Kanji tattoos

Saturday, November 8, 2014

Glow-in-the-dark-tattoos: Phosphorescence / Persistent luminescence / Radioluminescence

Hi, everyone. In this post I'll talk about an extremely popular misconception, which can be resumed into this single phrase: "Glow-in-the-dark tattoos that do not require a light source to glow because they contain phosphorus which can glow by itself". There is a lot of confusion here, so let's try to be clear step by step:

1. The ink that glow in the dark is (usually) phosphorescent ink, and it's different from the fluorescent ink (the Uv-reactive ink).

2. Phosphorescent materials/substances glow in the dark after the end of an excitation with ultra-violet light or visible light. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in a common fluorescent tube, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to re-emit energy occur less often (source: wikipedia). So, since the phosphorescent materials need to be charged by expusure to light, it's obvious that if you keep your phosphorescent tattoo away from any source of light, it won't glow in the dark.

3. There are indeed some substances that glow without exposure to light, but these substance aren't either phosphorescent or fluorescent, they are radioluminescent substances (and radioactive).

4. Phosphorescent ink is made from phosphors such as silver-activated zinc sulfide or doped strontium aluminate, calcium sulfide, alkaline earth metal silicate, etc. These substances are called "phosphors". The chemical element Phosporus isn't phosphorescent; it emits light due to chemiluminescence, not phosphorescence (pretty confusing, isn't it?).

5. In phosphorescence, the lifetime of the emission of light lasts from several seconds to several minutes. If a material/substance can keep glowing for several hours, it's not correct to refer to that material as phosphorescent materials/substances. The phenomenon that occurs in this case is called Persistent luminescence and is commonly but incorrectly referred as phosphorescence. In order to make the phosphorescent ink/paint to glow several hours, various activators are added (e.g. the glow-in-the-dark cosmetic creams used for Halloween make-ups are made using the ZnS:Cu phosphor - Zinc sulfide activated with Copper). These activators are one of the reasons why you should avoid a permanent glow-in-the-dark tattoo.

6. As I've said before, there are, indeed, some types of substances that glow by themselves - the radioluminescent materials/substances. Radioluminescent paint contains a radioactive isotope (radionuclide) combined with a radioluminescent substance. The isotopes selected are typically strong emitters of fast electrons (beta radiation), preferred since this radiation will not penetrate an enclosure. Radioluminescent paints will glow without exposure to light until the radioactive isotope has decayed (or the phosphor degrades), which may be many years. They are therefore sometimes referred to as "self-luminous" (source - wikipedia). Tattoos made with radioluminescent ink can be very spectacular, but you don't want radioactive substances on or under your skin, don't you?

Related posts:
Uv-reactive tattoos and glow-in-the-dark tattoos for the Halloween-party. Are they a good idea or a very bad one?

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