Helecon was a trade name for a phosphor fluorescent from the zinc-sulphide group that reacts to Ultra Violet light and fluoresces a bright orange red. The UV range it is sensitive to is 3000 - 4000 Angstroms. It was applied on and after 1965 to all postage stamps to allow for automatic cancellation of letters. The machinery would simply detect the presence of the stamp, somewhere on the envelope, and react accordingly.
There is some confusion as to the application of Helecon on Australian postage. The bottom line is that ALL stamps were 'heleconised' in 1965, and that NO stamps (except for a trial run of the 11d bandicoot) were heleconised prior to that date. Long running definitives, such as the QEII 2d, and some coils, are found with and without Helecon, simply because their final 1965 printings are on Helecon paper.
Most Helecon issues are considerably paler than their non-helecon cousins. When viewed under a UV lamp all Helecon issues are dramatically different in appearance.
Helecon ink versus Helecon paper.
Helecon ink was ONLY used on the 5d QEII red. All other stamps used Helecon coated paper. However, it is this issue, and primarily it's change in colour from Green to Red, that gives the game away. The 5d Red was the defining moment when all Australian postage changed to Helecon. There is confusion with these Green / Red varieties with / without helecon ink / paper. The bottom line is that, red stamps, coils and booklets contain helecon ink, green stamps and booklets (there were no coils) contain no helecon. There are exceptions, and this is properly explained on the relevant page.
The list of helecon issues is as follows