NIR

Near-infrared (NIR) refers to the portion of the electromagnetic spectrum just beyond the visible range. It generally includes wavelengths from about 750 nanometers to around 2500 nanometers (0.75 to 2.5 micrometers). Exact boundaries vary by field; some definitions use 0.7–1.4 μm or 0.75–2.5 μm. NIR is used because photon energies are higher than mid- to far-infrared but still low enough to interact effectively with certain materials, enabling various imaging and sensing techniques. Common NIR wavelengths include around 850 nm, 940 nm, 1.3 μm, and 1.55 μm.

Sources and detection in the NIR span natural and artificial emitters. The Sun emits across the spectrum including NIR, while artificial devices such as LEDs and laser diodes produce targeted NIR bands. In fiber optics, the 1.3 μm and 1.55 μm windows are important due to low attenuation in silica fibers, making NIR critical for telecommunications. Detectors for NIR often rely on semiconductor materials such as indium gallium arsenide (InGaAs) or indium antimonide (InSb); silicon-based sensors cover part of the NIR up to roughly 1100 nm.

Applications are diverse. NIR spectroscopy probes molecular vibrations and overtones, used in agriculture, pharmaceuticals, and quality control. NIR imaging enables non-invasive medical diagnostics, skin and tissue studies, and ophthalmology, as well as industrial inspection and remote sensing of vegetation and soils. Night-vision and surveillance systems frequently employ NIR illumination and imaging to operate with reduced visibility in darkness.

Limitations and challenges include atmospheric absorption by water vapor and carbon dioxide, which creates transmission windows and gaps. Thermal emission increases at longer wavelengths, affecting some measurements. Detectors require cooling or specialized materials to reduce noise, and calibration is important to account for spectral response and material properties.