This page provides some background in short-wave infrared imaging and details a few SWIR camera applications using the latest technology of InGaAs cameras currently available on the market.
A bit of background on Short-Wave InfraRed…
What is SWIR?
SWIR is the acronym for Short Wave Infra-Red and refers to non-visible light falling roughly between 1400 and 3000 nanometers (nm) in wavelength. The visible spectrum ranges from 400nm to 700nm, therefore SWIR light is invisible to the human eye. In order to detect SWIR wavelengths, we need dedicated sensors made of In GaAs (Indium Gallium Arsenide) or MCT (mercury cadmium Telluride) as silicon detectors are no longer sensitive to wavelengths larger than 1100 nm. In GaAs sensors are the primary sensors used in typical SWIR range. MCT is also an option and can extend the SWIR range, but these sensors are usually more costly and application dependent.
SWIR light interacts with objects similarly to visible light as it is reflective, consequently it exhibits shadows and contrasts in its imagery. Images from a SWIR camera are comparable to visible images in terms of resolution and detail.
Objects that are almost the same color while imaging in visible region can be easily differentiated using SWIR light, making objects easily recognizable. This is one tactical advantage of imaging in SWIR compared to visible region. Some of the natural emitters of SWIR are ambient star light and background radiance, therefore SWIR is an excellent application for outdoor imaging. Conventional quartz/halogen bulbs also act as a SWIR light source. Depending on the application, some sensors in SWIR cameras can be adjusted to have linear or logarithmic response to avoid saturation.
There are many advantages of using SWIR over a conventional visible sensor. Some applications that are not possible to image in visible range can be imaged using SWIR range. One example, is silicon wafer inspection, which is only possible due to silicon being transparent in the SWIR range. Other examples of materials that are transparent in SWIR region are; Sodium Chloride (NaCl) and Quartz (SiO2). Water vapor is also transparent in SWIR, making SWIR cameras more desirable when imaging through haze or fog. Applications where using SWIR is crucial are detailed in the following section.
SWIR Camera Sensor resolution and pixel pitch
InGaAs cameras bridge the gap between NIR wavelengths in the 950-1700 nm range, where silicon detectors are no longer sensitive. There is a still a gap however in the resolution of InGaAs sensors compared to that of silicon detectors, which can nowadays reach tens of megapixels and sometimes over 100 megapixels.
There are currently 3 ‘standard’ resolutions for SWIR InGaAs sensors on the market, with price increasing as the resolution goes up. Sensor pixel pitch is often related to the resolution with higher resolution sensors tending to have smaller pixels.
QVGA sensors offer a resolution of 320 (H) by 256 (V) pixels with a pixel pitch typically ranging from 20µm to 30µm.
VGA sensors are today the most standard resolution for InGaAs sensors with a resolution of 640(H) by 512 (V) pixels and a typical pixel pitch of 15µm.
As of 2020, only a few megapixel InGaAs sensors were on the market. These sensors have an XSGA resolution of 1280 (H) x 1024 (V) or 1.3 Megapixels. The standard pixel size varies from 12µm for the largest to 10µm for the smallest.
In 2021, Sony is releasing new sensors based on the SenSWIR technology with a pixel pitch of 5µm, making them the smallest pixel pitch on the market for InGaAs-based sensors. High resolution imaging is now possible with 1280 (H) x 1024 (V) or 640 (H) x 512 (V) resolution sensors with 5µm pixel pitch.
To find out more, visit our Goldeye G-130 and G-030 page here.
SWIR Camera Applications
SWIR cameras are used for a variety of applications in different aspects of the industry and research, ranging from inspection, quality control, identification, detection, surveillance, and more. Here we summarize some applications where SWIR range is commonly used. More applications are being discovered all the time.
Laser Beam Profiling
Measurement and analysis of laser-beam characteristics is essential for today’s laser-based applications. In general, beam characterization involves measurement of the beam spatial-energy density distribution, commonly known as the beam profile. Beam-profile analysis provides an understanding of the beam spatial characteristics such as size, shape, position, propagation, and mode structure properties. These measurements enable the required process-specific beam parameters, such as alignment, focus spot size, or beam uniformity (to name a few), to be achieved and maintained for optimum laser system performance.
