Infrared detectors have negative temperature performance coefficients: they become more sensitive when cooled. Marlow can provide thermoelectric coolers (TECs) that use single-stage cooling for near-room temperature operation through six-stages to cool to about 170 Kelvin     (-103°C).

  A typical six-stage production thermoelectric cooler pumps 16 milliwatts from 175 to 300 K, consuming four watts of input power. This is a temperature difference (delta T) of 125°C. The mercury cadmium telluride (MCT) infrared focal plane array (IRFPA) is mounted with thermal epoxy on the top stage of the thermoelectric cooler. When operated at 175 K, the IRFPA creates a thermal image in the 3-5 micron mid-wave infrared spectrum. When viewed in pitch black darkness by the thermal weapon sight (TWS), the user can "see in the dark." He does this by seeing heat photons that are passively emitted by the objects in front of the infrared camera rather than by seeing them in the visible spectrum where a light source must illuminate the scene.

  One infrared imaging system that uses an IRFPA is a forward-looking infrared (FLIR) system.

  Other materials that benefit from cooling, used by FLIR or other radiometric sensing systems, are lead sulfide (PbS), lead selenide (PbSe), silicon, indium-doped gallium arsenide (GaInAs), indium arsenide (InAs), gallium indium arsenide phosphide (GaInAsP), gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), and other quaternaries (four-element combinations). Many of these materials can also be made into emissive sources such as lasers and light-emitting diodes. While they also benefit from cooling, the TECs are used in the source applications mainly for temperature stabilization.

  It is important to differentiate between IRFPAs that can use the cooling capability of TECs (to about 175 K) and those that require much colder operating temperatures (down to 77 K). The latter are liquid nitrogen or Stirling cycle or Brayton cycle mechanical refrigerators capable of driving the IRFPAs to deep cryogenic temperatures. Others, such as liquid helium (4 K) or liquid hydrogen (28 K), require even lower operating temperatures. Marlow does not compete in these areas. The materials that require these extreme cold temperatures are: intrinsic silicon, mercury-doped germanium, long-wavelength mercury cadmium telluride (8-20 microns), indium antimonide, platinum silicide, lead tin telluride, and other derivatives of doped germanium.

  Marlow's devices can cool the short- and mid-wave mercury cadmium telluride materials that are doped to operate in the 1-5 micron spectral region at 175 K.

  Multi-stage thermoelectric coolers of more than three stages must usually be placed into vacuum enclosures that are pumped to a 10e-5 vacuum level for optimum operation. The base of the vacuum enclosure is the heat sink for the thermoelectric cooler and the cooler is usually soldered to this base with compliant solder.

Advantages of Thermoelectrics for this Application

  Thermoelectric coolers are lightweight, small, and rugged. They operate with good power efficiency, have no moving parts, cause no vibration, and after cleaning can be used in vacuum enclosures. They function in any orientation, require no connective bonding, need no connecting lines (except for two power leads). They can support ball, stitch or ultrasonic bonding.

Specific Marlow Advantages

  Marlow is well acquainted with the infrared detector industry. Our engineers, designers, and customer account managers have worked with forward-looking infrared and infrared detector manufacturers for a number of years. We have also accumulated over 25 years of custom design experience providing state-of-the-art thermoelectric solutions in the harshest and most demanding environments.

  Applications such as forward-looking infrared (FLIR) imaging and radiometric tracking that use thermal sensors need variable-temperature radiation sources for their calibration and check-out. These systems differ from visible-spectrum cameras. The latter use RETMA charts (visible black and white pictures or charts that depict varying resolution lines and circles on which a camera focuses) to calibrate the sensitivity and resolution of a pick-up sensor, detector, or photosensitive surface (i.e., a camcorder).

  An infrared sensor uses pixels (picture recording elements set in linear or X by Y matrix arrays) that record the scene being viewed. If the detectors are arranged in linear arrays, the array must be scanned with a mechanical or optical chopper to see the "object space." If the detectors are arranged in an X by Y matrix (staring array), the detectors need not be chopped, as all pixels are looking at the scene all the time. The scene is then integrated into a storage buffer that is interrogated to clock the picture out of the infrared focal plane array (IRFPA). This creates a TV-like picture for the viewer or tracker.

  The thermoelectric thermal reference source calibrates an IRFPA in the thermal (temperature variation) domain while the RETMA chart calibrates a visible camera in the visible spectrum.

Advantages of Thermoelectrics for this Application

  The TTRS is usually mounted in the telescope so that it can be "viewed" once or twice during each scan of the scene. Viewing the TTRS with the infrared detectors enables the calibration of the gain in the preamplifiers that are receiving the output of the detector signals for responsivity variations in the detectors. They also determine the linearity and offset of the difference in a two-color correction algorithm. The TTRS is usually viewed by rotating a mirror into the optical path that projects the image of the TTRS onto the IRFPA.

  The TTRS is normally heat-sunk to the wall of the FLIR housing to enable optimum heat-sinking for the thermoelectric heat pump.

  Thermoelectric coolers are small, rugged, low in mass and weight, consume little power, impart no vibration, have no moving parts, and are as reliable as solid-state semiconductors. Thus they are ideal for this application.

Specific Marlow Advantages

  Marlow has been the thermoelectric supplier of choice for most of the major system integrators for over 20 years. We have considerable experience in forward-looking infrared system requirements: three of our engineers have worked on FLIR systems at Raytheon/TI, Honeywell, and Lockheed Martin for many years. We specialize in custom designs to develop and manufacture best solution for a customer's particular boundary conditions and needs.

  We provide free-standing sealed and unsealed TTRS units. We also install them in hermetically sealed packages.