Dual bandpass filter day night imaging: keep color and see in IR

A color camera that runs around the clock faces a conflict. By day it needs near-infrared cut away to render color; at night, under an 850 nm LED, that same filter blocks exactly the light you are paying for. The usual fix, a mechanical IR-cut switcher, adds a moving part, a mode signal and a refocus check to every unit you build. A dual bandpass filter removes the mechanism: one piece of glass passes visible light plus one narrow infrared band, so the same camera renders color by day and sees the illuminator at night. Below are the passband numbers, the mounting arithmetic, the 850 versus 940 nm choice, and the cases where this filter is the wrong part.

Two passbands on one piece of glass

A dual bandpass filter does the work of two filters in one substrate. It passes the visible band, commonly 400-700 nm, for daylight color. It blocks hard between the bands. It then passes a second, narrow band centered on the night illuminator's wavelength, typically 850 nm or 940 nm, sized to cover the LED's own emission width of roughly 40-60 nm plus binning and temperature tolerance.

Compare the alternatives. A fixed IR-cut filter (hot mirror) passes visible and blocks all near-infrared: accurate color, dead night channel. Remove it at night, as a mechanical switcher does, and the sensor floods with every NIR wavelength: sensitive, but color-blind and open to stray light. The dual bandpass filter sits in one position permanently, so the optics never change between modes.

Why it replaces the switcher

The switcher's costs are mechanical: a shutter to move, a driver line to wire, a mode delay to sequence against each frame, dust and wear on the mechanism. The dual bandpass filter has none of these. The subtler benefit is optical: because the glass never leaves the beam, focus, white balance and calibration are identical in both modes. A camera calibrated for length measurement from image markers keeps that calibration day and night, instead of carrying a small focus error in whichever mode has the extra window in the beam.

Where to mount it, and the focus arithmetic

Two placements work, and they differ by one number.

Behind the lens, in the C-mount stack, the filter sits in the converging beam, and glass in a converging beam moves the image plane away from the lens. The shift comes from thickness and refractive index alone:

Typical filter glass has n ≈ 1.5-1.7. A 1 mm filter then shifts focus about 0.33 mm, a 3 mm filter about 1.0-1.2 mm, a 5 mm filter about 1.7-2.1 mm. In front of the lens the beam is still collimated, so the shift is zero; the cost is mechanical fit, because the filter must match the lens front thread or a step-up ring and clear any hood.

Worked example, with illustrative camera values: a 16 mm lens on a 5 MP, 2/3-inch sensor with 3.45 µm pixels (2448 × 2048, sensor width 8.45 mm), set to m = 0.05.

Field of view = 8.45 mm / 0.05 = 169 mm. Working distance ≈ f × (1/m + 1) = 16 × 21 = 336 mm. One object pixel = 169 mm / 2448 = 69 µm. With a 3 mm filter behind the lens, Δf = 3 × 0.5 / 1.5 = 1.0 mm. At the day iris f/8, depth of field ≈ 2 × 8 × 3.45 µm × 1.05 / 0.0025 = 23 mm, so the 1 mm shift is about 4% of it — focus before fitting the glass and you would never see the difference. At the night iris f/2.8 the same geometry gives about 8 mm of depth of field, still far more than the shift.

Push magnification to 0.3 and the picture flips: depth of field at f/8 falls to 2 × 8 × 3.45 µm × 1.3 / 0.09 ≈ 0.8 mm, less than the 1 mm shift. The rule that falls out: below roughly 0.1x, focus normally and the shift hides inside the depth of field; from about 0.2x upward, install the filter first and focus through it, or mount it in front.

The constant glass shift is not the only focus effect. Visible light and 850 nm do not focus at the same depth on an ordinary lens, because its color correction is tuned to the visible range. The machine vision lenses category includes an Ir-corrected type that holds one focus across both bands; on other lenses, check night sharpness against a target and refocus if the IR image falls behind. For your own shift-versus-depth-of-field numbers, use the calculator below.

Day color and the magenta question

Silicon keeps responding past 1000 nm, and the camera's color filter array leaks near-infrared into all three pixel colors, red and blue most. Two consequences follow.

At night with a plain IR-pass filter — the switcher's night position — every NIR wavelength reaches the red and blue pixels, and the mix renders as a magenta-tinted monochrome. The infrared contamination guide traces that leak channel by channel.

