Different luminescent features are widely used nowadays, but they have a relatively short history in the field of security printing. The first applications were in the sixties, in the US.
Before we jump into the application possibilities, let’s define luminescence, fluorescence, phosphorescence, and the relation between them.
Luminescence is the emission of light that is not caused by heat, it is also called „cold light”. It is in contrast to light emitted from incandescent bodies, like heated or molten iron, burning coal or wood, or the wolfram wire heated with an electric current in our household bulbs. Basically, we can say that luminescence is energy dissipated by a substance in the form of light. There are numerous types of luminescence that are showing us the different possible energy sources to start this glowing phenomenon (see figure 1.).
Maybe the two most known type of luminescence is photoluminescence and chemiluminescence. You have probably seen one of those glowing night stickers that do not show anything in daylight but shine in yellowish-green at night. These are the perfect examples of phosphorescence. Also, if you have ever visited a disco club with blacklights equipped, you sure did realize that the clothes which are normally white, turned to a strong blue color under the UV lights. That was because of the phenomenon called fluorescence and was triggered by the UV-light (this type is called UV-fluorescence) that excited the optical brightener in your clothes.
Another typical form of luminescence is chemiluminescence, which is mostly presented in glowing sticks that are used for parties or as a fishing accessory. In this example, a chemical reaction of the two components starts glowing when you mix them together by bending the stick.
Before we get started, let us clarify a few terms here. It will help us understand the basic physics of the „glowing” materials. I know physics sounds bad, I assure you, in a small amount, it can be fun and truly instructive.
Now that you got a few examples of the luminescent things around us, let’s try and understand the physics that’s turning them in these glowing colors. Phosphorescence and fluorescence are usually confused with each other as they are pretty similar phenomena, but if we get to the basics of how they work, we won’t mix them up again.
So basically, photoluminescence is when a substances glowing is caused by some kind of light. Both fluorescence and phosphorescence are based on that phenomenon but with one difference: time.
A fluorescent material is glowing just until the time it is excited by a source of light (usually UV-light), but a phosphorescent material is always producing an afterglow, seconds, minutes, or even hours after the excitation stops.
To see what’s the root of this difference we need to know some basics about the excited states of the electrons. The atoms of different molecules have a different number of electrons, and these electrons can absorb energy (in our case this energy is transmitted by photons, marked as Abs. (Absorption) in Figure 2.) therefore these electrons become electronically excited and transform into an excited state with higher energy.
The explanatory model of this phenomenon is in Figure 2. In basic conditions, the electrons are staying on their ground state (S0) in most of the molecules, but because of the excitation the electrons transfer to an S1/S2/Sn excited state, depending on the amount of energy transferred, and that amount is specified by the wavelength of the absorbed photon.
These types of excited states of the electrons are unstable, and they want to relax and get back to the ground state, where life is calm and predictable for them. In order to do that, the electrons can go through several types of relaxing processes. These processes will be the key to finding the basic difference between fluorescence and phosphorescence.