IR pass vs IR cut lens: NIR sensitivity for CS and M12 cameras

You fitted a starlight-class board camera at a gate, added a strong infrared illuminator, and the night image is still dark and soft. The question that follows is always the same: is there an IR-blocking filter inside these CS or M12 lenses, and can it come out? This explainer answers with numbers: where the cut really sits, what IR-pass and IR-corrected mean on a listing, what removing a filter does to focus and color, and how to set focal length, f-number and illuminator wavelength for near-infrared work on CS and M12 cameras.

Does the lens mount decide whether NIR reaches the sensor?

No: C, CS and M12 are mechanical standards only — C-mount is a 1 inch-32 thread at a 17.526 mm flange distance, CS is the same thread at 12.526 mm, and M12 is a 12 mm thread — and none of them carries, implies or needs an infrared filter.

The 5.000 mm flange difference between C and CS is why a C-mount lens needs a 5 mm spacer ring on a CS camera; it moves focus spacing and nothing else. The full adapter logic is in C mount vs CS mount: adapters, focus and clearance.

Whether a lens passes near-infrared is an optical choice made at design time. Photographic lenses almost always cut it; CCTV designs ship in both variants; most machine-vision lenses pass it — the machine vision lenses finder even carries an Ir-corrected type filter. A separate filter can also hide under the mount as a window, so an M12 module and a CS camera can carry identical filter stacks. Ask what the lens does and what the camera does, never what the mount does.

Where does the IR-cut filter actually sit?

Three places: inside the lens as a coating or filter glass; behind the lens as a discrete window, fixed or motor-switched; or on the sensor, because the cover glass of most color modules is itself the IR-cut filter.

Inside the lens, the coating edge sits deep in the red. Behind the lens, day/night CCTV modules carry a motorized dual-filter assembly: IR-cut glass in the path by day for honest color, clear glass at night, moved by a servo on a light-sensor or software command. If your module has one, you already own the switch — the lens only has to pass the band. The sensor side, including what mono and color cover glass each do, is covered in color camera near-infrared imaging: IR-cut filter options.

Can you remove the IR-cut filter instead of buying a lens?

Yes, and the gain is real — you recover the 700–1000 nm band the glass was absorbing — but you pay in color, focus and protection: the focus plane moves about one third of the removed thickness, and if that glass was the cover window the sensor sits bare.

A plane-parallel plate of thickness t and index n holds the focus plane a distance d farther from the lens than air; remove the plate and focus pulls in by d. With n ≈ 1.5, d ≈ t/3: 0.3 mm of cover glass is 0.10 mm, 0.7 mm is 0.23 mm. On an M12 barrel with a 0.5 mm pitch that is a fifth to nearly half a turn — the focus mechanics behind M12 lens thread pitch and focus shift, in numbers. Refocus with the night illuminator on, because glass optics put NIR focus slightly farther back than visible focus, then check corners and lock as in M12 lens focusing: adjust, check corners and lock.

The color cost comes from the Bayer array: its dyes leak near-infrared unevenly, the red channel most, so neutral surfaces turn pink under NIR. A night white balance or a monochrome night mode absorbs it.

If the removed glass was the window, the sensor surface is now exposed. Keep a plain window or a sealed lens in front of it; the lens-protection habits in rugged shop camera selection apply to any camera that loses its front glass.

What do IR-cut, IR-pass and IR-corrected mean on a listing?

IR-cut blocks everything past its edge, typically above about 650 nm; IR-pass has no cut coating and transmits deep into the near-infrared; Ir-corrected passes NIR and holds visible and NIR focus on one plane, which matters on small pixels.

Read the transmission curve first. Where there is none, the series and type names carry the information: starlight low-light lines are built to run with IR illumination, the machine-vision finder's Ir-corrected filter marks lenses that hold both bands in focus, and CCTV lenses commonly come in an IR variant of the same formula. If a lens you already like leaves the question open, close it with a discrete filter behind the lens — a near-infrared long-pass for night, an IR-cut window for day — and the lens only has to be sharp. For the underlying format and focal-length choice on board cameras, see M12 lens selection for board cameras; beyond 1000 nm the rules change again, in SWIR lens and UV lens basics and the band comparison SWIR vs NIR vs visible imaging.

Which illuminator wavelength for night work?

Use 850 nm unless the LED's dull red glow is itself unacceptable: at 940 nm a silicon sensor keeps roughly half or less of its short-wavelength NIR response, so you buy invisibility with illuminator power and lens aperture.

Silicon absorbs less as wavelength grows, and the charge a 940 nm photon generates sits deeper in the die, where a photodiode collects less of it. Starlight-class back-side-illuminated sensors soften that loss; they do not remove it. The visible difference is small: the shorter-wavelength diodes show a faint deep-red point when driven hard, while 940 nm diodes look fully dark. So decide in one line: a covert requirement takes 940 nm with about twice the optical power or a wider aperture, everything else takes the shorter band. Either way, budget the light as strobe pulses inside the exposure — pulsed IR illumination timing for low-light image capture covers the trigger side.

