So having previously reviewed incandescent light and fluorescent light sources, now would be as good a time as any for a log entry about Light Emitting Diodes (LEDs).

As with fluorescent lamps, LEDs are light sources which generate their output through luminescence. Diodes are a fundamental semiconductor device which allow current to pass in one direction through its terminals while preventing flow in the other direction¹. This flow in what is termed the forward direction will only occur at a minimum voltage threshold defined for a given diode based upon its design characteristics. The defining material characteristics are somewhat beyond the scope of a simple project log, but suffice it to say that an LED is a diode constructed with selected semiconductor materials and specific impurities such that as current flows through an LED electrons will cross a positive-negative or p-n junction where they will fall to a lower energy level (this property is common to all diodes). The difference in the energy level of the electron as it crosses the p-n junction is released, in part, in the form of heat. However, in the case of LEDs this energy level difference is specifically engineered such that the release can also include photons of a specific wavelength. The result is that LEDs are a source of nearly monochromatic light. LEDs experience losses to heat as a side-effect of the flow of current through the device, but this is a far cry from incandescent which were in an earlier log shown to more efficiently radiate heat as infrared than any other part of their visible spectrum. The exact LED output wavelength can be tailored based upon material selection². Rather than being limited to a couple of ultraviolet emission bands as defined by mercury plasma emissions in a fluorescent, an individual LED can be engineered for an output wavelength along the light spectrum from infrared to ultraviolet and points in between all based upon the selection and combination of appropriate component materials.

Although LED light sources can be tuned to produce light at lower energy levels within the visible spectrum, the most common white light LED design incorporates a blue LED and an associated yellow phosphor. The yellow phosphor is designed to emit a broad spectrum of visible light including reds, greens, and yellows. Some of the source blue LED light is intentionally allowed through the phosphor material resulting in a white light with color rendering performance often meeting or exceeding earlier fluorescent bulbs. The solution is simply termed a phosphor-coated LED. Despite both being monochromatic sources with a phosphor-expanded spectrum, note the differences between fluorescent and LED source outputs below.

Normalized visible light spectrum energy output of a typical fluorescent.5	Normalized visible light spectrum energy output of a typical LED.5

The most encouraging news regarding phosphors is that decades of research has made sure that the quantum efficiencies of current phosphors on the market can be more than 90% (but not yet exceeding unity in the visible spectrum)³. However, the conversion of a photon of energy from a higher wavelength to a lower wavelength results in a loss of energy which can significantly limit the maximum total efficacy of the system. In the case of fluorescent lamps the maximum system efficiency assuming no other losses could be roughly estimated by the ratio of the emitted wavelength (254 nm) to that of mean wavelength of visible light (550 nm)⁴. Based upon an ideal white light source as referenced in an earlier log (250 lumens per watt), fluorescent lights sources would be capped to around 115 lumens per watt. In fact, the highest efficacy noted for any fluorescent system is indeed around 100 lumens per watt. Unfortunately, compact fluorescent lamps compromised this peak efficacy through design modifications such as smaller tube diameters which generally reduced their efficacy to no more than 70 lumens per watt. Phosphor-coated LEDs possess an advantage here because their selected output wavelength is much longer, with blue LEDs commonly operating at the 460-nm wavelength. Although other factors contributing to overall efficiency, aka wall-plug efficiency, will normally factor into a consideration this fundamental difference in favor of LED-based designs is the reason that LED efficiencies of >200 lumens per watt are considered within reach.

Note that all of the LED light sources selected for this project are rated for over 90 lumens per watt. This was not an intentional selection in favor of high-efficiency designs, but rather reflects the general state of the current retail LED market. Even more impressive is that some of these choices do so while providing a claimed 90+ CRI (efficiency and color spectrum/rendering represent typical trade-offs in a commercial light source). This single data point is perhaps one of the biggest factors among many which has driven the rapid standardization towards LEDs as the preferred efficient lighting source.

Having discussed the fundamentals of artificial light generation, and in particular having highlighted some of the significant advantages presented by LED light sources, the next project logs will cover other key concepts at play in this project proposal. Specifically, both color temperature and Color Rendering Index will both receive further examination. The log entries should highlight the limitations of current LED sources while also establishing the team's interest in developing a solution around commodity LED lamps vs. a custom-built LED solution.

¹ Stevenson, Richard. “The LED’s Dark Secret.” IEEE Spectrum: Technology, Engineering, and Science News, IEEE Spectrum, 1 Aug. 2009, spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret.

² Scully, Taylor. “Everything about LEDs: Learn the Basics of LED Lighting and How to Power!” LEDSupply BLOG, LED Supply, 11 Mar. 2015, www.ledsupply.com/blog/what-you-need-to-know-about-leds/.

³ Kane, Raymond, and Heinz Sell. Revolution in Lamps: A Chronicle of 50 Years of Progress. Fairmont Press, 2001.

⁴ Ronda, Cornelis R. Luminescence: from Theory to Applications. Wiley-VCH, 2008.

⁵ “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/.