Infrared technique: period 1960 – 1970
Partly due to the success of the infrared guided missiles developed in America such as the Sidewinder (1955), the development of infrared detectors at TNO was continued. In 1960, this led to the first TNO thermal imager with a self-built indium antimonide (InSb) detector with liquid nitrogen cooling and a tube amplifier. Making a picture of approximately 100 x 100 pixels took about 10 seconds.
InSb crystal production
Retractor: InSb detectors require an InSb single crystal with a specific axis direction of the crystal and being highly pure. In this self-developed retractor installation, purified InSb was made. InSb was heated to just above the melting temperature in an H2 atmosphere to prevent oxidation. A seed crystal with the right axis direction hung above the surface of this melt and was carefully lowered until the melt. After contact, the crystal was slowly raised by the retractor in such a way that the cooling and crystallisation proceeded evenly.
The crystals were sawn into slices and blocks with the crystal saw (above) developed at the laboratory. After further processing, the crystals could serve as light-sensitive cells in various types of detectors.
Physics of heat phenomena
It soon became clear that infrared technology does not only consist of building cameras. The interpretation of infrared images requires another type of physical knowledge, something that is illustrated in the following picture. A number of heat transfer phenomena are responsible for the surface temperature of an object and its environment, and thus for the perceived contrast with a thermal infrared or a thermal imaging camera.
Many environmental factors influence the thermal (temperature) contrast and equilibrium. In addition, this is influenced by material properties such as:
- specific density
- specific heat
- the thermal conduction coefficient
- the surface roughness
- the spectral and specular reflection
If the thermal balance is in equilibrium, then the surface temperature
constant. If not, the temperature increases or decreases. Thorough knowledge of physical transport phenomena is a requirement for correct contrast interpretation of infrared emissions.
To further investigate the possibilities of infrared sensors, the main activity in the early 1960s was the realisation of a field camera with a self-developed InSb detector.
Thermal imager with a large field of view
In 1963, the first real thermal imager with a large horizontal field of view was developed:
- geometric resolution: 1 mrad
- thermal resolution: 0.1 ° C
- field of view: 6 ° vertical, 360 ° horizontal
- time for a recording of 6° x 9°: 10 seconds
- main mirror: D = 30 cm, f = 58 cm
- detector: InSb photo-Voltacel of 0.5×0.5 mm
Examples of night shots with this camera of ships and military vehicles can be seen below. The warm spots (white) on the ship’s hull of the cargo ship indicate the location of the engine room, the chimney, and the galley. The compartments of the oil tanker show that they are filled with hot oil. The vehicles, pictured a few hours after the engine is switched off, retain their heat for a long time. Striking are the traces in the terrain. The results gave rise to the development of more extensive infrared research activities when an Israeli warship (Eilat) was sunk by an Egyptian Russian anti-ship missile (SS-N-2 Styx) in June 1966.
Airborne infrared scanner FLORIS
In the following years, the efforts in the infrared field increased sharply, both in improving the recording equipment and in the acquisition of target signatures. In 1966, the Floris aircraft scanner was developed which allowed measuring the infrared signature of Russian warships from a Neptune Navy plane and especially of the infrared decoys that were developed behind the iron curtain. These data were of great importance to the Naval Intelligence Service. Those signatures were exchanged with intelligence services from other ‘friendly countries’.
This research also led to the development of an infrared radiation model for Dutch Navy vessels and the devising of countermeasures to limit infrared threats. All these activities were discussed in the Working Group Infrared and Electro-optical Countermeasures (Weed) and the ‘Chimney Group’. The Infrared group of the TNO Physics Laboratory was also represented in these communities as well as various Royal Navy divisions, including the operational department that easily could provide vessels for experiments.
The other armed forces also saw the importance of infrared. For example, at the NV Optische Industrie “De Oude Delft” the Orpheus infrared scanner was developed for a reconnaissance pod under the Lockheed F104 (Starfighter) in 1968. The Physics Laboratory TNO and NLR played a supporting role on this assignment by the Royal Netherlands Air Force (KLu).
Combination Brightness Amplifier Infrared Viewer (CHIK)
In 1969, TNO developed the CHIK (Combination Brightness Amplifier Infrared Viewer). CHIK’s single detector allowed to detect persons at 1.000 metres and recognise them at 500 metres. Tanks could be detected at 3.000 metres.
In later years TNO has frequently advised on the material procurement of equipment by the Armed Forces, including the introduction of the Leopard thermal imager of Zeiss (Royal Netherlands Army), the LION uncooled camera for the soldier (KL), the LANTIRN reconnaissance and laser designer pod for the F16 (KLu) and the SIRIUS 360 degree search scanner for the LCF frigate (Royal Navy). Knowledge needed to give the right advice was gained during the development of various technology demonstrators (prototypes) such as:
- the Seacat Infrared Sensor (SIRS) to capture of the Seacat rocket shortly after launch (Van Speijk frigate),
- the ETIS (Experimental Thermal Infrared Scanner) series for the Dutch Army and Navy applications, and
- the Multi-Detector Thermal Image-scanner for the F104 (Starfighter).
An additional aspect was the assistance in carrying out tests to determine the performance of equipment and manufacturing test equipment such as the Minimum Resolvable Temperature Difference setup, and the Subsonic and Supersonic Missile Simulators for the SIRIUS scanner. A great deal of knowledge was acquired through contacts with other countries, including the cooperation in the various NATO Research Study Groups and through bilateral contacts with England, Norway, America and France.
In 1969, TNO developed the Combination Brightness Infrared Viewer (CHIK). CHIK requirements were: at least 15 images/s, an image field of at least 1°x3°, 40 W max., 5x enlarged image, and a weigh of less than 6 kg.
The forward-looking infrared (FLIR) image was projected via mirrors (and in the later, more compact built CHIK II via reversing prisms) to the centre of the brightness intensifier. The infrared image was successively built by using a fast scanning rotor with a facet disc with 24 facets. The CHIK had an optical resolution of 0.5 mRad and thermal resolution of 0.6° C.
T. Nooijen (2015), Physics Research at RVO-TNO during the early cold war, Univ. of Utrecht (pdf)