The bright world of the security printing and brand protection industry – Part 1: The essentials

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.).

Figure 1.: Forms of luminescence

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.

Figure 2.: The excitation and relaxation processes of the electrons (Source: Wikimedia Commons)

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/S_n 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.

Vibrational relaxation happens extremely fast (<10^-12sec) and dissipates energy as heat which is passed on to the surrounding molecules while slightly heating the whole environment. In figure two it is presented as a blurry line between the thin lines of one excited state. Here, the energy is dissipated without the emission of light, therefore it is a non-radiative transition.

Internal conversion is also a non-radiative process where the energy dissipation is going through two different levels electronically excited state (e.g., S₂-S₁, marked with „IC” on Figure 2).

The electron may dissipate the absorbed energy only with the non-radiative relaxation processes we discussed above. When this occurs, the molecule cannot emit light thus all the energy is released as heat. The rate of the absorbed energy dissipated by radiative or non-radiative events gives us the so-called quantum yield or quantum efficiency of the fluorescent or phosphorescent molecule. The rate of the fluorescent/phosphorescent molecules to all the excited molecules gives us the quantum yield. When the quantum yield approaches one, the substance will have higher fluorescence intensity, when it approaches zero, the molecules do not fluoresce.

Intersystem crossing (ISC on Figure 2.) is a process, where the transition is between two different types of electronically excited states: the electron transfers from singlet state (S₁) to triplet state (T₁), as seen on Figure 2.

Phosphorescence is when the electron goes from the triplet (T₁) excited state back to ground state (S₀) through intersystem crossing. The transition between the triplet and the singlet state is „forbidden” but it does not mean that it is not happening it is just statistically much more unlikely, and it happens at a much slower rate than other relaxing processes. It means that the transition from T₁ to S₀ (phosphorescence) can happen on a timescale from microseconds to a thousand of seconds, resulting in the glowing phenomenon that can even last overnight.

And now we get to fluorescence, which is a relaxation process from the lowest excited state S₁ to the ground state S₀ involving light emission. This form of energy dissipation is very fast, it only lasts usually 10^-9 to 10^-6 seconds. During that time the molecule emits photons with a specific wavelength.

Excited molecule’s electrons usually relax to the lowest excited state possible by non-radiative processes like vibrational relaxation. This phenomenon is causing shifts between the excitation wavelengths and the emitted wavelengths, so that the emitted wavelengths are always longer, thus with lower energy. This event is called Stokes-shift and the extent of the shift is molecule-specific, and this is what determines these molecules’ incredible set of colors.

For our business that is a pretty important difference to clarify, because there is a lot of misunderstanding around them. Luminochem’s portfolio is containing UV fluorescent pigments and dyes, UV fluorescent pigment dispersions and even UV bi- and tri-fluorescent pigments thus they have no afterglow effect: they fluoresce just until they are excited by UV light. These materials are optimized for use in the high security printing industry as they have pale daylight color and outstanding chemical-physical properties. They are perfect for developing security inks, security features in banknotes, tax labels, tax stamps, legal documents and can be used for brand protection purposes.

We will show you the versatility and the possible uses of these products in the security industry in the next blog post but now get back to some more essentials that will help you fully understand the topics we will cover in the future.

If we go deeper and see what type of molecules have the ability of UV fluorescence, we will find two main groups.

Inorganic materials are a huge group of luminescent compounds. They are known for more than 150 years now. When you read this article on your computer or mobile phone the light of your screen is most likely emitted by inorganic materials. But they are widely used in lamps, LEDs, etc. Regarding their chemical composition, most of them have a host matrix crystal composed of a wide diversity of elements in different forms. These matrices are usually doped with a low percentage of metallic ions such as rare-earth elements, Zn, Cu, Ag etc.
Unfortunately, not too many of them can be used for document security purposes. Although they are extremely stable (usually) against light and chemicals, their relatively low luminescent intensity limits their use.

The other big group of UV fluorescent materials is organic compounds. Fluorescence is most characteristic in organic substances that have aromatic groups or contain alicyclic carbonyl, aliphatic, or highly conjugated double-bond structures.   In aromatic compounds, the quantum efficiency usually gets bigger with the number of rings connected.

