Bandpass Filters and IR-Cut Filters for Machine Vision: CWL, FWHM, and Day/Night Architectures
IR-cut, bandpass, dual-bandpass, and no-filter architectures for machine vision cameras. This guide covers when each applies, how CWL, FWHM, and OD blocking work, and how mechanical switching fits in.
Four filter architectures cover every machine vision imaging mode. An IR-cut filter blocks NIR above roughly 650nm to protect visible color. A bandpass filter passes one narrow band, such as 850nm ±15nm (30nm FWHM), to isolate an active illumination wavelength from ambient. A dual-bandpass filter passes visible plus one NIR band at once, for passive day/night. No filter passes everything the sensor detects, correct for NIR-only and RGBIR systems.
Which Filter Does Your System Need?
| Architecture | Passes | Best use case | Main tradeoff |
|---|---|---|---|
| IR-cut filter | Visible only, blocks NIR above ~650nm | Color inspection, label reading, any daylight-only system | Attenuates NIR to the filter's OD (optical density) blocking level, so it is effectively incompatible with IR illumination |
| Bandpass filter | One NIR band, e.g. 850nm ±15nm (30nm FWHM) | Active NIR illumination, barcode reading, presence detection | Blocks visible light, so no color imaging is possible |
| Dual-bandpass filter | Visible + one NIR band, simultaneously | Passive day/night with no moving parts | Some NIR bleeds into daytime color; some ambient bleeds into NIR mode |
| No filter | Full sensor spectral range (~400-1000nm) | Pure NIR inspection, RGBIR sensors, multispectral systems | Daylight color severely degraded if any visible imaging occurs |
| Mechanical IR-cut switcher | Visible in day mode, full NIR in night mode | True day/night where daytime color accuracy is a spec requirement | Moving parts, drive electronics, finite switching lifetime |
Each architecture is covered below, then written into the Commonlands lens part number. For the illumination-wavelength decision behind it, see NIR imaging machine vision. A filter is one part of the optical stack, not a fix for poor focus, working distance, motion blur, or glare from the illumination source.
IR-Cut Filters: When to Use One and When to Skip It
An IR-cut filter blocks near-infrared above roughly 650nm before it reaches the sensor. Silicon responds out to about 1100nm, and Bayer RGB dyes do not separate NIR from visible. A photon at 800nm passes red, green, and blue almost equally, so it contaminates every channel. Without it, daylight color shifts red, white balance drifts, and classification contrast drops.
Keep the IR-cut filter, or the 650/660nm IRC lens variant, whenever the system needs accurate RGB under ambient or white light. Typical cases include color inspection, label verification, or any model trained on visible images. Use the NIR (no-filter) variant for systems built on 850nm or 940nm illumination, RGBIR sensing, or multispectral imaging. An IR-cut filter in an NIR path makes the illuminator look dim, a common integration mistake: an NIR source needs the filter to transmit that wavelength, not block it.
A lens that runs without an IR-cut filter but still works in daylight must be IR-corrected, or visible and NIR focus at different distances and the image softens in sun. 650nm is the common cutoff. Designs needing more deep red use 680nm or 700nm and accept more NIR bleed. The filter does not bring the two bands to a common focus, so a day/night system needs the IR-corrected lens as well.
Commonlands fixed filters ship round and square, sized to the sensor: 7mm round, 7mm square, and 10mm round cover most M12 systems. Integrate them in a holder, a switcher, or bonded to the lens via custom filter gluing, the lowest-stack-height option and not field-removable.
Bandpass Filters: CWL, FWHM, and OD Blocking
A bandpass filter passes a defined band and attenuates everything outside it. Three parameters define it: center wavelength, passband width, and out-of-band blocking.
Center Wavelength (CWL)
CWL is the passband midpoint. Match it to the illuminator's peak emission from the LED datasheet, not the marketing name: many "850nm" LEDs peak between 840nm and 870nm with temperature and drive current. Since LEDs emit with a 20-50nm FWHM of their own, the CWL only needs to fall within that band, not sit on the nominal wavelength.
Full Width at Half Maximum (FWHM)
FWHM is the passband width at the 50% transmission points. Narrower rejects ambient more selectively. Wider tolerates CWL mismatch and angle-of-incidence shift but passes more ambient. For outdoor NIR with broadband ambient, 25-40nm FWHM is a practical starting range, narrow enough to reject sunlight and wide enough to capture the LED's own emission spread.
