The extended SWIR – Short-wave Infrared (eSWIR) – generally refers to wavelengths of light that start where traditional lattice-matched InGaAs stop absorbing, around 1700 nm, up to 2500 nm. Only a handful of detector technologies are capable of detecting photons in this spectral range, and solar illumination in that band is lower than at shorter wavelengths.
Despite the lower optical power incident on the Earth’s surface, eSWIR imaging finds utility in outdoor applications like wildfire detection, piercing through degraded visual environments (DVEs), and long-range detection and surveillance. It is also used in active illumination systems employing optical sources such as lasers or halogen for applications in security, waste sorting, art analysis, and gas detection. As the laser community finds more applications for longer wavelength emitters, Extended SWIR (eSWIR) detectors are becoming increasingly useful as beam profilers for characterizing emitter performance for sensitive applications such as medical devices.
Reduced atmospheric scattering in the Extended SWIR (eSWIR) band enables excellent imaging through smoke, haze, and other atmospheric particles, and enables high resolution long range imaging.[2][3]
Other use cases for eSWIR take advantage of is the strong water absorption peak around 1950 nm, which allows medical laser applications to improve laser ablation efficiency.[4]
Sometimes Extended SWIR (eSWIR) can be mistaken for a thermal imager, but the blackbody radiation of objects is not significant in the eSWIR band at room temperatures.[5] Objects need to be hot, typically 200 °C or greater, to emit enough Extended SWIR (eSWIR) photons for eSWIR sensors.
Despite this, eSWIR can be very useful for inspecting hot glass objects due to their thermal response and transparency; this combination can be used to examine stresses inside the object.
Current technologies capable of detecting eSWIR photons are extended (lattice-mismatched) InGaAs and Type-II Superlattice (T2SL) materials, HgCdTe, and Colloidal Quantum Dot photodiodes (CQD). All technologies except CQDs must operate with integrated coolers to reduce their dark noise at temperatures that can be down to 77 K.
A primary consideration for detector technologies is cost. An eSWIR CQD sensor, such as the Acuros -002 and -004 products, enables the lowest-cost solution at the highest resolution due to the simple manufacturing process and room temperature operation. CQD sensor technology also has the ability to scale up sensor resolutions. For example, the industry-leading Acuros 2.1MP format camera was first offered by SWIR Vision in 2018. Another consideration is the size, weight, and power of the solution; extended InGaAs and HgCdTe cameras come with large dimensions and high power consumption, but T2SL and eSWIR CQD cameras can come in low size, weight, and power (SWaP) packages.
From budget-constrained to mobile and large scale use-cases, Acuros eSWIR CQD sensors offer the most competitive solution. However, it may be beneficial to observe competing technologies for extreme sensitivity applications like methane detection.
RELATED LINKS
REFERENCES
[1] Wikimedia: Solar spectrum en.svg
[2] Hyperphysics
[3] Duke | Information Space Project
[4] National Library of Medicine
[5] E-education | Blackbody Radiation
[6] Spectralcalc
