In many overseas countries at the present time various luminescent substances are being incorporated in postage stamps to facilitate the automatic facing up and cancelling of letters. The general principle is that articles bearing such stamps pass along a conveyor belt past electronic detecting devices which sense the "tagging" substance in the stamps, actuate the postmarking machine, and finally channel the cancelled articles to a stacker, where they appear correctly "faced up" ready for the next sorting process.

The Australian Post Office has been experimenting on these lines for some time, and the first machines for automatic cancelling and facing will be installed in the Melbourne and Sydney mail exchanges in the near future. Eventually, similar machines will be in operation in other capital and provincial cities.

The Australian experiments on the treatment of adhesive stamps have centred around the use of substances which are usually inert but which when excited by ultraviolet rays emit light, which is detected by the sensing mechanism in the machines. The substance used in Departmental experiments is known commercially as "helecon", one of the zinc sulphide group. Helecon becomes luminescent in the orange-red spectrum when subjected to ultra-violet rays, and retains the luminescence for a fraction of a second after the UV light is removed. This property, known as phosphorescence, allows the machine to differentiate between a "tagged" stamp and the envelope paper, which quite often is treated by the manufacturer with a whitening agent which may also glow under UV light. However, the whitener ceases to emit light when the UV source is removed (i.e. the whitener is fluorescent but without residual phosphorescence) and the "tagged" stamp is detected during a period of UV lamp black-out. It follows, therefore, that the UV lamp must be used on a rapid on-off cycle, while the electronic sensing device nearby is on a continuous alert. The helecon used in stamp "tagging" is quite harmless to human health.

For Australian Post Office purposes, helecon has been incorporated in adhesive stamps in two different ways. A large proportion of the 5d. red Queen Elizabeth the Second stamps issued on 30th June, 1965, is printed on ordinary paper stock and the helecon is included in the red ink formulation. This class of stamp is being distributed in sheet form in New South Wales and Victoria only (the first two machines are being used in these States) at this time. The 5d. red Queen Elizabeth the Second stamps on issue in the other four States in sheet form will superficially appear the same but will not include helecon in the ink. However, stamps included in the various editions of stamp booklets will have helecon in the printing ink.

 The second method of providing for helecon content in adhesive stamps is the inclusion of the substance in the surface coating of the paper. Excluding the 5d., stamps up to and including the 4/- denomination are being printed on this kind of paper, the helecon having been incorporated during the process of manufacture. Specific dates-of issue are not being arranged in connection with printings of current issues on this paper but dates of initial distribution are being listed in the Philatelic Bulletin. 

It is appropriate to mention that while a number of experimental printings of stamps with helecon content were made for Departmental purposes before the present group of issues, none of the relevant stamps - with one exception - have been used or issued in such a way that they could pass into collectors' hands.

The exception relates to an experimental printing of approximately four million lld. stamps, for which a test shipment of paper containing helecon was used. As the printing proved satisfactory, the stamps were issued in the usual way for public use, initial distribution commencing in December, 1963. There was little or no apparent difference between stamps of this printing and stamps of earlier printings. Following exhaustion of stocks of the test printing, a further printing of the lld. stamps on ordinary white (non-helecon) paper was made, distribution commencing in September, 1964.

The only certain method of positively identifying stamps containing helecon in ink or paper is to subject them to ultra-violet light within the 3,000-4,000 Angstrom range, preferably viewing through a suitable filter to retard visible blue light. Under proper viewing conditions stamps containing helecon will fluoresce brightly in orange-red colours.

Extracted from APO Philatelic Bulletin 72, June 1965