In flat screen LCD televisions, there is a large light at the back of the TV. The light travels to the front screen through millions of tiny units called pixels (picture elements). One of these is shown in Figure 2.3.19 (a) and (b). Each unit has three cells, with red, blue, or green filters, each controlled independently. When the voltage across a liquid crystal is switched off, the liquid crystal passes the light through the particular filter. One can vary the picture contrast by varying the strength of the voltage applied to the liquid crystal.

Infrared spectrum

How are the SWIR wavelengths imaged? Imaging detectors have an absorbance layer that receives light photons and absorb their energies. To be a good detector they must be able to convert the energies into an electrical current, rather than into heat or even reflecting the photons away. In materials that do this conversion well the absorbance layer is typically a semiconductor crystal whose bandgap energy is less than the energy carried by the photon's wavelength.

Many crystals and solutions rotate the plane of polarization of light passing through them. Such substances are said to be optically active. Examples include sugar water, insulin, and collagen (see Figure 2.3.20). In addition to depending on the type of substance, the amount and direction of rotation depends on a number of factors. Among these is the concentration of the substance, the distance the light travels through it, and the wavelength of light. Optical activity is due to the asymmetric shape of molecules in the substance, such as being helical. Measurements of the rotation of polarized light passing through substances can thus be used to measure concentrations, a standard technique for sugars. It can also give information on the shapes of molecules, such as proteins, and factors that affect their shapes, such as temperature and pH.

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To examine this further, consider the transverse waves in the ropes shown in Figure 2.3.13. The oscillations in one rope are in a vertical plane and are said to be vertically polarized. Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves. For EM waves, the direction of the electric field vector E is analogous to the disturbances on the ropes (Figure 2.3.14).

SWIRcamera

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Polarizers are composed of long molecules aligned in one direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave (see Figure 2.3.13).

The lenses are filters which either block circularly polarized light or convert it to linearly polarized light which then is seen.

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In contrast, light that is plane polarized (also called linearly polarized) has E oriented in one specific direction in space (Figure 2.3.12).  The polarization direction is defined by the orientation of E (as opposed to B).

What is the SWIR wavelength region? The shortwave infrared (SWIR) has various definitions, depending on the author. Like most definitions, the meaning of the label has also evolved over time. SWIR is a sub-region of the electromagnetic wavelength spectrum that scientists historically have called the Infrared and is included in the region that spectroscopists referred to as the Near Infrared (NIR). As detector technology developed, the NIR region was bounded by where the human eye can no longer see and where silicon detectors become transparent and lose sensitivity, which is slightly longer than 1 micron (1 µm) in wavelength. For the purposes of this website, which is provided by Princeton Infrared Technologies (PIRT) a SWIR imaging detector company, the SWIR region runs from 1000 to 2600 nm (1.0 to 2.6 µm). This is bounded on the shorter wavelength end by the NIR region from 750 to 1000 nm (0.75 to 1 µm) and on the longer side by the dominant atmospheric absorbance band from 2600 to 3000 nm (2.6 to 3.0 µm). This longer wavelength SWIR boundary is further re-enforced by the end of the transmission window for glass and quartz optics, which is around 2.6 µm.

Polaroid sunglasses are familiar to most of us. They have a special ability to cut the glare of light reflected from water or glass. Polaroids have this ability because of a wave characteristic of light called polarization. What is polarization? How is it produced? What are some of its uses? The answers to these questions are related to the wave character of light.

Glass and plastic become optically active when stressed; the greater the stress, the greater the effect. Optical stress analysis on complicated shapes can be performed by making plastic models of them and observing them through crossed filters, as seen in Figure 2.3.21. It is apparent that the effect depends on wavelength as well as stress. The wavelength dependence is sometimes also used for artistic purposes.

Figure 2.3.16 illustrates how the component of the electric field parallel to the long molecules is absorbed. An electromagnetic wave is composed of oscillating electric and magnetic fields. The electric field is strong compared with the magnetic field and is more effective in exerting force on charges in the molecules. The most affected charged particles are the electrons in the molecules, since electron masses are small. If the electron is forced to oscillate, it can absorb energy from the EM wave. This reduces the fields in the wave and, hence, reduces its intensity. In long molecules, electrons can more easily oscillate parallel to the molecule than in the perpendicular direction. The electrons are bound to the molecule and are more restricted in their movement perpendicular to the molecule. Thus, the electrons can absorb EM waves that have a component of their electric field parallel to the molecule. The electrons are much less responsive to electric fields perpendicular to the molecule and will allow those fields to pass. Thus the axis of the polarizing filter is perpendicular to the length of the molecule.

For visible and NIR wavelengths, the Silicon (Si) molecule is a good detecting material. Depending on the detector structure Si can convert over 90% of photons into electrons, a measure known as Quantum Efficiency (QE). However, as the wavelengths lengthen past 900 nm (0.9 µm), the efficiency in capturing photons becomes weaker and photons start passing through Si so QE falls, and goes below 1 % past 1100 nm, even with processing tricks used to extend sensitivity.

The Sun and many other light sources produce waves in which E (and B, though it is not shown) are not preferentially oriented – they exist in every direction perpendicular to the direction of propagation (see Figure 2.3.11). Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization.

