The key difference between photoluminescence and fluorescence is that the photoluminescence occurs via absorbing and emitting photons having either different or equal wavelengths whereas fluorescence occurs via emitting a longer wavelength than the absorbed wavelength.
Luminescence is the process of light emission. We use the prefix photo- with the term luminescence when the light emission is due to the absorption and emission of photons. Sometimes the absorbed and emitted photons have the same wavelength. However, sometimes, the absorbed wavelength is higher than the emitted wavelength. We call this type of luminescence as fluorescence. Therefore, fluorescence is a form of photoluminescence.
CONTENTS
1. Overview and Key Difference
2. What is Photoluminescence
3. What is Fluorescence
4. Side by Side Comparison – Photoluminescence vs Fluorescence in Tabular Form
5. Summary
What is Photoluminescence?
Photoluminescence is a form of luminescence that occurs with a photoexcitation via photon absorption. This light emission occurs when a substance absorbs electromagnetic radiation and re-emits the radiation. The process initiates with photoexcitation. This means the electrons of the substance undergo excitations when the substance absorbs photons and electrons move to higher energy states from lower energy states. Following these excitations, there are relaxation processes as well. In the relaxation step, photons are re-radiated or emitted. The time period between the absorption and emission of photons may vary depending on the substance.
Figure 01: Schematic for the Excitation-Relaxation Processes of Photoluminescence
There are several forms of photoluminescence that differ from each other according to several parameters. When considering the wavelength of absorbed and emitted wavelengths of photons, there are two major types of fluorescence and resonance fluorescence. Fluorescence describes that the wavelength of emitted radiation is lower than the wavelength of absorbed wavelength. Resonance fluorescence describes that the absorbed and emitted radiation have equivalent wavelengths.
What is Fluorescence?
Fluorescence is a form of photoluminescence in which a substance emits light with a different wavelength to that of the absorbed wavelength. Usually, the emitted light has a longer wavelength than that of the absorbed wavelength. Therefore the energy of the emitted light is lower than the absorbed light.
Figure 02: Fluorescence of Different Substances under UV light-looks like a Rainbow
Most of the times, substance absorb light radiation in the UV range, emitting light in the visible region; thus, we can see a brilliant color emitting from these substances. We can see this color only when we expose the substance to UV light. However, the radiation emission stops soon after we take the substance away from the UV light source. There are many fields in which we apply the process of fluorescence, i.e. mineralogy, gemology, medicine, etc.
What is the Difference Between Photoluminescence and Fluorescence?
Photoluminescence is a form of luminescence that occurs with a photoexcitation via photon absorption. Fluorescence is a form of photoluminescence in which a substance emits light with a different wavelength to that of the absorbed wavelength. Though fluorescence is a form of photoluminescence, photoluminescence may refer to either the fluorescence or resonance fluorescence, which are different from each other depending on the wavelengths of the absorbed and emitted radiation. Based on this, the difference between photoluminescence and fluorescence is that in photoluminescence, the wavelengths of the absorbed and emitted photons can be the same or different. But, in resonance fluorescence, the wavelength of the absorbed photons are higher than the emitted photons.
Summary – Photoluminescence vs Fluorescence
Both photoluminescence and fluorescence are forms of luminescence; light emission. The key difference between photoluminescence and fluorescence is that the photoluminescence occurs via absorbing and emitting photons having either different or equal wavelengths whereas fluorescence occurs via emitting a longer wavelength than the absorbed wavelength.
Reference:
1. “Photoluminescence.” Wikipedia, Wikimedia Foundation, 18 Aug. 2018. Available here
Image Courtesy:
1.”Photoluminescence animation” By BlyumJ – Own work, (CC BY-SA 4.0) via Commons Wikimedia
2.”Fluorescence rainbow” By Maxim Bilovitskiy – Own work, (CC BY-SA 4.0) via Commons Wikimedia