أ 970 nm LED is a near-infrared emitter aimed at the weak water absorption band centered near 970 نانومتر. That weakness is the point. Where 1450 nm saturates in a fraction of a millimeter of water, 970 nm penetrates on the order of centimeters, so it measures moisture through the bulk of a sample rather than at its surface. It is also the last useful water band that silicon still sees, which keeps the detector a cheap photodiode or a standard CMOS imager instead of an InGaAs array. This guide covers the water band structure and why band strength decides your sample thickness, silicon and InGaAs detection at 970 نانومتر, reference-wavelength pairing for ratiometric measurement, what to specify (center wavelength, FWHM, radiant power, temperature coefficient, طَرد), and how 970 nm compares with 940, 1050, 1200, و 1450 نانومتر.

For where 970 nm sits in the wider near-infrared range and how wavelength selection works across the band, see our NIR LED guide.

The water bands, and why the weak one is useful

Liquid water absorbs across the near-infrared in a series of overtone and combination bands of the O-H stretch. They are not equal. Each successive overtone toward shorter wavelengths is roughly an order of magnitude weaker than the one below it, and that ladder of strengths is the whole basis of wavelength selection for moisture work.

Read the penetration column and the design rule writes itself. Absorption strength and usable sample thickness trade against each other directly. A strong band gives large contrast per unit of water but saturates almost immediately, so it reports on a surface layer. A weak band gives less contrast per unit of water but stays inside the linear part of the Beer-Lambert law through a thick sample, so it reports on the bulk.

That is why 970 نانومتر و 1450 nm are not competitors. They answer different questions. If the measurement is “how wet is the surface of this web, film, or coating,” the answer is 1450 نانومتر. If the measurement is “what is the moisture content of this grain kernel, this piece of lumber, this bottle of solvent, this volume of tissue,” 1450 nm never gets past the outer layer and 970 nm is the working wavelength.

One caveat on the penetration numbers. They are absorption-only depths in clear water. Real samples scatter, and in a scattering medium like grain, powder, or tissue the photon path is far longer than the geometric thickness while the useful depth is far shorter than the table implies. Treat the column as a ranking of the bands, not as a spec for your sample.

Why 970 nm keeps you on silicon

The second reason to choose this band has nothing to do with water. It is the detector.

Silicon’s response falls away past roughly 900 nm and is gone by about 1100 نانومتر, and every water band above 1100 nm therefore forces an InGaAs detector into the design. 970 nm sits inside the last stretch of usable silicon response. That single fact changes the bill of materials for a moisture instrument.

For a hyperspectral or multispectral instrument this is the decisive point. Adding a water channel at 1450 nm means adding an InGaAs sensor, its cooling, its optics, and its cost to a system that was otherwise silicon. Adding a water channel at 970 nm means adding one more LED to the existing illuminator and reading it on the camera already in the machine. The contrast is weaker, but the integration cost is close to nothing.

What a 970 nm LED is made of

أ 970 nm emitter is built on gallium arsenide, with a strained InGaAs quantum-well active region. That is the same material system behind the 980 nm pump diodes used in fiber amplifiers, and it is a mature, high-volume, well-behaved technology.

Three consequences matter for design.

Efficiency is good. GaAs-based near-infrared emitters convert electrical power to optical power far more effectively than the InP-based SWIR devices above 1100 نانومتر. Cost per milliwatt at 970 nm is a different order of magnitude from cost per milliwatt at 1450 نانومتر, and the available radiant power per package is much higher.

It is fully invisible. There is no residual glow at 970 نانومتر, unlike the faint red visible from an 850 nm emitter. For instruments deployed near people or in view of a color camera, that is a practical benefit.

It is spectrally broad and incoherent. Expect a full width at half maximum on the order of 40 ل 70 نانومتر, confirmed on the specific datasheet. There is no speckle and no modal noise, and the radiation pattern is Lambertian rather than a beam.

