuate it.
Step 4: Evaluate the transition slopes
Steep transition edges on both the UV and IR sides mean the filter cuts cleanly between blocked and passed wavelengths. This is important for accurate color rendition – gradual slopes allow spectral leakage that can affect your imaging. Look for filters with well-defined edge specifications.
Step 5: Check transmission efficiency
A good UV/IR cut-off filter should have high transmission (90%+) across the visible spectrum. Low transmission means less light reaches your sensor, requiring longer exposures or brighter illumination. Review the filter's transmission curve to ensure it's efficient where you need it.
Step 6: Consider blocking performance (optical density)
How well does the filter block UV and IR? For most machine vision applications, moderate blocking is sufficient. For demanding applications where even tiny amounts of out-of-band light could affect results, look for higher optical density (OD 2.0 or greater) in the blocking regions.
Step 7: Look for anti-reflection coatings
Quality UV/IR cut-off filters include AR coatings to reduce surface reflections that can cause ghosting or flare. Multi-coated filters offer better performance than uncoated or single-coated options.
Step 8: Select the right size and mount
UV/IR cut-off filters are available in threaded mounts (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. Some cameras also accept internal filter holders in the C-mount or CS-mount back-focus area.
A Few Practical Tips
- Don't assume your camera has built-in filtering. Many industrial and machine vision cameras ship without UV/IR filters – it's left to the integrator to add filtration appropriate to the application. Check your camera's documentation or test empirically: point the camera at a TV remote (IR source) or a UV flashlight to see if it responds to invisible wavelengths.
- Beware of cheap filters with soft edges. Low-quality UV/IR cut-off filters may have gradual transition slopes that allow UV or IR bleed-through. For critical color applications, invest in a filter with sharp, well-defined edges on both sides.
- Consider filter placement. UV/IR cut-off filters can be placed in front of the lens, behind the lens (in the back-focus area), or in a filter holder. Each position has trade-offs for image quality and convenience. Front-of-lens mounting is simplest but may be more susceptible to reflections.
- Watch for thermal effects. Some filters shift their spectral characteristics slightly at extreme temperatures. If you're operating in a high-heat environment, ask about thermal stability.
- Test under real conditions. The best way to confirm a UV/IR cut-off filter is working as expected is to test your system under actual production lighting conditions. Compare images with and without the filter to see the difference in color accuracy, sharpness, and contrast.
- One filter is often enough. Unlike setups that require stacking multiple filters, a UV/IR cut-off filter handles both ends of the spectrum in a single element. This reduces complexity, weight, and potential image quality issues from multiple glass surfaces.
UV/IR Cut-Off vs. Other Filter Types: A Quick Comparison
UV/IR cut-off filter vs. IR cut-off filter
An IR cut-off filter blocks infrared but may allow UV through, depending on the glass and coating. A UV/IR cut-off filter blocks both, providing more complete spectral isolation. For most color imaging applications, UV/IR cut-off is the safer, more comprehensive choice. Choose IR-only if you specifically need UV sensitivity for your application.
UV/IR cut-off filter vs. bandpass filter
A bandpass filter passes only a narrow range of wavelengths (e.g., 530nm ± 20nm), blocking everything else – including most of the visible spectrum. A UV/IR cut-off filter passes the entire visible range while blocking only UV and IR. Choose a bandpass when you need tight wavelength control for a specific LED or laser application. Choose UV/IR cut-off when you want natural, full-color imaging.
UV/IR cut-off filter vs. longpass filter
A longpass filter blocks short wavelengths (including visible blue and green) while passing long wavelengths (red and IR). A UV/IR cut-off filter does the opposite on the IR side – it blocks IR while passing visible light. Choose a longpass when you want to capture IR or eliminate short wavelengths. Choose UV/IR cut-off when you want accurate visible-light-only imaging.
When to Use a UV/IR Cut-Off Filter vs. Other Options
Choose a UV/IR cut-off filter when:
- You need accurate color imaging that matches human vision
- Your environment contains both UV and IR sources (sunlight, mixed lighting, hot machinery)
- You want a single-filter solution that handles both ends of the spectrum
- Consistency and repeatability across varying conditions are priorities
- Your inspection results will be compared against human visual judgment
Choose an IR cut-off filter when:
- UV contamination isn't a concern in your environment
- You have purely visible LED lighting with no UV component
- Cost is a primary factor and IR-only blocking is sufficient
Choose a bandpass filter when:
- You're using a single-wavelength LED or laser and want maximum rejection of everything else
- You need tight spectral control for a specific application
- Color accuracy across the full visible spectrum isn't required
Choose a longpass filter when:
- You want to capture infrared while blocking visible light
- You're doing IR imaging or thermal-related inspection
- You need to eliminate short wavelengths (blue/green) for contrast enhancement
Bringing It Together
A UV/IR cut-off filter is one of the most valuable additions you can make to a color machine vision system. For a single optical element, you gain accurate color rendition, sharp focus, and consistent imaging – free from interference at both ends of the spectrum.
The key is matching your filter to your camera and application. A quality UV/IR cut-off filter, paired with controlled visible LED lighting, forms the foundation of reliable, repeatable color imaging. It's a simple solution that eliminates an entire category of potential problems.
Need help selecting the right UV/IR cut-off filter for your application? [Explore our optical filter range →https://www.kupooptics.com/en/collections/uvir-cut-off-filters] or contact us for application support.