Some of Luminochem’s organic UV fluorescent pigments under UV-light

Although some of them are pretty sensitive against light or certain chemicals, their vivid and intensive fluorescent colors are the key to producing contemporary security features. Without organic fluorescent pigments and dyes, it would be impossible to design banknotes, tax stamps, ID cards, or passports. These security features should provide an aesthetical experience as well as the highest security available.

Luminochem produces mainly organic UV fluorescent pigments and dyes for security printing applications. These materials can be produced on a much wider scale of colors than the inorganic ones, thus providing much more opportunities to make high security pigments for any application with the highest protection for high security printing companies.

Let’s take a closer look at the trigger of the whole phenomenon: UV light. UV-light is the radiation that is passing the energy to the electrons of the UV fluorescent compound. But why UV-light? Generally saying UV light has just enough energy to make the electrons jump to a higher energy level, but not too much energy, which would cause the molecule’s bonds to break.  But how much energy is that? It depends on the wavelength of the UV-ray. According to the ISO-21348 standard ultraviolet radiation is between 10 to 400 nanometers, as seen in Table 1.

Table 1: The wavelength and energy of light from X-rays to visible light according to ISO-21348

The table shows well what we’ve already stated before: the shorter the wavelength of the radiation, the higher the energy it carries and passes on.

UV-A and UV-B have enough energy to cause sunburn if people are exposed to it for a longer time, but UV-C and radiations with shorter wavelengths can be extremely dangerous and can cause instant burn, eye damage, or even cancer if exposed. Luckily our atmosphere’s ozone and water vapor do not let these wavelengths through and protect us by absorbing them.

Actually, quite a few things around us have been found to be UV fluorescent: minerals, microbes, plants, fungi, and even marsupials. Speaking of the security printing industry, there are special security features, including UV fluorescent pigments and dyes that are activated in UV-A, UV-B, or UV-C radiation and are emitting the brightest colors of the visible spectra. These 3 types of UV radiations cover that wavelength range that can be used to excite these materials causing colorful UV-fluorescence without decomposing them.

So, what are the most important things we should know about these wavelengths of UV radiation?

Ultraviolet A, also known as UV-A, is often referred to as blacklight or Soft UV. This type of UV light, in fact, is not filtered out by the atmosphere (only about 10 %) and it is less dangerous given its relatively lower energy. Although it is enough to activate the melanin production in our skin and longer exposure causes the aging of the skin. This is the UV type that comes from the disco blacklight and illuminates your white clothes and the UV fluorescent paintings on the walls.

We have to mention that this so-called blacklight is usually referred to as 390-400 nm light. Luminochem’s UV fluorescent materials are excited by 365 nm UV-A light and most of them are inactive in the 390-400 nm region.

Ultraviolet B, UV-B or often referred to as medium-wave UV and is mostly absorbed by the Earth’s ozone layer, therefore only 10% of the radiation is reaching the surface and our body. UV-B is the radiation is why we have to use sun cremes. As it has a bigger amount of energy than UV-A it can cause sunburn, skin cancer, and eye damage if we are unprotected.

Ultraviolet C, UV-C, or shortwave ultraviolet radiation is in fact completely filtered out by the atmosphere. We are lucky to have these wavelengths dodged, as these are the shortest, these can cause the most damage to living things (instant burn, cancer, eye damage, etc.). That is why it is also called germicidal UV because it has so much energy, it can instantly kill microbes, so it’s often used to purify air, water, or laboratory equipment.  Nevertheless, in compliance with safety regulations, it is a safe technology that is widely used in laboratory conditions for authenticating security prints and for development purposes.

Luminochem’s UV fluorescent pigments and UV fluorescent dyes that are active in UV-C light are showing their true color under a 254 nm UV light and invisible under UV-A light.

Luminochem has a special set of products, the UV bi- and tri-fluorescent pigments that are active in UV-A, UV-C, and even in UV-B radiation and showing two or three different vivid colors offering a secondary or tertiary level of protection for high security prints.

Why are fluorescent features so widely used today for document, banknote, and brand security purposes? What are the technical, security, and economical drivers of this widespread use? How can a security material producer help its partners?

We try to give you answers to these questions in our next chapter of  The bright world of the security printing and brand protection industry.

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