IR-korrigierte Objektive für die industrielle Bildverarbeitung: Fokusstabilität bei Beleuchtung im sichtbaren und im nahen Infrarotbereich
Most standard lenses defocus under 850nm or 940nm illumination because their correction stops at the visible band. This guide separates chromatic focus shift from the thermal and filter-induced kinds, then shows which Commonlands M12 lenses hold focus across both.
An IR corrected lens extends chromatic focus correction into the near-infrared band, typically 850nm and 940nm, so visible and NIR light share one focal plane within the system's depth of focus. Most standard lenses are corrected across the visible band only, so they defocus when illumination switches to NIR, a wavelength-dependent case of focus shift called longitudinal chromatic aberration.
Removing an IR-cut filter does not fix this. The filter only selects which wavelengths reach the sensor. It does nothing to close the lens's visible-to-NIR focus separation. Simultaneous RGB/NIR capture needs adequate common-focus performance across both bands. A day/night camera can instead refocus between bands or use a filter design that compensates for the remaining focus shift.
Was ist ein IR-korrigiertes Objektiv?
An IR corrected lens is a lens whose optical design extends chromatic focus correction beyond the visible band into the near-infrared, typically holding the visible-to-NIR focus separation from 400nm through 850nm or 940nm inside the system's depth of focus, not driving it to zero. Most standard lenses are corrected across the visible band only and defocus under NIR when they were focused in visible light. Some ordinary designs do hold part of the NIR band. Only measured through-focus data at your wavelengths tells you which you have.
The bench test is direct: focus under visible light, switch to NIR without touching focus, and capture at the production aperture. Sharpness there is evidence, not proof, since stopping down hides a large shift. Measure best-focus separation between the two wavelengths, or through-focus MTF at each.
Correction is a matter of degree. An ordinary multi-element prescription already narrows the visible-to-NIR gap while achromatizing the visible band. Designs that target NIR narrow it further through element count and glass selection, and datasheets specify the wavelength range where focus holds. For the aberration family this addresses, see lens aberrations in machine vision.
Wie das optische Design die IR-Korrektur bewirkt
A single lens element cannot hold visible and NIR light at the same focal plane, because every glass type has its own dispersion curve, the relationship between refractive index and wavelength. IR correction pairs element materials with complementary dispersion so the combined system brings a wider wavelength range to a common focus than any single glass could. It is the achromatic doublet principle, extended to include 850nm and 940nm.
Dispersion curves are nonlinear and glass catalogs are finite, so a designer accepts a residual visible/NIR offset rather than eliminating it. A well-corrected lens holds that offset within the system's depth of focus at the intended aperture and format. Ask for the number rather than the label: maximum focus shift or MTF loss across wavelength, field, aperture, temperature, and conjugate, against the depth of focus you have.
Was ist eine Fokusverschiebung, und warum wird sie durch die Wellenlänge verursacht?
Focus shift is the movement of the plane of best focus after an optical condition changes, even though no one touched the mechanical focus setting. It is distinct from back-focus error, a static lens-to-sensor misalignment set once at assembly.
Three mechanisms move the focal plane, each with its own fix. A fourth condition, aperture, mainly changes how much shift the system tolerates rather than where the plane sits, although residual spherical aberration can shift best focus as the aperture changes. Wavelength is the chromatic case this pillar covers: glass refractive index varies with wavelength, so 850nm or 940nm NIR converges at a different axial distance than visible light. In a non-IR-corrected lens, that separation can be large relative to the sensor's depth of focus.
