Luminescence Definition and Examples

Luminescence is a process by which a substance emits light without becoming noticeably heated. The term comes from the Latin word “lumen”, meaning “light.” In contrast, incandescence is light resulting from heating a material so that it emits blackbody radiation.

Key Takeaways: Luminescence

• Luminescence is light emission that does not result from heat, unlike incandescence.
• It occurs when electrons absorb energy and release it as light while returning to a lower energy state.
• Types of luminescence include fluorescence, phosphorescence, chemiluminescence, bioluminescence, electroluminescence, and more.
• The color and duration of the light depend on the excitation source and the material’s properties.
• Applications span from nature (e.g., fireflies, deep-sea creatures) to technology, medicine, archaeology, and entertainment.

History

Early humans knew about light from some fungi and the aurora borealis. The phenomenon was formally observed in the early 1600s, with the discovery of the “Bologna Stone”. This barium sulfide-based substance, discovered by an Italian shoemaker and alchemist named Vincenzo Cascariolo, would glow in the dark after exposure to sunlight.

In the 19th century, British scientist Sir George Gabriel Stokes made significant strides in understanding this phenomenon. He coined the term “fluorescence” to describe the glow of fluorspar and uranium glass under ultraviolet light. The broader term “luminescence” was introduced in 1888 by German physicist Eilhard Wiedemann.

How Luminescence Works

At the molecular level, luminescence occurs when a substance absorbs energy, which promotes its electrons to higher energy levels. When these electrons return to their original, lower energy states, they release the excess energy in the form of light. The energy gap determines the color (wavelength) of the emitted light.

Luminescence vs. Incandescence

Luminescence and incandescence are two very different ways that materials produce light. While luminescence results from electron transitions that emit light without significant heat, incandescence occurs when an object becomes hot enough to glow. Understanding the difference between these two processes helps explain why a firefly glows coolly while a filament bulb burns hot. This comparison highlights their distinct energy sources, temperatures, and everyday examples.

Categories and Types of Luminescence

There are different categories of luminescence. They depend on the cause of the excitation, the duration of the emission, and the nature of the excited state. Here are the main categories:

Photoluminescence

This is light emission resulting from the absorption of photons. The absorbed energy excites electrons, which emit lower-energy photons when they return to a more stable state. There are two main types of photoluminescence:

• Fluorescence: Fluorescence occurs when a substance absorbs photons and re-emits them very quickly, within nanoseconds. An everyday example is a fluorescent highlighter pen, which glows under UV light.
• Phosphorescence: Phosphorescence is similar to fluorescence, but the substance re-emits the absorbed photons over a longer period, resulting in a sustained glow even after the removal of the exciting source. Glow-in-the-dark stars are a common example.

Chemiluminescence

This is light that results from a chemical reaction. An everyday example is the light from a glow stick, where a chemical reaction makes the stick glow.

Bioluminescence

This is a form of chemiluminescence that occurs in certain living organisms, allowing them to produce and emit light. Fireflies, some fungi, and many deep-sea creatures are bioluminescent. Technically, bioluminescence is a form of chemiluminescence that occurs in living organisms.

Electroluminescence

This is light in response to the passage of an electric current or strong electric field through a material. The screens of OLED TVs, night lights, and some types of digital displays use this principle.

Thermoluminescence

This light results from heating of a material. It’s used in archaeology for dating ancient artifacts. Thermoluminescence differs from incandescent light produced by heat.

Cryoluminescence

In contrast to thermoluminescence, cryoluminescence is the emission of light from cooling a material. Wulfenite is an example of a mineral that displays this type of luminescence.

Triboluminescence

Triboluminescence is light that results from friction or crushing of a material. It’s often seen when crushing sugar or certain types of crystals.

Radioluminescence

This is light from the bombardment with ionizing radiation. An example is the glow of radium dials on old watches and clocks. Tritium lights work in much the same way, where radiation makes a phosphor glow.

Table of Luminescence Types

Uses and Applications of Luminescence

Luminescence has a wide range of applications in nature, science, industry, art, and everyday life. Its ability to produce light without heat makes it valuable in sensitive environments and advanced technologies.

Natural Functions

• Bioluminescence in animals helps in communication, camouflage, mating, and predation.

Scientific and Medical Uses

• Tracking cellular processes.
• Imaging cancer cells with luminescent markers.
• Archaeological and geological dating (e.g., thermoluminescence).

Commercial and Industrial Uses

• OLED displays and electroluminescent panels.
• Emergency exit signs and safety strips.
• Glow-in-the-dark materials and novelty items.

Art and Entertainment

• Luminescent paints, costumes, stage props, and installations.

Advancements in Luminescence

Recent advances in nanotechnology and materials science produce new luminescent materials with unique properties. For instance, quantum dots are tiny particles that emit light of different colors based on their size. They find use in cutting-edge display technology to enhance color accuracy and brightness. As another example, genetically engineered luminescent plants are an option for sustainable, energy-efficient lighting. In medicine, luminescent markers hold promise for highlighting cancerous cells. In the field of safety and security, luminescent materials offer better visibility in low light conditions.

Fun Facts About Luminescence

• Shrimp and squid release bioluminescent ink to confuse predators and escape danger in the dark ocean depths.
• Scorpions glow under ultraviolet (UV) light because compounds in their exoskeletons fluoresce.
• The Bologna Stone, a barium sulfide mineral, captivated early scientists by glowing for hours after sunlight exposure.
• Glow sticks produce light through chemiluminescence.
• Adhesive tape emits X-rays when you peel it in a vacuum, thanks to triboluminescence.
• Some deep-sea creatures control their glow using light-producing organs called photophores.
• Fireflies use precisely timed flashes of bioluminescence to attract mates, and different species flash in distinct patterns.
• Crushed sugar cubes spark with brief flashes of light when you bite them in the dark due to triboluminescence.
• Tritium-powered devices glow continuously for years without electricity by emitting radiation that excites phosphors.

References

• Anctil, Michel (2018). Luminous Creatures: The History and Science of Light Production in Living Organisms. Montreal & Kingston, London, Chicago: McGill-Queen’s University Press. ISBN 978-0-7735-5312-5.
• Atari, N.A. (1982). “Pizeoluminescence phenomenon”. Physics Letters A. 90 (1-2): 93-96. doi:10.1021/ed100182h
• Harvey, E. Newton (1957). A History of Luminescence: From the Earliest Times Until 1900. Philadelphia: American Philosophical Society.
• Muraria, M.K.; et al. (Jun 2021). “Infrared radiofluorescence (IR-RF) dating: A review”. Quaternary Geochronology. 64: 101155. doi:10.1016/j.quageo.2021.101155
• Valeur, Bernard; Berberan-Santos, Mário N. (2011). “A Brief History of Fluorescence and Phosphorescence before the Emergence of Quantum Theory”. Journal of Chemical Education. 88 (6): 731–738. doi:10.1021/ed100182h