Infra-red image converters are available on the surplus market at a price of the order of £1 or so. They were used during the war for a variety of military purposes, and can be extremely useful to the amateur experimenter. These image converters will convert the infra-red radiation arriving from any objects into a visual image which can be seen on a small screen within the instrument. It is thus possible to use the apparatus to render any hot objects visible or to view any object illuminated with infra-red radiation even if no visible light whatsoever is present.
The image converter consists of two parts, namely the image converter tube itself and the power supply. These two pieces of equipment can be used to detect a source of heat in a darkened room, but if a complete picture is required on the screen of the image converter it is also necessary to use a lens to focus an infra-red image on to the internal coating of the front face of the image converter tube. The image formed is inverted, but this can be avoided if a suitable erector lens is employed in addition to the apparatus already mentioned. A filter which will transmit infra-red radiation but not visible light is extremely useful and, for some work, essential.
Image Converter Tube
One of the most commonly used image converter tubes is the CVI48 which is pictured above and shown diagramatically below.
An image converter tube.
These tubes consist of a cylindrical evacuated enclosure of Pyrex glass about 5 cm in diameter by 4 cm in length. There are only two connections to the tube; the anode is connected to a metal ring at the back of the tube and the cathode is connected to a ring of a conducting material cemented around the edge of the front face.
Principle of Operation
The internal face of the front window, the cathode, is coated with a semi-transparent silver- caesium oxide mixture. This has a bluish-grey tinge and acts as a photo-cathode, since it will emit electrons when excited by infra-red radiation of wavelength up to just over 1μ.
The emitted electrons are attracted to a fluorescent screen (usually of Willemite) which is kept at a positive potential of between 3 and 6.5 kV with respect to the cathode. The fluorescent screen may be marked with a graticule and is parallel with the cathode and 5 mm behind it. The brightness of any point on the screen depends on the number of electrons arriving at that point per second, this, in turn, being controlled by the intensity of the infra-red radiation striking the corresponding point on the cathode. The image converter may be considered as a photocell, the anode of which consists of a fluorescent screen.
The screen is normally observed from the end window which is remote from the cathode. Resolutions of up to 350 lines per inch can be obtained, equivalent to about 600 lines across the whole screen. The image is green.
The tube requires between 3 and 6.5 kV at approximately 10-9 amp (one thousandth of a microamp). A conventional transformer power supply can be used, but this has the disadvantage that it is not portable and may not be very safe. A vibrator supply could also be considered.
Diagram of a Zamboni pile. About eight of these piles are used in a typical surplus image converter.
Surplus image converters are usually supplied with EHT from a number of Zamboni piles, each of which really consists of a large number of very small dry cells. Each pile is cylindrical, about 9 in long and 0.75 in in diameter. The cellulose nitrate body is an insulator and brass caps are screwed on at each end.
In dry air the voltage between the brass caps is over 1kV when measured with an electrostatic voltmeter, but the maximum continuous current available is only about 10-9 amp owing to the high internal resistance of the piles (nearly 1010 ohms). Therefore if there is any appreciable leakage path (such as will occur on any day when the humidity is high), the terminal voltage of the pile will drop to a small fraction of its original value. In service instruments the Zamboni piles and connections to the image converter tube are completely sealed in a plastic container which effectively prevents leakage paths.
It has not been found possible to obtain any voltage reading from a Zamboni pile with even the most sensitive moving coil instrument, as the pile cannot supply the necessary current. If a Zamboni pile is giving a reading of, perhaps, 1 kV on an electrostatic voltmeter and both ends of the pile are touched simultaneously, the leakage path through the persons body immediately reduces the voltage to a negligible value. Nothing at all can be felt and the Zamboni pile is thus an absolutely safe method of obtaining an EHT voltage for low current tubes such as image converters.
Internally each Zamboni pile contains over a thousand paper discs pressed together by means of a spring. Each disc is coated on one side with a mixture of manganese dioxide, zinc chloride and gelatine and on the other side with tinfoil. Each coated disc is about 0.004 in thick and provides an EMF of 0.8 Volt per disc. Zamboni piles last for many years. A typical Zamboni pile is illustrated above.
The pile voltage is quite small at winter temperatures, whilst at high temperatures the discs tend to dry out and the internal resistance becomes larger. The writer has found that piles dated 1942 are still in working order.
Initial experiments may be carried out using any hot object such as a poker which is just below red heat or a piece of resistance wire carrying a current which is just not sufficient to cause the wire to emit visible light. An ordinary electric lamp bulb provides more infra-red radiation than visible light, but a filter is required to cut off the visible radiation before it can conveniently be used for image converter experiments.
Military applications include detecting the infra-red radiation from homing beacons, etc., in which a telescope is usually employed to receive the radiation. The image converter tubes even found an application in vehicle driving at night. There are many possible scientific and practical applications (e.g. in viewing the processes involved in the manufacture of panchromatic plates), and these image converters will provide the enthusiast with cheap and instructive experience in an unusual branch of electronics.