For lasers with beams that are nearly collimated (paraxial), measurement of the beam-profile characteristics involves sampling the spatial distribution of the beam energy (or power) density in a plane (x and y-axis) perpendicular to the beam propagation path.
The advent of two-dimensional InGaAs detector arrays offers a viable approach for measurement of short-term beam-profile variations as opposed to scanning slit based systems. The individual sensors (pixels) in the detector arrays provide measurement of the energy density at discrete locations across the entire beam measurement plane while the entire array samples the beam simultaneously, thus providing a virtually instantaneous two-dimensional beam-profile data set (or beam image).
This approach can therefore capture the beam characteristics of a single laser pulse. When coupled to a computer, the detector-array data can be digitized and graphically displayed for qualitative visual inspection of the beam mode profile structure using a variety of beam profile plots. Numerical analysis of the profile data provides information such as:
Position: peak position, centroid position;
Relative total energy or power;
Peak power or energy density;
Width and diameter: knife-edge, slit, D86, second moment (D4σ), cross-section widths, fitted widths and more;
Circularity/ellipticity, aspect ratio, beam orientation;
Divergence (requires far-field optical set-up or a focusing lens)
Propagation M2, beam parameter product (requires multiple profile measurements through beam waist).
Mode structure analysis: Gaussian fits, top-hat fits, uniformity, satellite or stray beams and more.
High-Speed SWIR Imaging
The potential applications for high speed imaging in the SWIR are nearly boundless, from tracking to small animal imaging fluorescence, high speed cameras can unlock the mysteries of the world which the naked eye cannot capture outside of the visible spectrum and process fast enough. Unlike some high-speed sensors in the visible that are specifically designed to reach very high speeds thanks to special ROIC architecture by design, high frame rates in the SWIR wavelengths is somewhat limited due to the current InGaAs and MCT sensors on the market.
Another important aspect of high speed imaging is the capability of using a fast shutter to limit motion blur. This directly translates to the ability of the sensor to work with small exposure times (typically in the microsecond range). Shutter efficiency and integrity is crucial to avoid compromising the dynamic range and noise figures.
Besides shot noise, InGaAs cameras are limited by their readout noise and dark current. In a high speed regime where the high frame rate directly translates to short exposure times, the dark current is typically negligible or much smaller compared to the readout noise of the sensor which itself does not depend on exposure time or the temperature of the focal plan array. Therefore, the sensitivity sensitivity is directly defined by the temporal noise level of the sensor.
Free Space Optical Telecommunications (FSO)
An FSO system in its simplest form is illustrated below. The data to be transmitted is converted to a binary format (1 and 0), then into light pulses (ON/OFF). A transmitter (laser source and focusing lens) sends the light pulses, aiming the direction of a receiver. The receiver collects the light pulses, which are then processed and converted. Note that the system can be used in the reverse direction. The system is interfaced at both ends with a physical network (cable, fiber). In more complex implementations, the laser beam can be modulated.
FSO can be used for ground to ground communication: outdoor wireless 2G/3G and 4G networks, to cover the edge of physical networks (“the last mile access”), CCTV surveillance networks, etc. More importantly, it can be used for ground to space (satellite) communications. FSO enables simultaneously establishing a large number of independent links with high throughput; two major advantages compared to the radio bandwidth which is limited by its low directionality and radio frequency throughput (< 40 GHz). For example, Earth-observation satellites only overpass ground stations for a couple of minutes per day, it is critical that the large amount of data they collected can be transmitted in a short amount of time. Even more so for military satellites which very often may only communicate with ground stations within a limited geographical zone. Finally, FSO is the best option for extra-terrestrial communication which may come in use in the next decades… In short, FSO is a fast-growing segment for telecommunications, both in civil and military fields.
FSO communications are limited by a series of factors. Fortunately, some can be overcome by using Short Wave InfraRed (SWIR) (900 – 1700 nm) rather than visible (400 – 700 nm) or Near InfraRed (700 – 900 nm) wavelengths. Optimized transmission Disturbances inherent to air/free space may influence the optical transmission:
- weather conditions such as fog, rain, snow…
- various other effects like water/dust absorption, scintillation, scattering
- physical obstruction: trees, birds or buildings.
These factors attenuate the transmitted signal, leading to a higher number of errors when detecting the signal. Laser power increase is not the solution as the laser power density is limited to class 1M in order to keep an eye-safe environment.
