ZEPHIR CAMERAS FOR SCIENCE AND INDUSTRY - COOLING SWIR SENSORS - WHY A TE4 AIR-COOLED CAMERA
Dark current is a critical parameter when one is looking to acquire a scientific and/or industrial imaging camera, especially in the short-wave infrared (SWIR) region. Very careful attention must be paid to the cooling method used to optimize this parameter. Multiple cooling technologies are available, each having certain benefits and drawbacks. In its new ZephIR line of SWIR cameras, Photon etc. uses a four stage thermoelectric
The ZephIR 1.7 is an InGaAs camera sensitive in the 800 nm to 1700 nm range. The ZepIR 2.5 and 2.9 are HgCdTe cameras sensitive in the 0.85 μm to 2.5 μm and 0.85 μm to 2.9 μm ranges respectively. With their integrated TE4 air-cooled systems, these cameras reach an operating temperature of -80 °C and possess dark currents of 300 e-/p/s s (ZephIR 1.7), 30 Me-p/s (ZephIR 2.5) and 340 Me-/p/s (ZephIR 2.9).
OVERVIEW OF COOLING METHODS - Thermoelectric Cooling
Thermoelectric (TE) stages are solid-state devices composed of two differents faces. Those stages uses Peltier effect to generate a temperature difference between the two faces. Semiconductors with different electron densities, n-type and p-type (see FIG. 1), are placed in series and connected with a conducting material on each side. The passage of an electrical current through the junction induces a heat flow from one face to the other, creating a cold and a hot side. The cold face absorbs heat which is carried to the other side where the heat sink is located. TE stages are usually connected side by side and sandwiched in two insulators. Water or air cooling is typically used to dissipate the heat accumulated in this process. The temperature that can be reached by TE coolers is related to the number of stages being used. Hence, for more effective cooling, it is possible to stack several stages. This is the case of Photon etc’s SWIR sensors, where four thermoelectric stages are cascaded together to lower the temperature. With four stages, a ΔT* of 120 °C can be reached. This results in a detector operating temperature of -80 °C (193 K) with proper heat extraction, at 25 °C ambient temperature.
Stirling coolers based devices operate on a closed Stirling cycle where a nearly ideal gas (usually helium) is being repeatedly compressed and expanded. A schematic of an ideal Stirling cycle and a stirling cooler is shown in figure 2. In order to obtain the change in pressure and temperature of the gas, two pistons are required: a displacer which put alternatively the gas in contact with a cold and hot reservoir and a working piston which is moved by the expansionand compression of the gas. A regenerator is also required and act as an internal heat
exchanger. Following the Ideal Gas Law, heat from the surrounding is being absorbed by the expanded gas during the expansion which makes it colder. When the gas is being compressed, heat is ejected from the gas to the atmosphere.
Four steps are needed in an ideal cycle: 1. Isothermal compression: heat ejected. 2. Isochoric process: the system is kept a constant volume. Heat is rejected to the regenerator. 3. Isothermal expansion: heat is absorbed
by the gas. 4. Isochoric process: the system is kept a constant volume. Heat is absorbed from the regenerator.Stirling cooled detectors can reach -210 °C (63 K).
LIQUID NITROGEN COOLING
Detectors can also be cooled with liquid nitrogen to reach -196 °C (77 K). In liquid nitrogen cooled system, the detector is placed in a cryostat that holds a dewar where the liquid nitrogen is stored. Different type of detector chamber are available. The detector can be connected to a copper cold finger inserted in the dewar. The finger carry the heat from the detector to the liquid nitrogen tank.
WHY A TE4 AIR-COOLED SYSTEM?
Each cooling method possess his advantages and downsides, the application will dictate the appropriate approach. Liquid nitrogen is used, for example, with MCT sensors working in the long wavelength infrared (LWIR - 8-15 μm) range to reduce thermal noise. It is also used for application requiring high cooling capacity and stability. Liquid nitrogen cooled sensor also possess long lifetime and relatively low initial cost. The main disadvantage is the regular need of liquid nitrogen supply, the limited autonomy and the time required to stabilize the
MAIN ADVANTAGES OF TE COOLED AIR SYSTEM:
› No moving parts
› Highly reliable
› Long lifetime
› No maintenance
› Low dark current
› Low readout noise
Unlike Stirling coolers, TE stages do not possess moving parts, which is a significant advantage for the overall durability and maintenance needs of the camera. No moving parts also means no vibration, which is perfectly suited for high magnification SWIR microscopy. Because of their long lifetime and reliability, TE cooled cameras are ideal for industrial process control or any other applications implying long cycles of operation.
Their small size is ideal to manufacture compact sensors that can be easily installed in either academic laboratories or industrial
An air-cooled system does not require a continuous flow of cold water in the camera. This greatly facilitates its integration in various environments.
LOW DARK CURRENT
The small bandgap of InGaAs (~0.75 eV at room temperature) and even smaller bandgap of HgCdTe (~0.15-0.43 eV) imply that electrons will be more likely to reach the conduction band and contribute to the dark current. For this reason, sensors based on InGaAs or HgCdTe possess a high intrinsic dark current at room temperature. For example, the dark current of InGaAs-based sensors approximately triples with every 10 ˚C increase. Cooling these sensors is crucial to attaining a good dynamic range and higher sensitivity. Photon etc. has thus integrated a four stage
Peltier module into their cameras reaching-80 °C, a temperature which significantly lowers the dark current of the camera. The ZephIR 1.7 dark current is typically 300 e-/pix/sec. Stirling cooling also provides really low temperature and offers a good solution for applications requiring long acquisition time with low dark current or low power consumption. Stirling cooling is efficient and compact however, it induces vibration, it has a limited lifetime and high initial cost to which rework cost need to be added later on. In industries where long life time and easy maintenance are essential, thermoelectric cooling seems to be the best suited option. It is also vibration-less and user friendly when compared to Stirling and LN2 cooling respectively, two advantages that are also often mandatory in advanced scientific imaging. This is why Photon etc. decided to go in this direction for its ZephIR line of cameras.
The suitable cooling method strongly depends on the application. ZephIR TE4 air-cooled systems are best suited for industrial application as well as demanding scientific imaging. This cooling system is more reliable, simpler and less expensive than other available cooling technologies. The TE4 cooled ZephIR InGaAs and HgCdTe are ideal cameras for state-of-the-art scientific research and a wide variety of industrial applications. They are perfectly suited for hyperspectral microscopy, fast broadband imaging, line-scanning systems, industrial sorting and quality control, thus broadening Photon etc’s scientific solutions to the SWIR
Photon etc. is represented by MD INNOVATION TECH GmbH, D-95659 Arzberg, www.md-innovationtech.com, in Germany, Austria, Switzerland, parts of South and East Europe.
MD INNOVATION TECH GMBH distributes hyperspectral imaging products / microscopy, IR cameras, high-end laser diode modules, laser sources and systems as well as customized laser systems for marking, engraving, cutting, welding, drilling, brazing, cladding. All our products have special features. The MD INNOVATION TECH sales regions cover Germany, Austria, Switzerland, BeNeLux, Scandinavia, parts of South and East Europe.
MD INNOVATION TECH GmbH
Phone: +49 9233 7157-745 or 746
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