| Condition that changes | Was bewegt | Typisches Symptom | Praktische Lösung |
|---|---|---|---|
| Wellenlängenverschiebung (VIS → NIR) | Chromatische Trennung der Brennebene | Blur in one illumination mode. Day/night performance gap | IR-korrigiertes Objektiv |
| Filter oder Umschalter eingesetzt | Die optische Weglänge nimmt zu | Defocus when filter is in. Varies between filter states | Compensated filter switcher. Back-focus re-set with filter in place |
| Temperaturanstieg oder -abfall | Thermische Drift der Bildebene | Die Fokussierung verschlechtert sich bei extremen Temperaturen | All-glass lens. Athermalized design (see ruggedized guide) |
| Blendenänderung (Abblenden) | Die Schärfentiefe vergrößert sich | Bei abgeblendeter Blende wirkt das Bild schärfer (das verschiebt das Bild, behebt das Problem aber nicht) | Diagnose bei Produktionsblende |
The longitudinal chromatic aberration behind this focus shift differs from lateral chromatic aberration, which spreads color fringing across the field rather than moving the axial focal plane. IR correction targets the longitudinal component.
Filter-stack shift is a separate, predictable effect
Inserting a plane-parallel glass plate into a converging beam shifts the paraxial focus away from the lens. A sensor cover glass or rear filter can produce this shift. The following approximation assumes near-normal incidence and air on both sides of the plate.
A 340μm shift can exceed the focus tolerance of a fast lens. Compare each filter state using t(1-1/n), rather than optical path length nt. Matching this focus displacement preserves the plate contribution when filters switch. Deliberately different displacements can compensate residual lens chromatic shift. Verify the complete lens, filter, and sensor assembly at both wavelengths.
The Commonlands CLA216-ICR-850BP compensates this: its glass thickness offset between the 650nm IR-cut and 850nm bandpass sides matches the two path lengths, so one focus setting stays valid across both.
Wie sich ein IR-korrigiertes Objektiv im Vergleich zu einem Standardobjektiv und einem entfernten IR-Sperrfilter verhält
These three things are often conflated. A standard lens with an IR-cut filter gives a sharp visible image because the filter stops out-of-focus NIR light from reaching the sensor, not because the lens itself is corrected. Remove that filter and NIR light gets through, but the optics still carry the same chromatic focus shift, so the NIR contribution is now visible and out of focus relative to the visible-light setting.
Commonlands labels the no-filter build NIR-pass (SKU suffix ANIR): the filter slot is empty, so visible and NIR both reach the sensor. This is not a NIR-pass longpass filter: visible light still gets through, and the band that reaches the sensor is set by the glass, coatings, and sensor QE, not by a defined passband.
An IR corrected lens in that NIR-pass build holds focus from visible through NIR in the optics, so no refocus is needed when illumination switches. Commonlands IR-corrected M12 lenses ship in both a 650nm IRC build (daytime visible) and this NIR-pass build (RGBIR and day/night).
| Objektiv- und Filtereinstellung | Optimaler Anwendungsfall | Hauptrisiko |
|---|---|---|
| Standardobjektiv + IR-Sperrfilter | Bildgebung ausschließlich im sichtbaren Bereich, keine NIR-Beleuchtung | IR-cut blocks NIR. Cannot use 850/940nm illumination |
| Standardobjektiv, IR-Sperrfilter entfernt | Nur schmalbandiges NIR, auf diese Wellenlänge neu fokussiert | Visible and NIR focus at different positions. Dual-wavelength use needs refocus |
| IR-korrigiertes Objektiv + 650 nm IRC | Bildgebung bei Tageslicht, zukünftige NIR-Fähigkeit möglich | Für den Dualband-Betrieb muss auf die NIR-Durchlass-Variante umgestellt werden |
| IR-korrigierte Linse, NIR-durchlässig (ohne Filter) | RGBIR-Kameras, Tag-/Nacht-Umschaltung, reine 850/940-nm-Systeme | Eine verbleibende VIS/NIR-Verschiebung kann bei hoher Auflösung dennoch von Bedeutung sein |
Wann ist die IR-Korrektur bei RGBIR- und Tag-/Nacht-Systemen von Bedeutung?
IR correction is not necessary in every application. A visible-only system with an IR-cut filter has no need for it. The cases where it becomes a design requirement share one factor: the system must produce a sharp image at two or more wavelengths from the same focus position.