Out-of-Band Blocking (OD Rating)
Optical density measures out-of-band attenuation: OD4 is 10,000×, under 0.01% transmission. OD4 handles most machine vision ambient rejection, including outdoor sunlight at NIR. Weak in-band signal against strong ambient (long working distance, low power, high gain) pushes toward OD5. Check the transmission curve where the sensor stays sensitive far from the passband.
Commonlands bandpass filters use thin-film interference coatings rather than absorption dyes, giving high peak transmission and sharp edges but making performance sensitive to the angle light enters at, covered in the angle-of-incidence section below.
850nm vs 940nm Bandpass Filters
Both wavelengths work with silicon CMOS sensors and LED illumination. The choice comes down to sensor sensitivity, visible glow, and illuminator power. Silicon has several times higher quantum efficiency at 850nm than at 940nm, so an 850nm system is brighter at the same power and exposure and can run at lower drive current.
The advantage is sensor-side: modern 850nm and 940nm emitters have comparable wall-plug efficiency. 850nm costs a faint visible red glow at higher drive, irrelevant in an enclosure but a problem in human-facing installs. 940nm is invisible to nearly all observers and color cameras, at the cost of lower sensor QE. It needs more power, exposure, or gain to compensate. For the CBP940, Commonlands specifies T>90% at 940nm. Neither wavelength is universally better.
Dual-Bandpass Day/Night Filters
A dual-bandpass filter passes two windows through one fixed element: the visible RGB range, roughly 400-680nm, and a narrow NIR band at 850nm or 940nm, blocking the 700-800nm region between.
The daytime tradeoff is that the NIR passband never closes, so ambient NIR at 850nm bleeds in and shifts color in a way post-processing cannot fully undo. Commonlands offers two levels. The CDB850 at greater than 90% at 850nm favors night exposure margin, while the CDB851 at about 60% favors daytime color, losing roughly a third of the night NIR throughput. The CDB941 covers 940nm for covert installs. Validate both under real outdoor daylight, not studio light.
A passive dual-bandpass fits when moving parts are unacceptable or there is no room for a switcher, as on outdoor perimeter cameras and robotics platforms. Pair it with an IR-corrected M12 lens to hold focus across both bands. A standard lens shifts focus between them. When daytime color accuracy is a hard specification, use the switcher covered next.
IR-Cut Filter Switchers: The Active Day/Night Architecture
The dummy window matters because glass in a converging beam shifts the focal plane. Leave the slot empty and the image goes soft. The CLA214-ICR matches a 0.21mm IRC filter with 0.21mm of dummy glass to hold focus in both modes. Some designs offset it on purpose: the CLA216-ICR pairs a 0.30mm IRC filter with 0.21mm dummy glass, and the 0.09mm difference sharpens NIR focus in night mode on a lens that is not IR-corrected.
A plain dummy window opens the sensor to the whole 700-1000nm range at night, ambient NIR included. Where that costs contrast, a hybrid switcher puts an 850nm bandpass in the second position: the CLA216-ICR-850BP carries a 650nm IR-cut on one side and an 850nm bandpass on the other, earning its keep against competing NIR sources such as other illuminators or solar background at dusk. Confirm drive, cycle life, and settling time on the datasheet, and debounce the day/night trigger so it does not burn switching cycles on every ambient flicker.
IR Filter vs No Filter: The M12A650 / M12ANIR Convention
The filter is the last field of a Commonlands lens part number. In CIL034-F2.3-M12A***, the *** names the filter: 650 for a 650nm IR-cut filter bonded into the slot, NIR for none. So CIL034-F2.3-M12A650 is the daylight color part and CIL034-F2.3-M12ANIR is the NIR part. Some lines cut at 660nm instead. The optics are identical, so the bonded-filter choice is made once at order time.
Pick the IRC variant for RGB color under ambient or white light, and the NIR variant for systems built around 850nm or 940nm illumination or an RGBIR sensor. Neither handles a camera that must do both at full quality: pair a NIR-variant lens with a dual-bandpass filter such as the CDB851, or an active switcher such as the CLA216-ICR-850BP. Confirm the exact designation on the product page, since P/N conventions vary slightly by lens family.
Angle-of-Incidence Shift in Compact Optics
Interference bandpass and dual-bandpass filters hold their spec only near normal incidence. Off-axis, the passband shifts to shorter wavelengths, a few nanometers at 10° angle of incidence and enough at 30° to miss the illumination peak. A long lens keeps chief rays near normal. A wide-angle M12 lens can hit 20-30° at the corners, so a narrow-FWHM passband walks off the illuminator peak and the corners darken while the center holds.
