The definition of luminescence

Acording to the Encyclopaedia of PWN (Państwowe Wydawnictwo Naukowe – Polish Scientific Publishers), luminescence (from „lumen”, Latin word for „light”), is glowing, the emmision of electromagnetic radiation in ultraviolet, visible and infrared ranges by atoms or particles during the transition from the extited state to a lower energy state, when the excitation is caused by factors other than the increase of temperature of the radiation source ( https://encyklopedia.pwn.pl/haslo/luminescencja;3934476.html ). One of types of luminescence is photoluminescence caused by the absorption of electromagnetic radiation from ultraviolet, visible or infrared spectrum. One of types of photoluminescence is fluorescence, which is a phenomenon of photoluminescence lasting only during the activity of an exciting factor (contrary to phosphorescence, which lasts even after the activity of that factor disappears) ( https://pl.wikipedia.org/wiki/Luminescencja ).

Explanation of the definition

Expressing the aforementioned with simpler words, the phenomenon of luminescence means the ability of selected materials to emit lightwaves (colourful „glowing”) after their excitation by various physico-chemical occurences. One of these can be irradiation by the white light (so-called „daylight”), UV, or infrared. Due to the irradiation by one of these types of light, the excited material starts to emit „its own” glow in colour (a few primary colours such as: red, green, blue, purple, yellow, etc.). This phenomenon is named photoluminescence. There are two types of photoluminescence. The first one occurs when the materials emits „its own” glow even when the excitation (irradiation) by the exciting factor stops. This is phosphorescence. Another type is when the glowing of the material takes place only when it is being irradiated by the exciting factor. That is fluorescence.

Nomenclature of paints using the luminescence and fluorescence phenomena

When it comes to nomenclature of paints using the luminescent and fluorescent pigments, there is bigger freedom, commonness of meanings and lack of scientific accuracy. Firstly, fluorescent paints are never included in a luminescent paints group. They are usually not even linked to that group. Secondly, even within the group of fluorescent paints – according to what has been mentioned above about the types of exciting light – there is a division to the paints seen in the daylight, in UV or in IR. The most popular and common group are fluorescent paints excited by the white light. These inks, with their unusual, bright colours, are a quite significant group within the inks available in the market and even if we treat them as special paints in our classification and nomenclature, they are often classified as ordinary inks by other participants of the market. In our typology, fluorescent paints excited by the white light are a wide and independent department of special inks (wide enough that we have decided not to link it with our luminescent paints’ department). On the other hand, luminescent paints use the entire rest of luminescent phenomena in their full extent and variety (including fluorescence after excitation by UV or IR light and another photoluminescent group – phosphorescence).

Why we distinguish between fluorescent paints and luminescent paints

Apart from the aforementioned reasons for our distinguishing between fluorescent paints from luminescent paints, the most important is the fact that the effect of luminescence (more exactly, its type – fluorescence), in the case of fluorescent paints excited by the white light is, firstly, immediately visible, and secondly, the excitation source is available everywhere. This fact is fundamental in the use of these paints in the paint industry because in majority of times, a potential spectator (a customer) expects the visual effect to occur immediately and without the need to bring out the effect by using any equipment, mechanism or algorithm of actions. Let us imagine an average user of the paint, who, in order to achieve the colourful effect,  plugs in an exciting device or turns off a light source just to see the colour. This is why fluorescent paints, excited by the white light, are often treated as ordinary paints. We classify them as special paints, but other than luminescent ones. The entire rest of luminescent paints are products for which we need sometimes quite complicated, relatively expensive or rarely available excitation method, or we need to „irradiate” them for a longer period of time (just like phosphorescent paints) to see the end effect.

Examples of applications of fluorescent paints

In order to better illustrate the differences between fluorescent and luminescent paints, we can use the examples of their applications. Fluorescent paints are characterised by their exceptional brightness and colour purity, vividness and richness of the colour and the „glowing” impression. These features make fluorescent paints significantly stand out with their brightness and expresiveness from other colorful paints. This is why they are used in such applications where it is required to highlight, emphasize, point out, or mark an element to make it more noticeable to the average spectator. They are used for painting evacuation paths, warning signs, elements that are to stand out (e.g. discount tags at supermarkets), communication tracts, barriers protecting adainst falls, road signs, horizontal road markings, elements of interior arrangements and decorations, theatre and event staging, escape rooms, details in art and advertising.

Examples of the use of luminescent paints

Luminescent paints are used wherever obtaining the final effect requires the use of an additional equipment, a lamp, a tool, a solution, an installation, etc. or an additional action (e.g. turning on the light) to see the effect. Invisible fluorescent inks and paints (which glow under UV) are used for creating wonderful decorations or a staging of events, at cluns or discos, escape rooms, in modern art, etc. After turning off the light and turning on the UV lamp (so-called „blacklight”), the alternative world full of colours and images which have not been seen before, gets revealed to the eyes of spectators. These products are also used for printing bank notes and other securities. Phosphorescent paints are also used where the lack of the artificial light makes it impossible to recognise what we would like to show our spectator. The ink or paint „charges” during the day and gives off that energy in the form of light during nighttime. Used for marking evacuation paths and nighttime pedestrian or bicycle routes, printing information boards visible at night, marketing effects which are to shock the spectator with the additional glow-in-the-dark effect, etc. Paints and inks glowing under IR are used for securing printouts, packagings, bank notes and securities. Luminescent paints from the discussed range can serve marketing or warning purposes, just like in aforementioned examples; in fact, there are many other applications of these paints, but this article may be too short to discuss them all 🙂 One thing is for sure: luminescent paints can be used for various applications where creativity and ingenuity are essential for the creation of an interesting solution. The only limit in this case can be only our imagination.

Summary

To sum it all up: there is a scientific explanation of differences and similarities between fluorescent and luminescent paints, however, it is being rather loosely treated by the paint market and different companies use different nomenclature, which is not always in accordance with logic or a scientific approach. The systematics we have adopted can be characterised regarding the functionality and ease of operation of the source of excitation („glowing”) and the caused effect. In this nomenclature, fluorescent paints are easily excitable by the daylight, are immediately visible and do not require any additional tools or equipment, whereas luminescent paints require additional action in order to bring out the effect they hide.

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