머신 비전을 위한 근적외선(NIR) 이미징: 850nm 대 940nm, 대역통과 필터 및 적외선 보정 렌즈

850nm vs 940nm: How to Choose the Right NIR Wavelength

Choose 850nm when a faint glow is acceptable and detection range or exposure time is the binding constraint. Choose 940nm when the illuminator must stay invisible, or when the system runs in bright daylight where suppressing ambient NIR matters more than sensor quantum efficiency. Commonlands stocks matched bandpass filters for both wavelengths.

850nm: 센서 양자 효율(QE)이 높으며 희미한 가시광선 발광이 나타남

Silicon CMOS sensors have higher quantum efficiency at 850nm than at 940nm, so for the same drive power an 850nm source gives more usable signal, supporting shorter exposures or lower illumination power. The tradeoff is glow: 850nm sits close to the edge of human vision, where deep-red sensitivity falls off steeply but does not vanish until roughly 750nm, and the short-wavelength tail of the LED's emission puts a small fraction of its output near that edge. At high drive currents this glow is detectable in dark environments.

940nm: 육안으로 보이는 발광은 없으나 실리콘의 양자 효율(QE)의 약 절반 수준

940nm sits far enough outside the eye's response that it produces no perceptible glow under normal conditions. The cost is sensor response: silicon QE at 940nm is commonly around half or less of its 850nm value, varying by sensor. A 940nm system therefore needs more power, longer exposure, or both to reach the signal-to-noise ratio an 850nm system achieves at the same range. High-throughput lines where exposure time is the binding constraint should verify this margin against the actual sensor QE curve.

대기 및 실외 환경 고려 사항

Outdoors, ambient sunlight carries strong NIR that competes with active illumination. Atmospheric water vapor absorbs solar radiation in a band around roughly 930nm to 970nm, which suppresses ground-level solar irradiance at 940nm relative to 850nm. That is a large part of why 940nm suits outdoor systems such as driver monitoring and face recognition: the ambient background is lower, partly offsetting the QE penalty. 850nm sees the full solar NIR floor.

These filters are not interchangeable: a 940nm bandpass filter blocks 850nm illumination and produces a dark image if paired wrong. See bandpass filter machine vision for how center wavelength and passband width factor into the selection.

Why NIR Reveals Contrast That Visible Light Misses

NIR imaging is useful because reflectance, transmission, and absorption vary with wavelength, so many materials look different in NIR than under white light. That difference is the entire basis for NIR machine vision, not a general-purpose upgrade to image quality.

잉크 및 인쇄 그래픽

Carbon-based inks absorb visible light and stay dark, and that absorption carries into the NIR band, so carbon-ink printing keeps its contrast against the substrate under 850nm or 940nm. Many dye-based inks that look opaque black to the eye are largely transparent in NIR, so a barcode printed in dye-based ink can effectively disappear in a 940nm image, revealing the substrate beneath.

That cuts both ways. Disappearing ink helps when inspecting a feature under a label, but it defeats an OCR or print-verification system reading dye-based characters. Carbon-based ink does not have this problem. Verify ink composition on the actual production substrate before committing to a wavelength.

Surface glare and coatings

Specular reflections from glossy or metallic surfaces saturate visible pixels when ambient light is not controlled. Narrow-band NIR with a matching bandpass filter rejects the broadband visible glare, so the sensor mainly captures the NIR scattered from the surface. It does not remove specular reflection from the NIR source itself, but it removes the visible glare that dominates typical factory lighting.

Thin films and coatings that look uniform under white light can show different NIR reflectance by composition or thickness. Biological and food-grade materials often show NIR contrast tied to water content, which is why NIR appears in fill-level inspection, food sorting, and pharmaceutical packaging.

NIR이 하지 않는 것

NIR is not universally better than visible. Color discrimination and legibility of visible ink are usually better served by visible light. NIR does not penetrate opaque materials the way X-ray does, and useful penetration depth varies by material and must be validated on real samples. A datasheet contrast difference does not guarantee the same result on production parts.

Filters, Lenses, and Switching Architectures for NIR Systems

Beyond sensor and illumination choice, an NIR system needs three hardware decisions: filter type, lens IR correction, and whether it must switch between visible and NIR modes at all.

필터 선택

The IR-cut filter is the most misunderstood element: it blocks NIR and must be out of the optical path, but removing it is necessary, not sufficient, because without a bandpass filter the sensor still sees ambient visible light. A single bandpass filter passes only the illumination band. A dual-bandpass filter passes a visible and an NIR window at once for RGBIR (red, green, blue, infrared) sensors but cannot fully reject either. An electronic switcher moves an IR-cut and a bandpass filter in and out for true day/night separation.

렌즈의 적외선 보정이 중요한 경우

If a system runs only in NIR, focus at the NIR wavelength and IR correction is unnecessary. If it must stay sharp at both visible and NIR from one focus position (day/night cameras, RGBIR sensors, dual-mode inspection), an IR-corrected lens is required. A standard lens has chromatic focus shift between visible and NIR from dispersion in the glass. Removing the IR-cut filter does not close that gap, since the shift is in the lens design, not the filter.

Sensor selection

Not every sensor is a good NIR candidate. Many consumer and some machine vision sensors have an NIR-blocking layer in the pixel stack itself, independent of any external IR-cut filter, capping sensitivity at 850nm and 940nm. Pull the sensor's spectral QE curve and read the value at each wavelength rather than assuming a generic silicon response. RGBIR sensors depend on a dual-bandpass filter to define the NIR passband, or leakage degrades daylight color.