SWIR LED Illumination for Moisture Detection and Water Content Measurement
Water is almost perfectly transparent to visible light. In the short-wave infrared it is not. Around 1450 nm, liquid water absorbs strongly enough that wet regions go dark in the image while dry regions stay bright, and that single property is the basis for moisture detection, water and ice discrimination, drying-process control, and a large share of food and agricultural inspection.
TeraPower LED is the authorized United States distributor of REVOX, Inc., a Japanese manufacturer of high-power industrial LED components. This page explains how SWIR illumination is used to make moisture visible, how to choose between 1450, 1550 and 1650 nm, and what to specify when you design the illuminator.
We supply bare COB LED emitters (components), not finished illuminators or inspection machines. They are intended for machine builders, system integrators, and research groups that design their own illumination heads.
Why water is visible in the short-wave infrared
The O–H bond in water absorbs infrared light at a set of characteristic wavelengths. In liquid water the practically useful features sit near 970 nm, 1200 nm, 1450 nm and 1940 nm, arising from overtone and combination modes of O–H stretching and bending. The 1450 nm feature is the one most SWIR inspection systems are built around, because it absorbs strongly and still falls comfortably inside the response range of a standard InGaAs sensor.
REVOX describes this absorption mechanism explicitly, as distinct from the transmission mechanism that near-infrared inspection relies on: some near-infrared wavelengths are absorbed by moisture, and that absorption can be used for inspection. The published example is mandarin orange inspection. A bruise is where the fruit will begin to rot, but it is difficult to judge by eye; under near-infrared illumination the region where the cells have been crushed appears black.
That is the entire mechanism in one sentence: where there is more water, less light reaches the camera, and the region reads as dark.
What a moisture image actually measures
Two jobs look similar on screen and have very different requirements.
Contrast inspection answers a yes/no question: is this region wetter than its surroundings? Bruise and dent detection, leak and seal inspection, fill-level checks through opaque packaging, and water-versus-ice discrimination are all contrast problems, and a single wavelength inside the absorption band is enough.
Quantitative measurement answers how much water is present. Single-wavelength intensity cannot do that on its own, because source output, surface geometry, colour and camera gain all move the signal as well. The established approach is a two-wavelength ratio: illuminate with one band inside the water absorption feature and one just outside it, then use the second as an intensity reference so that everything except water absorption divides out. With 1450 nm as the measuring band, a reference near 1300 nm sits outside the feature, and we supply 1300 nm in the same package and the same 16-chip configuration. Both bands can therefore be built into one head with matched optics and a matched thermal design.
Choosing a strong absorption band has one further consequence: strong absorption means shallow penetration. At 1450 nm the signal is dominated by water at and near the surface. If you need to measure water through the bulk of a thick or dense product, a weaker feature stays inside the linear part of the Beer–Lambert relationship over a longer path, at the cost of less contrast per unit of water. Decide which depth you are measuring before you choose the wavelength.
SWIR requires an InGaAs camera
Silicon sensors are limited by the 1.12 eV bandgap of silicon, which places the response cutoff near 1100 nm, with quantum efficiency already falling steeply before that point. Every device on this page sits beyond that limit, so a standard CMOS or CCD camera will see nothing at all. InGaAs sensors cover roughly 900–1700 nm and are the correct choice here.
If the inspection has to keep an existing silicon camera, the only water features within reach are the weak bands near 970 and 1200 nm, and the available contrast is far lower. See NIR COB LED emitters for our 740–1300 nm range, and the custom section below for wavelengths we do not stock.
Choosing between 1450, 1550 and 1650 nm
| Wavelength | Principal absorber | Typical use | Radiant flux (Typ.) |
|---|---|---|---|
| 1450 nm | Water (O–H) | Moisture detection and measurement, water/ice discrimination, drying-process control, bruise and dent detection, fill level | 2.61 W @ 16 A |
| 1550 nm | No strong feature | General-purpose SWIR imaging and reference illumination | 2.17 W @ 16 A |
| 1650 nm | Hydrocarbons and polymers (C–H) | Plastic identification and sorting, polymer inspection | 1.49 W @ 16 A |
1450 nm is the moisture band and the default for everything described on this page. 1550 nm sits between the major water and hydrocarbon features, has excellent atmospheric transmission, and has by far the widest selection of matched lenses, filters and fibre components because it is also the telecommunications band. 1650 nm overlaps C–H absorption from hydrocarbons and polymers, so it is the plastics wavelength rather than the water wavelength.
One useful pairing: 1450 nm for the water signal and 1650 nm for the polymer signal lets a single multi-spectral head distinguish wet product from its packaging.
Why an LED instead of a halogen lamp
SWIR imaging has traditionally used broadband halogen or tungsten lamps. LEDs replace them with instant on/off switching, microsecond-level strobe control that can be synchronised to a line-scan camera, no warm-up drift, and a service life measured in tens of thousands of hours.
For moisture work there is a further reason that outweighs all of those. A halogen lamp radiates heat onto the sample, and heat changes the quantity being measured. Hold a web, a food product or a coated surface under a broadband lamp long enough to expose it and the drying has already started: the surface water you are trying to quantify is leaving while you measure it. An LED emitting in a 138 nm band around 1450 nm puts a small fraction of that thermal load on the target, and when strobed it is only on for the exposure itself.
