These can be seen with the naked eye when viewed flat and held up to the light at eye level.
They were first introduced during the 1950s on the Wilding series.
When viewed under a ultra violet light, either short wave for old phosphor or long wave for issues after 1993. They give off various degrees of fluorescence and phosphorescence (afterglow).
Old phosphor : B
Most early Machins have this type of phosphor they consisted of either a centre band, two side bands, or a single side band. These were either green or blue in colour.
New Phosphor : A
A phosphor was brought into use in the mid 1980s, it is a variation of the phosphor used on the AOP paper issues (see phosphor coated paper ). The bands on these stamps fluoresce a little fainter than the previous printings, and are violet in colour.
C Phosphor :This phosphor had a additive called cartax mixed with the phosphor ink. This in turn allowed the stamps to fluoresce a yellow green colour when viewed under long wave ultra violet light. This is also termed AY phosphor by some specialist groups.
Early printings of this type were the 18p printed by Enschede in 1991. Several trials were carried out with various amounts of phosphor ink overprinted, this in turn resulted in several variations of yellow colour in the fluorescence. All these variations are catalogued and are all collectable.
If you are interested I have a few Enschede cylinder blocks of 6 (2 x 3) with Yellow phosphor bands ( no varnish) surplus to requirements, several values, dot and no dot panes are for sale at
discount prices. Email me for more information. firstname.lastname@example.org
D & D2 Phosphor (Long wave afterglow or Novaglow)
A trial printing D2 was released, this was termed nova glow by the printers in 1994 on the trial run of 38p stamps, again printed by Enschede. These stamps had a blue or light violet afterglow, this can be detected when viewed in a dark room under a long wave UV lamp, switch off the light after a few seconds for the desired effect. D phosphor was a intermediate blue phosphor without the afterglow.
Some of the earlier stamps included a layer of varnish under the phosphor, this was either 2 mm or 4 mm wide ( more about varnish later ).
Stamps with a D (blue fluorescence) have since been issued by all printers and now seems to be the norm on all current issues including Self adhesives. Questa, recent De La Rue stamps and other certain printings have the D2 aftergow.
Width and length of Phosphors :
As mentioned above all stamps of this type have either one or two bands running vertical from top to bottom of the perforations. Some have extra bands or no bands these were printed in error, and are highly collectable.
First class stamps printed in sheets normally have 9 mm when split reverting into two 4.5 mm bands these are set each side of the stamp. The 10 mm 10p stamp printed on the Chambon Press a exception to the rule having two 5 mm bands. Booklet stamps can also found to contain 8 mm & 4 mm bands.
Second class stamps from sheets have one 4.5 mm centre band, or on more recent issues a 4 mm band. This can also be off-set of centre.
Varnish under the bands
As mentioned above, some of the stamps printed by Enschede have a varnish strip printed under the phosphor ink. These were applied to eradicate problems, with the paper being to absorbent, this in turn caused the phosphor to have a weak signal. Two widths of varnish were tried first a 4 mm varnish then later a 2 mm version. Both varieties are collectables although the 2 mm type is hard to distinguish as the layer tends to correspond with the inter stamp margins.
To view these varnished bands they must be held up to the at a slight angle, the varnish seems a little lighter than the actual phosphor that has no vanish present.
Nearly 99% of variations come from booklet panes, they can consist of bands set either left or right, in different widths. Also variations of centre bands come in various widths including settings to the left or right of centre. Check for 8 mm & 4 mm bands, these are normally found on stamps from vending machine booklets.
These are found on stamps from many booklet panes . They consist of short bands bottom, short bands at the top, inset to the left or right of the perforations. Short bands both top and bottom. Two band, centre band and side band varieties are catalogued, all are collectables.
Inset refers to bands that are inset from the perforations. Most of the inset bands are found from booklet pane material and recent sheet printings by De La Rue . Three types exist, these are inset left, inset right or inset from both left and right .
These are either short from the perforations, top or bottom ( it should be noted that combinations of inset and short bands also exist )
Notched Bands :
These can be obtained from mixed value and prestige booklet panes, they consist of small notches in the top or bottom of the band. Four types are catalogued. Notched either top left, top right, bottom left or bottom right. It should also be noted that stamps exist with combinations of short, inset and notched bands. These can be very desirable stamps and are overlooked by a lot of collectors, they will no doubt be rarities of the near future.
Douglas G.A.Myall. Deegam Publications has devised and written a reference system for these stamps, The Deegam SIN System. Douglas has kindly given his permission for these to be reproduced on my web site . http://www.gbmachins.co.uk
Phosphor front and back :
Phosphor is normally applied to the face of the stamp over the printed image, when phosphor is also found on the reverse of the stamp, this has been applied in error. Some experts claim that wet ink has been transferred from one sheet to another during the stacking process. Another reason has been suggested that a printing roller has picked up ink by mistake and deposited it to the rear of the sheets.
Phosphor under ink :
Some stamps have a milky appearance, this is caused by the phosphor being printed first, then the stamp image printed over the top. The best way to test for these variations is is to hold the stamp up to the light, the bands will appear milky and lighter than the none phosphor area.