The tell is wavelength dependence: ordinary vignetting stays constant as illumination wavelength changes, while an AOI cast tracks the passband. Before blaming the lens, swap in a wider-FWHM filter at the same CWL. If corner falloff improves, AOI shift was the cause. This applies to the bandpass side of the Commonlands hybrid switcher too. For how sensor format relates to chief ray angle, see sensor size and lens compatibility and the angle of view calculator.
Top Machine Vision Filters by Use Case
Commonlands stocks the single-band CBP850 and CBP940 for active NIR, the CDB850 dual-bandpass for passive day/night, the CSP650 IR-cut for daylight color, and the CLA214-ICR switcher for day/night that must meet a color spec.
| # | Filter | Spectral spec | Primary use case | Link |
|---|---|---|---|---|
| 1 | CBP850 single bandpass | CWL 850nm interference bandpass | Active 850nm illumination; highest silicon QE of the NIR options | CBP850 850nm bandpass filter |
| 2 | CBP940 single bandpass | CWL 940nm, T>90% | Covert illumination with no visible red glow | CBP940 940nm bandpass filter |
| 3 | CDB850 dual-bandpass | Visible + 850nm, T>90% NIR, blocks 700-800nm | Passive day/night with no moving parts | CDB850 dual-bandpass filter |
| 4 | CSP650 IR-cut | Shortpass, blocks NIR above ~650nm | Daylight color accuracy on NIR-sensitive CMOS sensors | CSP650 650nm IR-cut filter |
| 5 | CLA214-ICR / CLA216-ICR switcher | 650nm IR-cut swapped for matched dummy glass: 0.21/0.21mm (CLA214), 0.30/0.21mm compensated (CLA216) | Active day/night where daytime color is a specification | CLA214-ICR IR-cut filter switcher |
| 6 | MidOpt (Midwest Optical) | Broad off-the-shelf CWL catalog, visible and NIR | Threaded C-mount thread-in filters across many wavelengths; typically higher per-unit cost than bare filter glass | midopt.com |
Frequently Asked Questions
What is a bandpass filter in machine vision?
A bandpass filter passes a defined wavelength band, such as 850nm plus or minus 15nm (a 30nm full width at half maximum), and blocks wavelengths outside that band. Placed between the scene and the sensor, it isolates the illumination wavelength a system uses. It also rejects ambient light, other illuminators, and off-wavelength interference. That improves contrast and signal-to-noise ratio.
What does an IR cut filter do in a machine vision camera?
An IR cut filter is a shortpass filter that blocks near-infrared light, typically above 650nm, before it reaches the image sensor. CMOS sensors remain sensitive to NIR, and Bayer dyes pass NIR into all three color channels. Without blocking, NIR contaminates all three channels and causes washed-out color and incorrect white balance in daylight imaging.
What is a dual-bandpass filter in machine vision?
A dual-bandpass filter passes two spectral bands through one fixed element: the visible RGB range and a narrow NIR band such as 850nm, blocking the 700-800nm region between them. This lets a camera produce color images by day and NIR-illuminated images at night from the same static filter, with no mechanical switching component.
What is an IR cut filter switcher in machine vision?
An IR cut filter switcher is a small electromechanical assembly that moves an IR cut filter in and out of the optical path. In day mode the filter blocks NIR for accurate color. In night mode it is removed, replaced by a glass-matched dummy window, so the sensor regains full NIR sensitivity for illuminated night imaging.
What does M12A650 mean compared with M12ANIR?
M12A650 and M12ANIR are Commonlands lens variant designators for the filter installed in the M12 filter slot. M12A650 has a 650nm IR-cut filter glued in place, for daylight color imaging. M12ANIR has no filter. It passes the full spectral range the sensor detects, for NIR imaging, RGBIR sensors, and day/night operation with IR illumination. The lens optics are identical between variants. Only the filter installation differs, and it is not field-removable. Confirm the current designation on the product page, since P/N conventions can vary slightly by lens family.
Need Help Choosing a Filter for Your Build?
Describe the illumination wavelength, sensor format, ambient conditions, and whether daytime color accuracy is a hard requirement. Commonlands engineering can help you choose between IR-cut, bandpass, dual-bandpass, and switcher architectures.