Temperature matters a second time over: the position of the water absorption peaks shifts slightly as temperature rises. Any moisture measurement that has to stay repeatable across a production shift is better off not being lit by a lamp.
Budget for low radiant flux
This is the main design constraint of SWIR LED illumination, and it is better confronted at the start. Radiant flux runs from 2.61 W at 1450 nm to 1.49 W at 1650 nm at 16 A, roughly an order of magnitude below a comparable near-infrared device: our 740 nm emitter in the same package delivers 15.09 W. Calculate the irradiance you need at the working distance before committing to a lens and an exposure time, and plan on secondary optics to concentrate the 120° output. An illumination design that worked at 740 nm will not transfer to 1450 nm unchanged.
Spectral half-width of 132–138 nm
These emitters are spectrally broad: 138 nm half-width at 1450 and 1550 nm, 132 nm at 1650 nm. For general illumination that is an advantage, because the band covers the water feature comfortably even as the feature shifts with temperature or composition. For spectroscopic work that needs clean band separation it is a limitation and a bandpass filter is required. Re-check the flux budget after the filter loss; with 2.61 W to start from, filter losses matter more here than anywhere else in the catalogue.
Plan the thermal path
Each device dissipates 27.2 W at 16 A with a thermal resistance of 1.0 °C/W, so the heat sink is a real design element rather than an afterthought. Junction temperature is limited to 125 °C and the operating range is −40 to +100 °C. Every device includes a built-in 10 kΩ NTC thermistor, so junction conditions can be monitored in the finished assembly instead of estimated. Both output and peak wavelength drift with junction temperature, and in a quantitative moisture measurement that drift appears directly in the result.
SWIR COB LED emitters we supply
| Peak wavelength | Model number | Radiant flux (Typ.) | Wavelength range | Spectral half-width |
|---|---|---|---|---|
| 1450 nm | 1450L-120D-16C-2827COB-01 | 2.61 W @ 16 A (2.06–3.5 W) | 1420–1480 nm | 138 nm |
| 1550 nm | 1550L-120D-16C-2827COB-01 | 2.17 W @ 16 A (1.55–3.00 W) | 1520–1580 nm | 138 nm |
| 1650 nm | 1650L-120D-16C-2827COB-01 | 1.49 W @ 16 A (1.19–2.00 W) | 1620–1680 nm | 132 nm |
All three share the same envelope: 16 chips on a 28.0 × 26.8 × 3.2 mm package with a 4.5 × 4.5 mm emitting area, 16 A forward current, 1.2 V typical forward voltage, 27.2 W power dissipation, 1.0 °C/W thermal resistance, 120° viewing angle, and a built-in 10 kΩ NTC thermistor. Radiant flux is measured at Tc = 25 °C. Full specifications, characteristic curves and outline drawings are on each product page and on the SWIR COB LED emitters collection.
Wavelengths outside this catalog
REVOX plans and designs the LEDs itself and procures and assembles the bare chips in-house, with a wavelength range from 235 nm to 1900 nm available in steps as fine as 5 nm, and the ability to combine several wavelengths on a single device.
For moisture work that matters in two concrete ways. A measuring wavelength and its reference wavelength can be built as one mixed-wavelength array instead of two separate emitters, which removes the alignment and matching problem between them. And a water feature we do not stock as a standard product can still be quoted as a custom device. If your measurement depends on a band outside 1450–1650 nm, send us the target wavelength and we will confirm what is feasible.
Chip count, array layout, substrate and package dimensions are configurable as well. Send us the target wavelength, the optical power you need, and the annual quantity, and we will quote the device.
Standard products cover 365 to 1,650 nm. Wavelengths outside that range, and other chip counts, array layouts or package sizes, are made to order. Tell us the wavelength and quantity you need and we will confirm feasibility with REVOX.
What to include in a request for quote
- Target wavelength and acceptable tolerance. State whether you need a specific peak or a band, and whether a reference wavelength is required alongside it.
- Required irradiance at the work surface, and the working distance. With SWIR flux this is the constraint that decides whether the design closes.
- Duty cycle and drive mode. Continuous, strobed with the camera, or pulsed; this drives the thermal design and the choice of driver.
- Annual quantity and target date. Pricing and lead time both depend on volume.
Ordering and sourcing information
Minimum order quantity. Production starts at a single unit. There is no minimum order quantity, so an evaluation build and a production run are both quotable.
Lead time. Approximately one month when parts are in stock at the factory, and 1.5 to 2.5 months when they are not.
Shipping. Orders are shipped either direct from REVOX in Kanagawa, Japan, or through our Torrance, California office. We do not hold U.S. inventory; devices are built to order, which is what drives the lead time above.
Technical support. Technical questions are handled from the United States in English, and we connect you directly with the design team in Japan when a question needs the manufacturer.
Pricing. All products are quoted individually, because pricing depends on wavelength, optical power, chip configuration, and quantity. Use the Request a Quote form on any product page, submit the Custom Order Form for non-standard requirements, or email sales@terapowerled.com.