As detector technology has developed, there are now low noise Vis-SWIR InGaAs imaging detectors that cover the visible to the middle of the SWIR range, 400 to 1700 nm (0.4 to 1.7 µm), such as the PIRT MVCam, and 1280SciCam. In the past, to extend beyond 1700 nm required changing the InGaAs crystal, this introduced excess dark current and image non-uniformities that made such imaging detectors unattractive to the market, causing some to commercialize low quantum-efficiency (QE) technologies such as colloidal quantum dots (CQD). This is now changing. PIRT has just introduced, a megapixel camera that responds from 400 to 2050 nm (0.4 to 2.05 µm) utilizing a completely different detector structure, one using InGaAs/GaAsSb type-II super lattice (T2SL) technology. This is yielding superior image quality with QE response of >30% at 1550 nm and >20% at 1900 nm (at 1.55 and 1.9 µm) with no imaging artifacts.

LWIR

Only the component of the EM wave parallel to the axis of a filter is passed. Let us call the angle between the direction of polarization and the axis of a filter θ. If the electric field has an amplitude E, then the transmitted part of the wave has an amplitude E cos θ (see Figure 2.3.18). Since the intensity of a wave is proportional to its amplitude squared, the intensity I of the transmitted wave is related to the incident wave by I = I0 cos2 θ, where I0 is the intensity of the polarized wave before passing through the filter.

SonySWIR

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Another interesting phenomenon associated with polarized light is the ability of some minerals and other crystals to split an unpolarized beam of light into two polarized beams (Figure 2.3.22). Such crystals are said to be birefringent.

Watch the first 6 minutes of the video below to see a practical overview of plane polarized light, using crossed polarizers, and how a third polarizer (which is how many minerals act) can be used to increase light output from crossed polarizers.

Swirmwir lwir

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Figure 2.3.17 shows the effect of two polarizing filters on originally unpolarized light. The first filter polarizes the light along its axis. When the axes of the first and second filters are aligned (parallel), then all of the polarized light passed by the first filter is also passed by the second. If the second polarizing filter is rotated, only the component of the light parallel to the second filter’s axis is passed. When the axes are perpendicular, no light is passed by the second.

For the NIR-SWIR range there are various materials that have been tried for imaging. Commercial production currently uses these molecules for detection: InGaAs, NIR extended Si, Ge, Si-Ge, PbSe colloidal quantum dots (CQD), InGaAs/GaAsSb Type 2 Superlattices, InSb, or HgCdTe. In general, for the 1000 to 1680 nm (1.0 to 1.68 µm) range, the material known as lattice matched InGaAs (which is really In.53Ga.47As) offer the best combination of high QE, dynamic range, and image quality with low Size, Weight and Power. When imaging applications need to cover 1000 to 2100 nm (1.0 to 2.1 µm), new solutions based on Type II superlattice structures that have layers of InGaAs alternating with GaAsSb are now available that provide the best QE and image quality of commercially available cameras. In the future one will see Type 2 Strained Layer Superlattices which are more complicated combinations of InGaAs/GaAsSb and these will allow detection to 2600nm. Several alternative products claim to reduce cost compared to InGaAs designs which manually hybridize the photodiode layer to the Read-Out Integrated Circuit (ROIC) for each Focal Plane Array (FPA). However, PIRT uses wafer-scale bonding to substantially reduce the assembly costs, while the so-called low-cost competitors have discovered they still need costly thin-film processing to achieve reliable, and consistent imaging performance.

Polarizing filters have a polarization axis that acts as a slit. This slit passes electromagnetic waves (often visible light) that have an electric field parallel to the axis. This is accomplished with long molecules aligned perpendicular to the axis as shown in Figure 2.3.15.

Electromagnetic waves are transverse waves consisting of varying electric and magnetic fields that oscillate perpendicular to the direction of propagation and perpendicular to each other.

Why are the SWIR, NIR-SWIR and Vis-SWIR bands valuable? The SWIR wavelength range is important because, chemical signatures due to chemical absorption are imprinted on reflected or transmitted light. The images are easier for operators to understand because viewing NIR and SWIR images are like viewing visible light images. There is also reduced scattering in the SWIR band allowing for sharper images while looking through the atmosphere at objects far away. For special cases, the SWIR band has value for thermal imaging as well, such as when object temperatures are >100°C, or need to be viewed through glass or quartz optics or windows.

NIRSWIR

MWIR

20231220 — Infrared camera lenses capture things that the human eye cannot see. This means that anyone without an infrared camera lens will not see any photos you take.

Each of the separated rays has a specific polarization. One behaves normally and is called the ordinary ray (o or ω), whereas the other does not obey Snell’s law and is called the extraordinary ray (e or ε). Birefringent crystals can be used to produce polarized beams from unpolarized light. Some birefringent materials preferentially absorb one of the polarizations. These materials are called dichroic and can produce polarization by this preferential absorption. This is fundamentally how polarizing filters and other polarizers work. We will use the property of birefringence to help us identify and distinguish minerals in thin section!

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While you are undoubtedly aware of liquid crystal displays (LCDs) found in watches, calculators, computer screens, cellphones, flat screen televisions, and other myriad places, you may not be aware that they are based on polarization. Liquid crystals are so named because their molecules can be aligned even though they are in a liquid. Liquid crystals have the property that they can rotate the polarization of light passing through them by 90 degrees. Furthermore, this property can be turned off by the application of a voltage, as illustrated in Figure 2.3.19. It is possible to manipulate this characteristic quickly and in small well-defined regions to create the contrast patterns we see in so many LCD devices.