Bandpass and IR-Cut Filters for Machine Vision
Choose the wavelengths the camera needs, then check transmission, blocking, and the angles at which light reaches the filter.
A bandpass filter transmits a selected wavelength band and attenuates light outside it. An infrared-cut filter reduces near-infrared light to help a color camera record visible colors. Choose the filter using the illumination spectrum and sensor response. It can reject unwanted wavelengths, but still passes background light within its transmission band.
Choose the Wavelengths You Need
A filter changes which wavelengths reach the sensor. Start with the useful illumination and the ambient light you need to reject. Bandpass filters can operate in ultraviolet, visible, or infrared bands; they are not limited to night imaging.
| Filter Choice | Purpose | Main Check |
|---|---|---|
| IR-cut | Reduce near-infrared contamination in visible-color images. | Visible transmission and blocking over the sensor's NIR response. |
| Single bandpass | Isolate a selected illumination band. | Useful signal and remaining in-band background. |
| Dual bandpass | Pass two separate wavelength ranges. | Whether unwanted light in either band affects the task. |
| No added filter | Keep the spectrum allowed by the rest of the optical stack. | Existing sensor filters, ambient light, and color requirements. |
A mechanical switcher changes which filter occupies the light path. It is an alternative to using one fixed filter in every mode.
IR-Cut Filters and Daylight Color
Silicon sensors can respond to near-infrared (NIR) light as well as visible light. The red, green, and blue filters on a color sensor may also transmit NIR, in amounts that vary with wavelength and sensor design. That extra signal can shift color under daylight.
An IR-cut filter reduces this contribution. Check its transmission curve instead of treating 650nm as a universal boundary: cutoff and blocking depend on the design. For NIR illumination, the complete optical stack must transmit the chosen band.
Removing an IR-cut filter can change focus as well as color. A lens corrected across visible and NIR wavelengths can help both bands share a focus setting; refocusing is another option. Optical correction does not separate NIR from the visible color channels.
Read CWL, FWHM, and Blocking
Center wavelength (CWL) locates the passband. It is commonly defined as the midpoint between the half-maximum crossings. Match the full filter curve to the illuminator's spectrum over temperature and drive current, rather than matching nominal wavelength labels alone.
Full width at half maximum (FWHM) is the width between the wavelengths where transmission reaches half its peak. If peak transmission is 80%, those crossings are at 40% transmission, not 50%. Newport's filter definitions explain the convention.
A narrower band can reject more broadband background, but may also reject useful light if the source or filter shifts. Choose bandwidth from the required spectral overlap, including the sensor's wavelength-dependent sensitivity. There is no universal bandwidth for outdoor machine vision.
Blocking must cover the wavelengths where unwanted light and sensor sensitivity overlap. An OD≥4 specification means transmission is at most 0.01% over its stated blocking range. It says nothing about leakage outside that range. See Edmund Optics' filter guide for the OD relation and filter types.
Compare 850nm and 940nm as Complete Systems
Compare the selected sensor's sensitivity, the LED's emitted power, and filter transmission at each wavelength. Those measurements determine the useful signal; a wavelength label alone does not establish the required power, exposure, or gain.
940nm emitters often reduce visible red glow compared with 850nm sources, but “invisible” is too broad a guarantee. For example, the ams OSRAM SFH 4726CS A01 datasheet describes reduced red glow. Test the actual emitter and ambient conditions.
Dual-Bandpass Day and Night Imaging
A dual-bandpass filter transmits two separate bands. Day/night versions often pass visible light and a selected NIR band. The filter stays fixed, but the night scene still needs enough light in a band the camera can detect.
The NIR passband remains open during daylight, so sunlight can affect color. The error depends on the scene, filter, sensor, and processing. An RGBIR sensor separates visible and infrared samples to help manage this; it does not make color accuracy automatic.
Test daylight color and night sensitivity separately. If daytime color requires stronger NIR rejection, consider switching filters between modes.
