People familiar with photochemistry often use the terms luminescence, photoluminescence, fluorescence, and phosphorescence interchangeably to describe materials that emit light. However, these terms do not mean the same things.
These terms are not well understood and can be confusing. Our experts explain photoluminescence and the differences between fluorescence and phosphorescence. We also discuss how materials possessing these properties are used in different applications.
Luminescence and Photoluminescence
Let’s start with photoluminescence. Photoluminescence is a specific type of luminescence. Simply put, luminescence is the emission of light that does not occur from heating. Often, people say “luminescence” when referring to “photoluminescence,” but there are many other types of luminescence, such as bioluminescence and chemiluminescence.
Photoluminescence occurs when a material emits light after absorbing photons (or, more simply put, light). Think about those sticky glow-in-the-dark stars that many of us stuck on our bedroom walls as kids. The light those stars emit is photoluminescence.
The light (or photons) that causes photoluminescence is excitation. During the day, the natural light in the room would “charge” the stars so they would glow in the dark. In this case, that natural light is the excitation. The glowing light from the stars is the emission.
In many cases, a photoluminescent material is combined with another material, where it remains invisible until it is excited by ultraviolet light. Only then will the material produce a visible color. The excitation wavelength required and the emission colors can vary greatly depending on the properties of the photoluminescent material.
In regards to light, light travels in waves. The length of these waves determines the colors we see. Wavelengths are measured in nanometers (nm). For example, blue has a wavelength of 400 nm and red has a longer wavelength of 700 nm. In photoluminescence, the wavelength of the excitation light is independent of the wavelength of the emitted light. Often, the excitation light is invisible and the emitted light is a bright, visible color
There are two main types of photoluminescence: fluorescence and phosphorescence.
The Difference Between Fluorescence and Phosphorescence
The technical differences between fluorescence and phosphorescence are complex, but the main difference between these two properties is the duration of emission after excitation.
Fluorescent material stops emitting light in a very short time after the excitation. In many cases, the color emitted by fluorescent materials will seem to disappear instantaneously when the excitation stops. Fluorescent materials are used for security features because the emission must be seen or detected only while it is excited.
In contrast, phosphorescent material can emit a visible color for a long time after the excitation, creating a glowing effect or “afterglow.” The phosphorescent afterglow may persist for a few seconds, minutes, or hours depending on the properties of the material, the intensity of the excitation the amount of time it was exposed to the excitation light source.
Excitation and Emission
The specific wavelengths required to excite a photoluminescent material and the colors these materials emit vary greatly depending on the properties of the material. Wavelengths are measured in nanometers (nm).
Excitation: Ultraviolet (UV) wavelengths are often used for excitation. Typically, UV excitation light sources use longwave UV at 365 nm (UVA 315 to 400 nm) or shortwave UV at 254 nm (UVC 200-280 nm).
Emission: Photoluminescent materials typically emit colors in the visible spectrum. The visible color range is violet (400 nm) to red (750 nm). However, it is notable that some materials emit outside of the visible color spectrum in the UV or the near IR range (above 750 nm).
Photoluminescent Products and Applications
Fluorescent and phosphorescent materials have significant differences and are used differently in an application. The majority of our photoluminescent products are fluorescent materials because they are most effective for document security, anti-counterfeiting, process control, and brand authentication applications.
Fluorescent materials are ideally suited to be integrated with printable inks, varnishes, adhesives, or plastics as well as other security materials to create layered features. Many finished products are designed to have little or no color under normal light and will emit an intense color only when they are exposed to invisible ultraviolet light. These materials are only detectable for as long as the excitation is maintained.
Phosphorescent materials can integrate with a variety of media. The long emission duration or afterglow of phosphorescent materials makes them suitable for a different range of applications such as safety equipment, exit signage, and entertainment applications, to name a few.