RGBIR-Sensoren
RGBIR sensors capture RGB and NIR-sensitive pixels through the same lens at one focus distance. A lens with significant chromatic focus shift leaves the NIR channel softer than the RGB channels, most visibly on high-resolution sensors with small pixel pitch.
Where the channels come off one exposure, the common case, there is no chance to refocus between them, so the lens must hold both wavelengths at once. Some sensors and drivers instead use channel-dependent gain, staggered or multiplexed exposures, or separate processing paths, so read the datasheet before assuming.
Tag-/Nachtkameras
A day/night camera uses visible illumination in daylight and 850nm or 940nm LED illumination at night. Without an IR-corrected lens, a fixed-focus camera is sharp in one mode and soft in the other, since it has no motorized refocus on mode change. It can pass a daytime acceptance test, then image softly the moment NIR illumination activates, so validate focus under both modes before deployment.
Decide the IR-correction requirement before choosing a focal length or mount. It turns on one question: must the system stay sharp at two wavelengths from a single focus position? Confirm the destination format with sensor size and lens compatibility, since residual shift bites hardest on small, high-resolution formats.
Die besten IR-korrigierten M12-Objektive für die Tag- und Nacht-Bildverarbeitung
The three lenses below are candidates for visible/NIR systems. Confirm the residual focus shift at your wavelengths, aperture, working distance, and temperature. An unfiltered NIR-pass variant transmits NIR but does not, by itself, establish common visible/NIR focus.
Commonlands ranked these three by how broadly each covers day/night and RGBIR work: focus held across visible and NIR, image circle against common 1/2" and 1/1.8" formats, and a fast fixed F/2.0 aperture for 850/940nm LED scenes. To match a focal length to your sensor, run the lens field of view calculator.
| Rang | Objektiv | EFL | F# | Bildkreis | Am besten geeignet für |
|---|---|---|---|---|---|
| 1 | CIL046 | 4,4 mm | F/2,0 | 8,9 mm | Wide 100° field for RGBIR and day/night sensors up to 8MP |
| 2 | CIL122 | 12 mm | F/2,0 | 9,3 mm | Narrower 43° field for day/night and robotics on 1/1.8" 8-12MP sensors |
| 3 | CIL161 | 15.6mm (nominal 16mm) | F/2,0 | 8,0 mm | Enges 28,8°-Sichtfeld für Tag- und Nachtbetrieb bei kleineren 1/2"-Sensoren mit 3–5 MP |
Order the 650nm IRC variant for daytime visible use, or the NIR-pass variant for RGBIR or day/night sensitivity without refocusing. Where a system physically switches filters, pair an IR-corrected NIR-pass lens with the CLA216-ICR-850BP, whose glass thickness offset compensates the path-length change between filter states.
Commonlands is not the only source: Sunex and Edmund Optics also ship IR-corrected designs, and either can be the better call for a C-mount format these M12 lenses do not cover. Confirm IR correction as a distinct line item on the datasheet regardless of mount, since day/night and security marketing language is applied inconsistently across suppliers.
Eine Validierungs-Checkliste zur Fokusstabilität im sichtbaren und im NIR-Bereich
A datasheet claim of "IR corrected" is a starting point, not a guarantee for a specific system. Pixel pitch, aperture, working distance, and the NIR wavelength all affect whether the residual shift stays acceptable. Confirm a lens holds focus across visible and NIR in the real deployment configuration, not on the bench.
Commonlands provides measured MTF data per lens and can supply depth-of-focus figures on request.
- Stellen Sie den Fokus unter der Hauptlichtquelle ein und notieren Sie die mechanische Fokusposition.
- Wechseln Sie zur sekundären Wellenlänge, ohne den Fokussiermechanismus zu verstellen.