That broad linewidth is usually treated as a drawback, and at 970 nm it mostly is not. The water band here is itself broad, on the order of 60 ل 80 nm wide, so an LED with a 50 nm FWHM sits inside the feature rather than smearing across it and the neighboring baseline. Compare that with narrow, sharply peaked absorption features in gas sensing, where an LED’s linewidth genuinely destroys the measurement and a laser is required. Water in the near-infrared is a broad-feature problem, and a broad-source solution fits it.

Two related parts are easy to confuse with this one. أ 980 nm pump laser diode is a narrow-line, high-brightness, fiber-coupled device for amplifier pumping, not a diffuse illuminator. And a 940 نانومتر LED, which the catalog stocks in far greater depth, sits just below the water band on a comparatively flat baseline, which makes it a poor moisture emitter and an excellent reference emitter. That second point is the basis of the next section.

Ratiometric measurement and reference-wavelength pairing

A single-wavelength transmission reading at 970 nm is not a moisture measurement. It is a measurement of everything that attenuated the beam: ماء, but also scattering from particle size and packing density, surface texture, sample thickness variation, window fouling, LED output drift with temperature and age, and detector responsivity drift.

The standard fix is a two-wavelength ratio. Measure at 970 نانومتر, where water absorbs, and at a nearby reference wavelength where it essentially does not. Everything that is common to both channels divides out, and what remains tracks water content.

Bracketing with references on both sides is better than a single reference when the sample’s baseline slopes with particle size or color, which is common in agricultural and powder work. Two references define a local baseline under the 970 nm peak, and the depth of the peak below that baseline is the moisture signal.

Three implementation details decide whether the ratio actually cancels anything:

Share the optical path. The channels must see the same sample volume through the same window at the same geometry. Separate optical paths reintroduce exactly the common-mode errors the ratio was meant to remove.

Multiplex in time, not just in space. Driving the emitters in sequence and reading one detector removes detector-to-detector responsivity differences. Modulating and using synchronous or lock-in detection also rejects ambient light and 1/f noise, which matters on a factory floor with hot lamps and daylight.

Calibrate against a reference method. The ratio is a proxy. It has to be regressed against oven-drying, Karl Fischer titration, or whatever your industry treats as the primary method, on your material, over your moisture range. That calibration is material-specific and does not transfer between products.

Where 970 nm is the working wavelength

The thread through that list is thickness. Every one of those samples is millimeters to centimeters deep, and the measurement is about what is inside. Push toward thinner samples, الطلاءات, films, and surface layers and the correct wavelength moves up the band ladder to 1450 نانومتر أو 1940 نانومتر.

Optical safety at 970 نانومتر

This is the one place where 970 nm is less comfortable than the SWIR moisture wavelengths, and it is worth stating plainly because it catches teams migrating down from 1450 نانومتر.

The eye-safety margin engineers associate with SWIR comes from water absorption in the cornea and aqueous humor above roughly 1400 نانومتر, which stops light before it reaches the retina. 970 nm is well below that threshold. It is inside the retinal hazard region, it is focused by the eye onto the retina, and because it is completely invisible it triggers no blink reflex and no aversion response.

أ 970 nm illuminator therefore needs the same treatment as an 850 أو 940 nm illuminator: risk-group classification under IEC 62471, or laser product classification under IEC 60825-1 if the source is a laser, evaluated at the radiant intensity and viewing distance the product actually presents. High-power arrays for in-line inspection deserve particular attention, since it is easy to assemble an emitter array whose combined output exceeds the exempt group limits even though each die is harmless.

Specifying a 970 نانومتر LED

The temperature coefficient deserves emphasis because it interacts with the measurement in a way that is easy to misdiagnose. ال 970 nm band has steep flanks. A source whose center wavelength walks 5 nm as the instrument warms up is sampling a different part of the absorption feature, and the reading drifts even though the sample has not changed. If the reference channel is a different material system or sits on a flat part of the spectrum, it will not drift the same way and the ratio will not save you. Stabilize the emitter, or characterize the drift and correct for it in firmware.