Switch Filters and Check Focus in Both States
A day/night switcher can place an IR-cut filter in the path for visible imaging and replace it with clear glass or an NIR bandpass filter for night use. A bandpass option rejects out-of-band background while still passing background within the illumination band.
Glass in a converging beam changes the focus position. Thickness and refractive index both matter, so equal glass thickness does not guarantee equal focus. Different thicknesses can be used as part of a compensation design, but verify the intended behavior with the lens supplier.
Check focus, image quality, drive requirements, and settling time in both states. The switcher collection provides mechanical options; the assembled camera determines the optical result.
Check the Filter Installed in the Lens
Confirm the installed filter in the selected lens variant and drawing. A no-filter lens may still sit in front of a sensor with its own IR-cut stack. List every cover glass, filter, and window between the scene and the sensor.
For a bonded filter, confirm assembly and replacement requirements before ordering. Include its thickness and clear aperture when checking focus travel and mechanical clearance.
Check Incidence Angle and the Full Ray Cone
An interference filter's spectrum changes when light arrives at a different angle. Increasing angle from normal generally shifts spectral features toward shorter wavelengths and can separate the responses to two polarizations. The shift depends on the coating design, as Semrock explains.
Use the filter's specified operating angle, which need not be zero. Each image point sends a cone of rays through the filter, so the center ray alone is insufficient. Semrock's modeling tool accounts for incidence angle, cone half-angle, and polarization.
Compare center and corner signal using the actual source and aperture. A wider filter that improves the corners suggests a spectral-overlap issue, but does not prove angular shift is the only cause. Check lens illumination and sensor response too.
Specify the Filter for the Camera
Send the source spectrum, sensor response, ambient conditions, and required color or detection accuracy. Include the filter position, ray angles, clear aperture, and thickness limits. Request transmission and blocking data over the complete relevant wavelength range.
Compare samples from the optical-filter collection in the final camera. Record useful signal, background, focus, and corner behavior at the operating temperatures.
자주 묻는 질문
What does a bandpass filter do?
It transmits a selected wavelength band and attenuates light outside it. The band can be ultraviolet, visible, or infrared. Background light within the passband still reaches the sensor.
What does an IR-cut filter do?
It reduces near-infrared light before it reaches the sensor, helping visible-color cameras avoid NIR contamination. The cutoff and blocking range are design-specific. Check the full transmission curve against the sensor response.
Is FWHM measured at 50% transmission?
FWHM uses half of the peak transmission. For an 80% peak, measure the width at 40% transmission. Some specifications use other edge definitions, so check the supplier’s convention.
What does OD4 mean?
OD4 corresponds to 0.01% transmission. A requirement of OD≥4 means no more than 0.01% transmission over the specified blocking band. It does not specify blocking at every wavelength.
Can a fixed dual-bandpass filter replace a switcher?
It can serve both modes when the remaining daylight NIR and night background meet the application limits. A switcher allows different filtering in each mode. Test color, sensitivity, and focus before choosing.
References and Further Reading
- Newport, Bandpass Filter Transmission Definitions. CWL and FWHM relative to peak transmission.
- Edmund Optics, Optical Filters. Filter classes and the relation between optical density and transmission.
- Semrock/IDEX, What Is Angle Tuning?. Design-dependent wavelength shift and polarization effects.
- Semrock/IDEX, MyLight Tutorial. Modeling incidence angle, cone angle, and polarization.
- ams OSRAM, SFH 4726CS A01 Datasheet. A 940nm emitter example with reduced red glow.
- J. Burge, University of Arizona, First Order Optics. Pages 6–7: focus shift and aberrations from a plane-parallel plate.
- H. Angus Macleod, Thin-Film Optical Filters, 5th ed. (2017). Further reading: bandpass filters, tilted coatings, and filter specifications. The link is the publisher’s contents listing.
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 compare filter options.