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There are numerous requirements for inexpensive TV imaging systems to cover
the near infra red spectrum out to 2000nm. Applications include covert
surveillance, semiconductor inspection, laser and thermal imaging. At the
heart of such a system is the image pick-up device: usually in the form of a
vacuum tube of the Vidicon type or a solid state sensor array.
Unfortunately there are very few photoemissive or photoconductive materials
that can be employed as sensors for cameras across the 1000-2000nm band.
Image tubes using the Si (Ag/O/Cs) photocathode work to i500nm but have poor
sensitivity. Most silicon devices in the form of Vidicon targets or CCDs
are not sensitive beyond iiOOnm. Only the PtSi Shottky barrier imagers
cover this spectrum, but they are expensive and require cooling to reduce
dark currents to tolerable levels.
For over 50 years lead suiphide has been used to detect near infra red
raaiation to its limit of 3300nm, but because of its low resistivity (iO
iO ohm-cm) it cannot store charge information long enough to be useful a3
an imaging material. On the other hand lead oxide has a resistivity (iO
ohm-cm) high enough for imaging but its long wavelength detection limit is
only 650nm.
In i949 Frank and Raithel (i) compromised by combining PbO and PbS in the
hope of making a more viable IR photoconductive target. Their method was to
evaporate first a PbO film and then subject it to sulphur vapour at elevated
temperature followed by further heat treatments. Although the spectral
response they obtained extended from the visible to about i800nm the
sensitivity was non uniform, dark currents were high and the Image retention
was severe.
Later work by Heimann and Kunze (2) and Hon et. al. (3) claimed to improve
the transient response or lag, but picture qualities still were deemed to be
very poor. More recent developments by Kawai et.al. (4) extended the
spectral limit of their Vidicon to 2700nm by using a very lengthy version of
the original Frank and Raithel process. There remained however many
practical problems concerned with reproducibility in the production process
and the stability of target and tube performance.
R Registered Trade Mark of EEV Co. for photoconductive camera tubes with
lead oxide targets.
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