Luminescence

Luminescence is the emission of optical radiation by a substance. This radiation can be ultraviolet, visible, or infrared light. Unlike incandescence, luminescence does not require heat to produce light. Incandescence is the light produced by hot matter, like a glowing stove. Because luminescence often happens at low temperatures, it is called "cold light." This process is vital for modern technology and scientific discovery.

The mechanism of luminescence begins when a system gains energy. This energy can come from light, electricity, or chemical reactions. When the substance absorbs this energy, its particles enter an excited state. These particles move to higher electronic or vibrational energy levels. To return to a stable, lower-energy state, the substance must release the extra energy. It often does this by emitting a photon, which is a particle of light. However, some energy may be lost as heat through non-radiative processes. These processes include vibrational relaxation or internal conversion. These competing pathways can reduce the overall efficiency of the light production.

Scientists categorize luminescence based on how the energy is released. Fluorescence is a type of photoluminescence that happens very quickly. It occurs essentially only while the substance is being hit by light. Phosphorescence is a slower form of photoluminescence. It involves a change in what scientists call spin multiplicity. This often results in a long-lived afterglow that people see in "glow-in-the-dark" items. Many of these items actually use persistent luminescence. This is a process where light is released from "traps" in the material. This can allow the glow to last for minutes or even hours.

There are many different ways to trigger luminescence. Photoluminescence is caused by absorbing light. Electroluminescence occurs when electrical energy creates excited states. This is the technology used in LEDs and OLED displays. Chemiluminescence is produced by a chemical reaction. A famous example is luminol, which reacts with hemoglobin. If this happens inside a living organism, it is called bioluminescence. Other types include mechanoluminescence, which comes from mechanical action like rubbing. There is even sonoluminescence, which is light produced by collapsing bubbles in sound fields.

The history of this field is tied to our understanding of physics. The German physicist Eilhard Wiedemann introduced the term "luminescence" in 1888. He wanted to classify light that was not caused by heating. Before quantum theory was fully understood, scientists studied fluorescence and phosphorescence. These early studies helped build the models we use for excited states today. Now, researchers use advanced tools to study these light patterns. They use luminescence spectrometers to measure the spectrum of emitted photons. They also use fluorimeters to measure the intensity of the light.

Luminescence is measured using two main figures of merit. The first is quantum yield, which is the fraction of excitations that produce photons. The second is lifetime, which is the time scale of the decay. Scientists also look at the Stokes shift. This is the difference between the light absorbed and the light emitted. In many systems, the emitted light has a longer wavelength and lower energy. This means the color of the light changes during the process.

This science has massive importance across many different fields. In medicine, fluorescent proteins and dyes allow doctors to image cells and tissues. In the world of electronics, LEDs provide direct electroluminescence for lighting and screens. Safety signage often uses persistent luminescent pigments for visibility. In earth science, luminescence helps us understand deep time. Scientists use thermoluminescence and optically stimulated luminescence (OSL) dating. These methods measure trapped charges in minerals. This allows archaeologists to estimate when a mineral was last heated or exposed to sunlight.

Finally, luminescence connects to many different physical materials. It can happen in organic molecules like dyes. It also happens in inorganic phosphors, which use activator ions in a lattice. Semiconductors use the recombination of electrons and holes to create light. Even defects in minerals can emit light under UV or electron beams. These properties are used in everything from radiation detection to mineral identification.