The use of SWIR band lasers is extremely pertinent because of their ability to go through obstacles such as fog or some types of plastics. The recent rise of eye-safe lasers in the SWIR band has allowed a major improvement. A camera based on an InGaAs detector array must be used at the receiver end, as visible cameras are not sensitive to SWIR wavelengths.
The following figures illustrate the advantage of using SWIR cameras in earth-to-earth and earth-to-space configurations. Visibility is increased compared to visible range imaging, demonstrating how SWIR signal propagates efficiently.
In conclusion, Free Space Optics benefits from using SWIR wavelengths (typically 1550 or 1330 nm), rather than shorter infrared ones (typically 785-850 nm).
Learn more reading this application note.
Gated Imaging for Security and Defense
SWIR is considered one of the most versatile technologies for the defense & security sectors. SWIR cameras can provide valuable information such as the ability to identify or recognize a target compared to MWIR & LWIR band cameras. It also brings better vision through harsh weather conditions such as fog or smoke. With a low readout noise and a high dynamic range, SWIR cameras can cope with the challenging requirements of the defense and security industry.
Gated imaging provides the ability to image a specific depth of a scene (i.e 3D imaging). There are multiple applications for gated imaging, including; observation through severe weather conditions or other obscurants, estimation of distance and localization of obstacles (i.e. drone detection) with background suppression, and others. Imaging devices must be fast enough to cope with reflected light from a laser source. SWIR cameras offer precision with the shortest effective exposure time, the shortest rise time, and highest dynamic range on the market. For this reason, these cameras can cover a broad number of situations in the field.
We recommend the WiDy SenS Gated SWIR InGaAs camera for this application. Its minimum exposure time of 100 nanoseconds and its high shutter efficiency combining high extinction rate and low jitter makes it the ideal candidate for range imaging using a pulsed illuminator in the SWIR.
Small Animal Imaging in the NIR-II & SWIR
Small animal imaging is one of the main research areas for preclinical studies, including but not limited to; drug discovery, drug effectiveness, and early detection of cancer. Over time, imaging in the SWIR range became more profitable for scientists studying small animals. The short wave infrared (SWIR) range has several advantages compared to visible and infrared wavelengths in the domain of in vivo imaging. SWIR light provides higher depth of penetration while maintaining high resolution, low light absorption and reduced scattering within the tissue which makes it desirable to study living organisms. One of the biggest advantages of the SWIR range is that the auto fluorescence is negligible. This low level auto fluorescence increases the contrast and sensitivity compared to conventional imaging in NIR and visible ranges. Some of NIR fluorescence imaging contrast agents such as; ICG (indocyanine green), IRDye800CW and IR-12N3, has a non-negligible long tails passing 1500 nm region (NIR-II/SWIR) . InGaAs (indium gallium arsenide) based SWIR cameras fill the gap for imaging in NIR-II/SWIR wavelength range (900-1700nm) where silicon detectors are no longer sensitive.
We recommend following the products for this application: C-RED2 Cooled InGaAs camera thanks to its low dark current and low readout noise.
Learn more by reading this application note.
Carbon Nanotubes Imaging using InGaAs cameras
SWIR cameras can be used for detection of single-walled carbon nanotubes requiring fast frame rates. Single-walled carbon nanotubes (SWCNT) have been established as remarkable fluorophores for probing the nanoscale organization of biological tissues [1,2]. They are stiff, quasi-one-dimensional nano structures, with a small diameter (~1nm) which enables excellent penetration into complex environments, and a large length (100nm to 1µm) which slows down their diffusion and thus allows the tracking of single fluorescent particles. Finally, their bright and stable near-infrared (NIR) fluorescence allows long-term tracking deep in biological tissues without suffering from biological autofluorescence. For example, single-walled carbon nanotubes could be detected in distant regions of the brain extracellular space (ECS) following their injection into the lateral ventricles of young rat brains, and the tracking of their diffusion. This yields novel and quantitative insights about the local morphology and viscosity variations within the brain ECS[1,2]. Such studies require a camera capable of tracking single-walled carbon nanotubes at high speed, making SWIR cameras desirable. One limiting factor for the spatial resolution of such diffusion analyses is the ability to observe displacements of SWCNTs over short time lags.
Learn more by reading this application note.