Introduction to Infrared Vision

Introduction

All objects with an absolute temperature over 0 K emit infrared (IR) radiation. Infrared radiant energy is determined by the temperature and emissivity of an object and is characterized by wavelengths ranging from 0.76 (the red edge of the visible range) to 1000 μm (beginning of microwaves range). The higher the temperature of an object, the higher the spectral radiant energy, or emittance, at all wavelengths and the shorter the peak wavelength of the emissions. Due to limitations on detector range, IR radiation is often divided into three smaller regions based on the response of various detectors.

SWIR: 0.9-1.7 μm

SWIR is also called the «reflected infrared» region since radiation coming from a light source is reflected by the object in a similar manner as in the visible range. SWIR imaging requires some sort of illumination in order to image an object and can be performed only if some light, such as ambient moon light or stars light is present. In fact the SWIR region is suitable for outdoor, night-time imaging.

SWIR imaging lenses are specifically designed, optimized, and anti-reflection coated for SWIR wavelenghts. Indium Gallium Arsenide (InGaAs) sensors are the primary sensors used in SWIR, covering typical SWIR band, but can extend as low as 0.550 µm to as high as 2.5 µm.

A large number of applications that are difficult or impossible to perform using visible light are possible using SWIR InGaAs based cameras: nondestructive identification of materials, their composition, coatings and other characteristics, Electronic Board Inspection, Solar cell inspection, Identifying and Sorting, Surveillance, Anti-Counterfeiting, Process Quality Control, etc… When imaging in SWIR, water vapor, fog, and certain materials such as silicon are transparent. Additionally, colors that appear almost identical in the visible may be easily differentiated using SWIR.

MWIR: 3-5 μm / LWIR: 8-14 μm

MWIR and LWIR regions are also referred to as “thermal infrared” because radiation is emitted from the object itself and no external light source is needed to image the object. Two major factors determine how bright an object appears to a thermal imager: the object’s temperature and its emissivity (a physical property of materials that describes how efficiently it radiates). As an object gets hotter, it radiates more energy and appears brighter to a thermal imaging system. Atmospheric obscurants cause much less scattering in the MWIR and LWIR bands than in the SWIR band, so cameras sensitive to these longer wavelengths are highly tolerant of smoke, dust and fog.

• MWIR collects the light in the 3 μm to 5 μm spectral band. MWIR cameras are employed when the primary goal is to obtain high-quality images rather than focusing on temperature measurements and mobility. The MWIR band of the spectrum is the region where the thermal contrast is higher due to blackbody physics; while in the LWIR band there is quite more radiation emitted from terrestrial objects compared to the MWIR band, the amount of radiation varies less with temperature (see Planck’s curves): this is why MWIR images generally provide better contrast than LWIR. For example, the emissive peak of hot engines and exhaust gasses occurs in the MWIR band, so these cameras are especially sensitive to vehicles and aircraft. The main detector materials in the MWIR are InSb (Indium antimonide) and HgCdTe (mercury cadmium telluride) also referred to as MCT and partially lead selenide (PbSe)
• LWIR collects the light in the 8 μm to 14 μm spectral band and is the wavelength range with the most available thermal imaging cameras. In fact, according to Planck’s law, terrestrial targets emit mainly in the LWIR. LWIR systems applications include thermography/temperature control, predictive maintenance, gas leak detection, imaging of scenes which span a very wide temperature range (and require a broad dynamic range), imaging through thick smoke, etc… The two most commonly used materials for uncooled detectors in the LWIR are amorphous silicon (a-Si) and vanadium oxide (VOx), while cooled detectors in this region are mainly HgCdTe.

IR detectors performance parameters

Signal-to-noise ratio (S/N)

It is defined as the ratio between the signal power and the noise power

P = Incident radiant power received by the detector [W] NEP = Noise Equivalent Power. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth [W] The higher this ratio, the best signal you get. To improve the S/N, infrared detectors must be cooled. Several cooling methods are available including thermoelectric cooling, cryogenic cooling (e.g. using liquid nitrogen) and mechanical cooling such as stirling coolers.

Responsivity R

Responsivity is the ability of the detector to convert the incoming radiation into an electrical signal. Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input.

S = Signal output [V] P= Incident radiant power received by the detector [W/cm2] A = Detector Active Area [cm2]

Noise equivalent power (NEP)

A photodetector produces some noise output with a certain average power even when it does not get any input radiation. This noise output is proportional to the square of the r.m.s. voltage or current amplitude. The noise-equivalent power (NEP) of a detector is the optical input power (P) which produces an additional output power identical to the noise power for a given bandwidth (Δf). In other words, the NEP is the light power required to obtain a signal to noise ratio S/N of 1, that is, the light level required to produce a signal current equivalent to the noise current. The units of NEP are watts per square root hertz. NEP indicates the lower limit of light detection: a smaller NEP corresponds to a more sensitive detector.

P = Incident radiant power received by the detector [W] A = Detector active area  [cm2] Δf = Noise bandwidth [Hz] S/N = Signal to Noise ratio

Specific detectivity D* (D-star)

D* is the photo sensitivity per unit active area of a detector. D* is conveniently used to compare the performances of various detector types since it is area-independent. D* is the signal-to-noise ratio at a particular electrical frequency, and in a 1 Hz bandwidth when 1 Watt of radiant power is incident on a 1 cm² active area detector. In other words it is equal to the reciprocal of the noise-equivalent power (NEP), normalized per unit area.