What focal length covers the scene at night?

For a 1080p starlight-class module — a 1/2.8-inch sensor, 5.57 × 3.13 mm from 1920 × 1080 photosites at 2.9 µm — a 16 mm lens frames about 2.8 m of width at 8 m; choose focal length from the field of view, not from the mount.

Worked example: cover a 2.5 m gate at 8 m with that module.

1. Wanted magnification: m = 5.57 mm / 2500 mm = 0.00223.
2. Focal length: f = 8000 mm × 0.00223 / 1.00223 = 17.8 mm.
3. Nearest common M12 length below it, 16 mm: m = 16 / (8000 − 16) = 0.00200, so the field is 5.57 / 0.00200 = 2780 mm, about 2.8 m wide.
4. Object-space pixel size: 2.9 µm / 0.00200 = 1.45 mm per pixel.
5. A 25 mm lens instead: m = 25 / 7975 = 0.00313, field 1780 mm ≈ 1.8 m, pixel 0.93 mm — tighter, but one person fills the frame.

Motion sets the exposure. A person walking at 1.5 m/s crosses 49.5 mm during a 33 ms frame at 30 fps — 34 pixels of smear at 1.45 mm per pixel. Cap the exposure at 5 ms and the blur falls to 7.5 mm, about 5 pixels; the missing light comes from strobing the illuminator inside that window.

Diffraction sets the f-number. The Airy disk diameter is 2.44 × λ × N, and λ is larger in NIR than in daylight: with λ = 0.85 µm at f/1.4 the disk is 2.44 × 0.85 µm × 1.4 = 2.9 µm — one pixel on this sensor. At f/2.8 it is 5.8 µm, two pixels, and the 2.9 µm pitch is wasted. Hold f/1.4 to f/2 for NIR on 2.9 µm pixels; the same argument drives M12 lens light gathering: pick a lower f-number at the same focal length.

What should you order for each case?

Match the order to the day/night split: night-only takes a starlight or IR-pass lens with band-matched light; a camera that must also give daytime color takes an IR-pass lens plus a switchable IR-cut filter; a lens of unknown NIR behavior is settled by a discrete filter behind it.

For the wide case, the M12 board lenses category carries the starlight low-light series:

For the narrow case, a longer board lens is the cleanest swap: the LS-MBL-0031 is a 25 mm M12 lens rated at 1/1.8-inch format and 5 MP. That format over-covers the 1/2.8-inch module — the safe direction, as same lens on a different sensor: how HFOV changes shows. On CS and C-mount bodies, fixed and varifocal options both work:

A C-mount lens on a CS camera takes the 5 mm ring from the first section. Size the full stack — lens, camera, light, controller — in the system configurator so the parts agree before you order.

When is the IR-pass route the wrong answer?

It fails three ways: when daytime color from the same camera matters, when the image must measure rather than watch, and when the site demands invisible 940 nm light on a tight power budget.

• Day color. A fixed IR-pass lens on a color sensor runs pink all day once the cover-glass cut is gone. Keep the module's mechanical day/night filter, or accept a mono camera.
• Measurement. Board-lens distortion figures are quoted for visible light, and NIR focus sits elsewhere; for gauging in NIR use an Ir-corrected or telecentric machine-vision lens — the lens finder filters both.
• Covert on a budget. 940 nm halves the signal; if you cannot double LED power or open the aperture, a shorter-wavelength unit hidden behind a bezel beats a dark 940 nm scene.
• Fisheye modules. The filter often lives in the module stack, not the lens; pulling it changes back focal length — the constraint set in fisheye lens selection for M12 board cameras.
• Thermal confusion. This page is reflected NIR from your own illuminator, not the 8–14 µm heat cameras see; glass lenses and silicon sensors cannot image that band at all.

What should you check before ordering?

Ten minutes with this list catches the three failures that kill most NIR builds: a hidden cut filter, a lens format too small for the sensor, and an f-number that diffraction has already defeated.

• Identify the cover glass: mono modules are clear, color modules usually cut — this is the filter most builds miss.
• Set the day/night split before the lens order: night-only is a fixed IR-pass lens; day color adds a switchable IR-cut filter.
• Match formats upward: a 1/1.8-inch lens on a 1/2.8-inch sensor is fine; the reverse vignettes the corners.
• Derive focal length from field of view and working distance, as in the worked example above.
• Hold f/1.4–f/2 in NIR on 2.9 µm pixels; the Airy disk reaches one pixel at f/1.4 in that band.
• Prefer the shorter wavelength over 940 nm unless the glow must be invisible; with 940 nm, budget roughly double the light.
• After any filter change, refocus at the working wavelength with the illuminator on, then check the corners and lock.
• If the cover glass came out, keep a plain window or a sealed lens ahead of the sensor.