This entry will differentiate the light output of fluorescent sources from incandescent lights. Highlighting these distinctions is a valuable step even for a project built upon LED lighting. Although primary light generation in an LED light source is completely distinct from that of either incandescents or fluorescents, the similarities LED light sources share with fluorescents are strong enough to justify an examination of the unique properties of fluorescent lamps. Let's review!
Fluorescent light sources generate radiation through a completely different mechanism from incandescent filament bulbs and produce a radiation output spectrum which is also distinct. Fluorescent lamps are a gas discharge light source in which an electrical voltage excites mercury vapor in a low-pressure tube. This excitation results in the promotion of electrons within the mercury atoms temporarily to higher levels of energy. When those electrons return to their normal state energy is released in the form of photons at specific energy levels or wavelengths. These are notable in the case of mercury for being in the ultraviolet spectrum. Rather than a continuous spectrum of wavelengths, fluorescent lamps instead exhibit an output radiation spectrum with peaks at a few wavelengths in the invisible UV range1 (see the figures below). This process, distinct from the emission of light through heating which defines incandescence, is known as luminescence.
|Normalized visible light spectrum energy output of a typical halogen incandescent source.4||Normalized (internal) output spectrum of a mercury vapor fluorescent (all peaks in UV).5|
At this point fluorescent sources have been defined by the method through which they convert electrons into photons. However, what has been reserved until now is the ultimate method by which these sources produce white light given the limited spectra output relative to the continuous band emitted by sources like the sun. For fluorescent sources the simple answer is phosphors. Through the process of photoluminescence substances known as phosphors, typically composed of various inorganic compounds in combination with rare earth metals, are used to absorb energy output by LED and fluorescent sources and convert them into lower energy wavelengths2. For fluorescent light sources this is critically important since the lamp must minimize the amount of UV radiation emitted to keep UV output within safe limits. Early fluorescent phosphors admitted UV photons and radiated visible light in the blue and orange parts of the visible spectrum. This light output (figure below) effectively produces a white light source but its associated color spectrum differs significantly from that of incandescent sources and drives a worthwhile discussion on the color rendering performance of the light (reserved for a later post). More modern fluorescent phosphor formulations known as triphosphors emit visible light in the blue, green, and red areas of the spectrum with an associated improvement in color rendering (usually requiring an increase in cost and complexity of manufacture)3.
|Normalized visible light spectrum energy output of a typical fluorescent.4|
LEDs present a slightly different opportunity since their light source is materially different and, as such, are worth a separate post!
1 Schubert, Fred. Light Emitting Diodes. 2nd ed.
2 Jones, Eric D. “Light Emitting Diodes (LEDS) for General Illumination: a OIDA Technology Roadmap.” Mar. 2001.
3 “How Is White Light Made with LEDs? | LED Lighting Systems | Lighting Answers | NLPIP.” Lighting Research Center, Rensselaer Polytechnic Institute, 2003, www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whiteLight.asp.
4 “Efficiency of LEDs: The Highest Luminous Efficacy of a White LED.” DIAL, DIAL GmbH, 4 Aug. 2017, www.dial.de/en/blog/article/efficiency-of-ledsthe-highest-luminous-efficacy-of-a-white-led/.
5 Jüstel, Thomas. “Fluorescent Lamp Phosphors: Is There Still News?” Mar. 2007, www.fh-muenster.de/ciw/downloads/personal/juestel/juestel/Fluorescent_Lamp_Phosphors-PGS-050307.pdf