Detecting fluorescence differences in Scandinavian stamp papers is a subject about which I receive many inquiries. This can be a highly technical subject, however, armed with some basic information it is relatively easy to master detecting fluorescent papers.
Use of fluorescent or phosphorescent reactive papers for stamps has come about for two basic reasons: increased productivity of mail processing and improvement in brightness of papers and appearance of the inks used to print stamps.
Fluorescence refers to reactivity of a substance when under ultra-violet light, specifically while the light source is present. Phosphorescence is the same except that the reactivity continues for some period of time after the light source is removed; the ultra-violet light excited the molecules of the substance and the reaction continues for some period of time after the light is no longer present.
Unfortunately, in philately, the two terms have often been used interchangeably and also tend to be used variously by catalog and album makers operating in different parts of the world. From a practical standpoint, however, all we philatelists care about is whether or not we see a reaction while viewing the stamp under ultra-violet (UV) light.
In the early 1960s when post offices sought new technologies for canceling and sorting the exploding volume of mail, the concept of using stamps that reacted under ultra-violet light came to the fore. Using a UV-reactive paper (or coating) allows automated facer and canceler equipment to locate which side and corner of the envelope bears the postage, rotate the envelope as required, and then place the cancellation properly on the stamp. This resulted in huge labor savings; so much so that in the 1970s Sweden discontinued the use of fluorescent papers supposedly so that more people could be employed in the process of canceling mail.
Likewise, post offices have used fluorescent inks for bar-coding mail sorting information. Once imprinted with a bar code, the mail can be sorted and resorted by machine. By using UV-reactive ink, the machines can more easily see the bar code without being confused by the other markings, etc., on the envelope.
Papermakers have, for several decades, added "paper whiteners" to make the daylight-visible brightness of their paper brighter. In many cases, brighter papers make the inks printed on them appear more vibrant, add contrast, etc. The "paper whiteners" are, in actuality, typically UV reactive. When viewed under UV light, they may look white, yellowish, or bluish. The use of such brighteners is typically not for postal purposes. In some case the post office has been completely unaware that there is more than one version of the paper. However, a paper that includes such brighteners is a different paper compared to one that does not and are thus catalog-listed in most specialized catalogs and are sought by specialists. Fortunately or unfortunately (depending upon your perspective) the Scandinavian catalogs for Sweden have not done a good job (or any job) of listing the so-called whitened papers: there are issues from the 1950s to the 1980s that exist but are not listed by the specialized catalogs; it is a great opportunity for further research.
When catalog publishers and album makers refer to either fluorescent or phosphorescent stamps they are, as a practical matter, simply referring to reactivity under UV light. In Scandinavian philately, I am NOT aware of any stamp issues which have been issued in BOTH fluorescent and phosphorescent versions.
The terms "ordinary paper", "nfl", and "non fluor", all refer to stamps which do NOT react under UV light, or for which the reaction is dull or insignificant.
The visual appearance of reactive SCANDINAVIAN stamps, under UV light, may range from bluish to white to yellow. (Stamps of other countries also use orange and green, and probably other colors too.) The difference, intensity, or texture of the color can signify different papers or coatings. Differences can also be the result of aging, soaking to remove stamps from envelopes, extended exposure to light, contact with other reactive materials (including other stamps, particularly in the case of Canadian stamps). It must be emphasized that soaking stamps (to remove them from envelopes) may cause either the fluorescence to be reduced/lost or cause fluorescence to be acquired/changed from the other stamps/paper being soaked at the same time. When the difference signifies a different paper it usually results in a listing in the specialized catalogs; Finnish definitive stamps are a good example of well-researched listings.
In the case of Denmark, a number of the fluorescent stamps of the 1960s exist with both white and yellow fluorescence, though I am not aware of any catalog that recognizes this fact. In the case of Norway, very minor differences in appearance (that are the result of different papers from different papermakers) are all cataloged by Norgeskatalogen.
When attempting to identify which of the different possibilities of a UV-reactive stamp issue that you have (when there is more than one fluorescent issue involved), it is usually important to compare both issues in the same viewing. In many cases, particularly comparing examples of whitened papers to fully flourescent examples, seeing just one of the two possibilities will result in more confusion than answers. Furthermore, the terminology may be relative: the difference between "white fluor" and "yellow fluor" may be obvious when you see both at the same time, but the may be VERY misleading if you see only one at a time (the "white" may look yellowish and the "yellow" may look whitish).
In SCANDINAVIAN philately, all UV-reactive stamps must be viewed using LONG WAVE ultra-violet light. In contrast, the stamps of many other countries (including the U.S.) require the use of SHORT-WAVE ultra- violet light. If you only collect Scandinavia, you only need the less costly LONG-WAVE light. However, if you collect other countries as well, you will probably be better off buying a dual light source.
If you are seeking a dual (long-wave and short-wave) light, check with the philatelic supply houses. They have a variety of sizes and models.
A very important note of CAUTION: UV LIGHT CAUSES "SUNBURN" AND CAN DO SERIOUS DAMAGE TO THE EYES. NEVER, NEVER, NEVER LOOK INTO A UV LIGHT. MINIMIZE THE TIME THE LIGHT IS ON. COVER EXPOSED SKIN. DO NOT HOLD STAMPS IN HAND WHILE USING LIGHT. AVOID USING LIGHT WHERE THERE ARE REFLECTIVE SURFACES; REFLECTED UV LIGHT CAN CAUSE JUST AS MUCH HARM AS DIRECT UV LIGHT. DO NOT USE WHILE OTHER PEOPLE ARE PRESENT. Short-wave UV light is much more harmful than long-wave. -- From "Scandinavian Philatelic E-News," published by Jay Smith & Associates and available at no charge. To subscribe go to http://www.JaySmith.com.