What is the difference between fluorescence / phosphorescence?
Fluorescence, phosphorescence, reflective, photoluminescent principle... There are so many scientific terms used in everyday life to designate an object that emits light in the dark. But what is the difference between all these concepts, and in what context should they be used?
In this article, we will review each of these different concepts and explain the main luminescent mechanisms.. Generally speaking, luminescence covers all emission phenomena and it is therefore easy to confuse the different terms, particularly when it comes to the difference between fluorescence and phosphorescence.
What is photoluminescence?
Before explaining what are called fluorescent and phosphorescent materials, it is important to understand the definition of photoluminescence. Photoluminescence involves two different types of reaction: phosphorescence and fluorescence. A fluorescent product is also considered to be photoluminescent. This will also be the case for phosphorescent materials.
Definition of photoluminescence
Photoluminescence is a physical phenomenon in which a substance absorbs photons of light and then re-emits it in the form of energy. In more synthetic terms, it is the process by which a molecule absorbs luminance and emits light in a different wavelength. This phenomenon is observed in many materials, such as crystals, semiconductors, fluorescent and phosphorescent dyes, as well as certain biological substances in science.
The basic process of photoluminescence is linked to the electronic energy level in the atoms or molecules of the materials. When a substance is exposed to a luminous source, some of its electrons absorb the energy and are excited to higher energy levels. In short, the light excites the photons present in the molecule, which in turn re-emits the energy.
However, these high energy levels are generally not stable, and the excited electrons eventually return to their lower initial energy states by emitting luminance.
In other words, this lower energy state allows excess energy to be released in the form of luminance. The power difference between the electronic levels determines the wavelength of the light emitted.
There are two main types of luminescence: fluorescence and phosphorescence.
Steps in the process of photoluminescence
- Absorption: When luminous source strikes a substance, its photons are absorbed by the electrons of the atoms or molecules of the material. This absorption causes the electrons to move from a lower energy state (the ground state) to a higher energy state (an excited state).
- Excitation: In the excited state, the electrons are not stable and will typically stay in this state for a very brief period (on the order of nanoseconds to microseconds).
- Relaxation: Before emitting luminance, electrons in the excited state can lose energy through non-radiative processes such as vibrational relaxation, where the energy is transferred to the lattice of the material in the form of heat.
- Emission: Eventually, the excited electrons return to a lower energy state (which can be the ground state or another lower excited state), and the excess energy is emitted as a photon. This emitted luminance is what we observe as photoluminescence. The energy, and therefore the wavelength and color, of the emitted photon is characteristic of the material and is typically different from the wavelength of the absorbed light.
What is fluorescence?
In the case of a fluorescence state, the electron which is present in the support absorbs energy from the luminous source and immediately re-emits this power in the form of luminance. In fluorescence, the excited electrons return to their fundamental states, so energy is emitted immediately after being excited.
The light re-emitted generally has a longer wavelength (and therefore lower power) than that of the initial luminance absorbed. One of the characteristics of fluorescence is that luminance emission is instantaneous and ceases immediately after excitation.
Fluorescence is widely used in fields such as scientific research, cell biology, substance detection and medical imaging.
Some examples of daily fluorescence:
- Yellow waistcoats and safety waistcoats
- Markers and highlighters using fluorescence
- Small "cracking" glow sticks with progressive activation power
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Memo Main characteristics of the state of fluorescence:
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What is phosphorescence?
Unlike fluorescence, which ceases immediately when the lightning source is switched off, phosphorescence lasts for a more or less variable period.
Phosphorescence is similar to fluorescence, but in this case the emission of brightness continues for some time, even after the lightning has been switched off. The excited electrons remain in high energy states for some time before returning to their ground states and emitting bright. The excited electrons remain in high energy states for some time before returning to their fundamental states and emitting light. Depending on the nature of the phosphorescent object, the relaxation process can last from a few minutes to several hours.
This delay is due to the more complex electronic transitions in the molecule, which require more time to return to their fundamental state. This is why phosphorescent objects can glow in the dark and emit light for some time after being exposed to light, even after lightning excitation has stopped.
Phosphorescence is observed in certain pigments used for luminous products in the dark.
Some examples of daily phosphorescence:
- Watch hands
- Stickers and various luminous products in children's bedrooms
- Signs and "emergency exit" panels in the event of a power cut
- Road paint in underground passageways using phosphorescence
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Main characteristics of the state of phosphorescence:
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In short, fluorescence emits light when the material is subjected to lightning excitation and will cease when the lightning source is switched off, whereas phosphorescence will continue to emit light in the dark. As the fluorescent object itself does not emit light as such, it is used on a one-off basis, whereas the object using phosphorescence continues to emit luminance even if the light source is switched off.
What is a reflective object?
A reflective object is a product whose material reflects part of the light that strikes it. When incident light from the lightning hits the surface of a reflective object, some of it is reflected back in different directions, including towards the observer. This allows the observer to perceive the object in question as a function of the light reflected from its surface.
Reflective objects can vary in their degree of reflection. Some reflective products reflect luminance very sharply and specularly, meaning that light is reflected in a specific direction and images can be clearly reflected. Other objects may have a more diffuse reflection, reflecting light in many directions, creating a more diffuse and less sharp appearance.
Reflective surfaces are commonly used in many areas, including the design of mirrors, reflective surfaces for lighting, reflective signage materials for road safety and in optical technologies where the reflection of luminance is essential for the formation of images and visual information.
It is important to remember that a reflective material is not luminescent. It is simply a mirror effect of the light exposed on the material.
What about LuminoKrom® paint?
By the end of this article, you're probably wondering where our LuminoKrom® road paint fits in and what concept our marking uses. Quite simply, it's a luminescent paint with pigments designed to exploit the phenomenon of luminescence. In other words, our patented innovation exploits the principle of phosphorescence and therefore of photoluminescence more generally.
Our LuminoKrom® paint and its intelligent pigments have the ability to glow in the dark after being exposed to sunlight or virtual lightning. This technology is particularly well suited to improving safety and visibility in a variety of dark environments, such as cycle paths with no street lighting or industrial and underground sites where there is a power cut.
Our almond green phosphorescent road paints
LuminoKrom® stores luminance from the sun or other lightning during the day and releases it as a glow in the dark for 10 hours.
Our photoluminescent markings have been designed to be durable and resistant to outdoor conditions, including weathering, UV and wear and tear from road traffic. This ensures that markings and signs remain visible and effective over the years.