By day, the risk is leakage from the filter itself. If the IR passband sits close to the visible edge, or the blocking between bands is shallow, infrared tails ride into the daylight image and wash the color out. Distance between the bands is the cure: with the visible edge at 700 nm and the IR band at 850 or 940 nm, the blocking region stays wide and its edges can be steep without distorting either passband — the reason wider separation gives better color. The fixed IR-cut choices and their cutoff positions are compared in the color camera near-infrared imaging guide.

One residual: by day the IR band also passes its own 40-60 nm slice of sunlight, riding alongside the ~300 nm visible band. Daylight is weaker out at 850 nm than at its visible peak, so the practical effect on color is minor on most parts.

850 nm or 940 nm

Match the filter band to the LED first — a 940 nm LED behind an 850 nm bandpass returns black night frames — then choose:

850 nm, the default. 850 nm LEDs are among the brightest infrared LEDs made, and silicon sensors respond strongly there, so night exposures stay short. The LED shows a faint deep-red glow when powered.

940 nm, the second choice with two advantages. The emission is invisible, which matters where people work nearby or the camera should not reveal its position, and the band sits farther from the visible edge, giving the best day-color margin. The cost is sensitivity: close to silicon's ~1100 nm limit the sensor responds several times less than at 850 nm, so budget longer exposures, more LED drive, or both.

The band-choice logic for defect contrast sits in the SWIR vs NIR vs visible guide, and the monochromatic lighting guide explains why one wavelength beats broadband light at night.

Where the setup earns its place

The filter pays for itself wherever one fixed color camera must work in ambient light day and night: dock doors and parcel portals, outdoor gantries, warehouse approaches, vehicle gates, yard corners around a machine cell. Day frames ride on sunlight; night frames ride on an IR flood or strobe — the lights finder lists the illuminator families.

Exposure still changes between modes. Keep a day profile (short shutter, smaller iris, capped gain) and a night profile on the LED. The day and night license plate capture guide walks that switch, and the automatic iris, exposure and gain guide covers the settings machinery. Strobe the LED to stop motion: a belt at 0.3 m/s blurs 0.6 mm across a 2 ms exposure — 9 pixels at the worked example's 69 µm object pixel — and a 50 µs strobe pulse cuts that to 15 µm, under one pixel.

Three lenses that fit the common day/night sensor formats:

The LS-FFL-0567 runs an F2.8-22 iris, useful when depth of field has to swallow the filter shift. Check thread and spacing against your filter holder with the lens configurator, and narrow by sensor format and iris range in the lens finder. If night frames also need real color — reading a label, telling wire colors apart — the filter is not the tool; sequence visible and IR wavelengths on one camera instead, as the multi-wavelength sequencing guide shows.

When the usual answer is wrong

• Monochrome camera. There is no day color to protect, and a simple IR longpass passes more of the LED than a dual bandpass does. Skip it.
• Color detail needed at night. Infrared carries no color. Fit white light for night and keep an ordinary IR-cut filter in place.
• Night scene lit by halogen or tungsten. Those sources radiate much of their energy past 700 nm, where the filter blocks. Expect long exposures; an IR-pass switcher collects more of that light.
• Magnification above about 0.2x. The behind-lens shift exceeds the depth of field, as the worked example shows. Mount in front or focus through the glass.
• An LED already installed at another wavelength. Buy the filter to match the LED you have, or replace the LED — never split the difference.

Checks before ordering

• LED peak wavelength and spectral width from its datasheet, and a filter IR band that covers peak, binning tolerance and temperature drift.
• Mounting position decided — front of lens or C-mount stack — with the thread or holder sized to the lens.
• Sensor format inside the lens image circle, with the coverage margin the calibration target field of view guide recommends.
• Lens chromatic behavior settled: Ir-corrected type, or a planned night focus check.
• Day white balance after installation, and night-frame handling decided in software: luminance-only or a locked white balance.
• Strobe duration checked against line speed, using the motion arithmetic above.

The decision

One color camera, daylight by day, IR LED by night: fit a dual bandpass filter matched to the LED — 850 nm as the default, 940 nm when the faint glow matters or day color needs the margin. Mount it in front below 0.1x magnification; from 0.2x upward, install it first and focus through the glass. Treat night frames as monochrome and keep two exposure profiles.

Monochrome camera, night only: a longpass filter, no dual bandpass. Color detail needed at night: white light plus an IR-cut filter instead. More lens-side reading sits on the lenses guide topic.

Send the camera, lens and LED wavelengths through the contact page and we will check the passband pairing against your sensor before you order glass.