- Nehmen Sie bei beiden Wellenlängen und im Arbeitsabstand eine Auflösungsprüfkarte oder ein Zielobjekt mit feinem Raster auf.
- Messen Sie die Schärfe bzw. die MTF in der Bildmitte und an mindestens zwei Eckpositionen für jede Wellenlänge.
- Führen Sie die Überprüfung bei der tatsächlichen Betriebsblende durch. Führen Sie die Überprüfung nicht bei abgeblendeter Blende durch, um die Verschiebung zu kaschieren, falls in der Produktion mit einer größeren Blendenöffnung gearbeitet wird.
For reading the MTF data this produces, see how to read MTF curves. Keep the recorded focus positions and MTF results with the production documentation, so later changes can be checked against that baseline.
Technical references
Häufig gestellte Fragen
Commonlands stocks IR-corrected M12 lens variants for NIR illumination. These answers explain when that correction is necessary.
Was versteht man in der Bildverarbeitung unter einem IR-korrigierten Objektiv?
An IR corrected lens extends chromatic focus correction into the near-infrared, typically 850nm and 940nm, so visible and NIR light share a common focal plane within the system's depth of focus. Most standard lenses are corrected for visible wavelengths only and defocus under NIR. Measured through-focus data is what settles it for a given lens. IR correction is a property of the lens optical design, not of any filter.
Ist das Entfernen eines IR-Sperrfilters dasselbe wie die Verwendung eines IR-korrigierten Objektivs?
No. An IR-cut filter only blocks which wavelengths reach the sensor. Removing it lets NIR light through but does nothing to the lens's chromatic focus shift, which still brings visible and NIR light to different focal planes. Closing that gap requires an IR corrected lens design, independent of any filter.
Benötige ich für RGBIR-Kameras ein IR-korrigiertes Objektiv?
Yes, for most RGBIR applications. These sensors typically capture RGB and NIR pixels from one frame through the same lens at one focus position, so a lens with significant chromatic focus shift leaves the NIR channel softer than RGB, most visibly on high-resolution sensors with small pixel pitch.
Inwiefern unterscheidet sich die chromatische Fokusverschiebung von der thermischen Fokusverschiebung?
Chromatic focus shift moves the focal plane with wavelength (visible versus NIR) and is addressed by IR-corrected optical design, which holds the residual visible/NIR offset within the system depth of focus. Thermal focus shift moves the focal plane with temperature, driven by barrel expansion and glass or plastic dn/dT, and can be reduced through athermal optical and mechanical design. All-glass construction alone does not establish thermal stability. See the ruggedized lens guide for the thermal mechanism and mitigation.
Wie sollte ich den Fokus im sichtbaren und im NIR-Bereich bei einem Bildverarbeitungssystem überprüfen?
Set focus under the primary wavelength, then switch to the secondary wavelength without touching the focus mechanism, and capture a resolution target at both. Measure MTF at center and corners at the real working distance and aperture, not stopped down, and document any residual focus offset against the depth of focus (set by F# and pixel pitch).
Benötigen Tag-/Nachtkameras sowohl ein IR-korrigiertes Objektiv als auch einen Filterumschalter?
Often, yes. The IR-corrected lens keeps the optics focused across visible and NIR. A filter switcher (or a fixed NIR-pass configuration) controls which wavelengths reach the sensor at a given time. Systems that physically swap an IR-cut filter for a bandpass filter also need the combined lens and filter focus shifts to fit within the focus tolerance at both wavelengths. Compare plate displacement t(1-1/n), not optical path length nt. See the bandpass filter machine vision guide for filter-switcher selection.
Benötigen Sie Hilfe bei der Auswahl eines IR-korrigierten Objektivs?
Send your sensor, working distance, and illumination wavelengths to our San Diego engineering team. We will confirm whether an IRC or NIR-pass variant fits your system, and flag any filter-switcher compensation your day/night design needs. Commonlands lenses are MTF characterized, and orders placed before 12 PM PT ship the same day.