How 970 nm compares with neighboring wavelengths

Two boundaries fall out of that table. The silicon boundary sits between 970 و 1050 نانومتر, and it is a cost boundary as much as a technical one. The eye-safety boundary sits near 1400 نانومتر, between 1200 و 1450 نانومتر. 970 nm is the only genuine water band that falls on the cheap side of both, which is exactly why it exists as a sensing wavelength rather than as a curiosity, and it is also why the eye-safety work is on you.

FAQ

What is a 970nm LED used for?

Moisture and water-absorption sensing, almost exclusively. 970 nm coincides with a weak overtone band of the O-H stretch in water, weak enough that light still penetrates roughly two centimeters of pure water, so it measures moisture through the bulk of a sample rather than at its surface. Typical uses are grain and seed moisture meters, lumber moisture measurement, water-in-fuel and water-in-oil detection, tissue water fraction in biomedical instruments, plant water status sensing, and adding a water channel to a silicon-based multispectral imaging system. It is not an illumination part.

Why use a 970nm LED instead of 1450 nm for moisture measurement?

Sample thickness. ال 1450 nm band is roughly sixty times stronger, which sounds better but means it saturates within about 0.4 mm of water. It reports on a surface layer and tells you nothing about the interior of a grain kernel or a piece of lumber. 970 nm stays in the usable, roughly linear part of the Beer-Lambert law through centimeters of material. The second reason is cost: 1450 nm requires an InGaAs detector, while 970 nm works with an inexpensive silicon photodiode or a standard CMOS imager. Choose 1450 nm for films, الطلاءات, and webs. Choose 970 nm for bulk samples.

Can a silicon photodiode detect a 970nm LED?

نعم, and that is one of the main reasons to pick this wavelength. A silicon PIN photodiode holds roughly 0.5 A/W at 970 نانومتر, past its peak near 900 nm but still comfortably usable. Silicon CMOS imagers are weaker: quantum efficiency at 970 nm is typically under 10% on a standard front-illuminated sensor, rising to roughly 20 ل 30% on an NIR-enhanced or deep-depletion variant. For imaging at this wavelength, specify the NIR-enhanced sensor version and check its published QE curve rather than assuming.

What reference wavelength should I pair with a 970nm LED?

Something close by where water barely absorbs, so that scattering, path length, window fouling, and source aging cancel in the ratio. On the short side, 850, 910, أو 940 nm all work and are well stocked. On the long side, 1050 nm brackets the band from above, though it is past the silicon edge and needs its own detector. If the sample’s optical baseline changes with particle size or color, which is normal for grain and powders, use references on both sides of 970 nm so the two of them define a local baseline and the depth of the peak below it becomes the moisture signal.

Is a 970nm LED eye-safe?

Not inherently, and it should be treated with more caution than a SWIR moisture emitter. Wavelengths above roughly 1400 nm are absorbed in the cornea before reaching the retina, which is where the eye-safety reputation of 1450 نانومتر و 1550 nm comes from. 970 nm is below that threshold, so it reaches and focuses on the retina, and because it is entirely invisible it produces no blink reflex or aversion response. Classify a 970 nm illuminator under IEC 62471 at the radiant intensity and viewing distance the product actually presents, and pay particular attention to multi-die arrays, whose combined output can exceed exempt-group limits even when each emitter is individually harmless.

Where 970 nm fits in the portfolio

970 nm is a measurement wavelength, not an illumination wavelength. It is selected because water absorbs there weakly, and weakly is what a bulk measurement needs. If your problem statement contains the words moisture content, water fraction, kernel, bulk sample, water-in-oil, or tissue hydration, and the sample is millimeters to centimeters thick, this is the band to evaluate first. If it contains coating, film, web, or surface moisture, the right answer is 1450 نانومتر. If it contains covert illumination or proximity detection with no water in the problem at all, 850 و 940 nm are cheaper and better stocked.

For available packages, datasheets, and sample requests across the near-infrared range, see the NIR LED product category.