P = Incident radiant power received by the detector [W] A = Detector active area  [cm2] Δf = Noise bandwidth [Hz] S/N = Signal to Noise ratio
In general the measurement conditions of D* are expressed in the format of D* (X, Y, Z), where X is the temperature [K] or wavelength [μm] of a radiant source, Y is the chopping frequency [Hz], and Z is the noise bandwidth [Hz]. The units of D* are centimeter-square root-hertz per watt, sometimes referred to as “Jones” units. The higher D*, the better the detector. D* values are very high

Noise equivalente temperature difference NETD

NETD is a widely used performance parameter that characterizes the sensitivity of thermal imaging sensors. NETD is the amount of incident signal temperature that would be needed to match the internal noise of the detector (such that the signal-to-noise ratio is equal to one). Essentially, it specifies the minimum detectable temperature difference. Typically NETD is expressed in units of Kelvin (K). Cooled infrared camera systems typically have low noise levels, in the range of 10 – 30mK. Uncooled infrared cameras systems are typically noisier, in the range of 30 – 120mK.

One important parameter that needs to be taken into account when specifying the NETD value of a thermal imaging camera is the lens aperture (or f-number). In fact, the lens f-number will directly affect the sensitivity of the camera. NETD values of different detectors can be compared only by using a lens with the same f-number.

Optical coatings

ANTI-REFLECTIVE AR COATING

Anti-reflective (AR) coatings are thin films applied to surfaces to reduce their reflectivity through optical interference. An AR coating typically consists of a carefully constructed stack of thin layers with different refractive indices. The internal reflections of these layers interfere with each other so that a wave peak and a wave trough come together and extinction occurs, leading to an overall reflectance lower than that of the bare substrate surface. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce ghosting. Perhaps the simplest, most common anti-reflective coating consists of a single layer of Magnesium Fluoride (MgF2), which has a very low refractive index (approx. 1.38 at 550 nm)

HARD CARBON ANTI-REFLECTIVE HCAR COATING

HCAR is an optical coating commonly applied to Silicon and Germanium designed to meet the needs of those applications with optical elements exposed to harsh environments, such as military vehicles and outdoor thermal cameras. This coating offers highly protective properties coupled with good anti-reflective performance, protecting the outer optical surfaces from high velocity airborne particles, seawater, engine fuel and oils, high humidity, improper handling, etc.. It offers great resistance to abrasion, salts, acids, alkalis, and oil.

ATHERMALIZATION

Any material is characterized by a certain temperature expansion coefficient and responds to temperature variations by either increasing or decreasing its physical dimensions. Thus, thermal expansion of optical elements might alter a system’s optical performance causing defocusing due to a change of temperature. An optical system is athermalized if its critical performance parameters (such as Modulation Transfer Function, Back Focal Length, Effective Focal Length, …) do not change appreciably over the operating temperature range.
Athermalization techniques can be either active or passive. Active athermalization involves motors or other active systems to mechanically adjust the lens elements’ position, while passive athermalization makes use of design techniques aimed at compensating for thermal defocus by combining suitably chosen lens materials and optical powers (optical compensation) or by using expansion rods with very different thermal expansion coefficients that mechanically displace a lens element so that the system stays in focus (mechanical compensation).

Basic optics definitions

FOCAL length

Lenses are commonly identified by their focal length. Focal length and field of view (FOV) are related by the following formula:

Where
d = Focal Plane Array diagonal (mm),
f = focal length (mm),
FOV = field of view (degrees). FOV is the angular subtense (expressed in angular degrees or radians per side if rectangular, and angular degrees or radians if circular) over which the optical system will integrate all incoming radiant energy.
According to the above formula, as the focal length increases, the field of view for that lens will be narrower and viceversa. For instance, long range thermal infrared surveillance applications require long focal length lenses.

F/#
The f/number determines the light gathering power of the lens and therefore affects the sensitivity of the optics-camera system. The f/number of an optical system is the ratio of the focal length of the lens to the  diameter of the front lens element.

f = focal length
A = diameter of the front lens element
As the focal length of a lens is increased, the diameter of the front lens element must be increased to keep the system f/number constant. Sensitivity of IR cameras can be increased by choosing the appropriate lens. Uncooled cameras equipped with uncooled microbolometer detectors are typically less sensitive than cooled cameras equipped with quantum detectors. Therefore a camera equipped with a low-sensitivity detector must be run with a lens that has a low  f/number (i.e. wide aperture) to have comparable sensitivity to a cooled camera. However, using such wide-aperture lenses limits the depth of field that can be obtained by the imaging system. In contrast, a cooled camera system can be  operated at higher f/numbers  without significantly compromising system sensitivity.
Long range thermal infrared surveillance applications require long focal length lenses, and the cost of lenses increases rapidly with focal length for uncooled camera systems and rather slowly for cooled systems

Spatial resolution
Diffraction limits the resolution possible with an objective lens. Each point of the object to be viewed is imaged as a spot pattern called the an Airy disk. Its diameter is given by the following formula

Where
λ = wavelength of the radiation
F/N = F/#
Two adjacent points can be  resolved (i.e. distinguished) when the center of the Airy disk for the first point occurs at the first minimum of the Airy disk of the second. This is known as the rayleigh resolution limit:

Clearly, as the FN / wavelength increases, the resolution limit increases proportionally. Therefore, in order to achive a similar resolution limit, LWIR lenses working at λ = 10 μm will require a much lower FN (larger apertures) than MWIR lenses working at λ = 4 μm.