Lighting – Light Emitting Diodes [Part 1 – Theory]

Efficient lighting is achieved when most of the energy conversion occurs when electrons transition between high and low orbitals. Maximize electron energy and minimize molecular/atomic energy.

Despite the daunting physics behind the devices, the operation of the first forms of electrical lighting have been pretty simple. Dump a lot of energy into a source of matter (solid or gas) until it glows.

The innovation behind the light emitting diode [LED] is very different in this aspect, requiring only a fraction of the energy in comparison to incandescent and fluorescent light sources. This is observable in all aspects of technology, as LEDs are being incorporated into just about everything.

A very popular use for LEDs is in the detection of current within a circuit (connected in parallel, of course). This gives the user an extremely easy visual signal with minimal power consumption and user effort required (since multimeters are so Old School)!

LEDs are essentially electrical “gates” (technically called diodes) that allow electrons to only flow through if given a decent enough “push” (a couple AAA batteries will do just fine). When the electron passes through, it “churns” the gate which releases “energy” in the form of a particle of light.

That was a bad analogy; way too abstract. Let’s try that again.

The production of light within an LED occurs at the interface between a two-layer system. These layers play the roles of electron orbitals, similar to the gas-discharge system described previously. One layer acts as a “high orbital” state at a specific energy, while the other acts as the “low orbital” state.

With an applied current, the majority of the electrons progress, traversing high-to-low energy states. These electrons release light particles [photons] when they make the transition. Since the materials do not change, the color of light emitted is (relatively) constant. LEDs only give off a single color, which is determined by the energy difference between the LED’s two layers.

A rule of thumb; a higher energy differential is typically harder to manufacture. Blue LEDs were truly worth a Nobel prize, as seen within the past year.

Note: Most developed LEDs consist of >3 layers. While only one interface between two of these layers results in light production, additional layers play significant roles in optimizing efficiency. A layer can assist in an electron’s transition between metal and semiconductor, reducing device heating.  Other layers can decrease the amount of leaky current (kind of like duct tape around a bad hose).

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“White LEDs” are a misnomer to some extent, because white is a mixture of colors that a single LED cannot accomplish alone. There are two ways to make an LED white.

  1. Place three LEDs (red, green, and blue) in extremely close proximity to each other and turn them on. One can change the LED’s color by changing the current flow between each of the LEDs, allowing for a nice nightlight effect!
  2. Take a blue LED and give it a phosphor coating. A large portion of the blue light undergoes fluorescence in the phosphor and is re-emitted as green/yellow/orange/red light. This is the more popular method since phosphor coatings are typically cheaper to incorporate.

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In the next post, I will (attempt to) write about the fabrication process behind LED manufacturing. And I hope it’s quite the treat!

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Lighting – Fluorescence

In the last post, I talked about using plasma as a source of lighting. Exciting electron flow in a low-pressure gas, electrons release energy in the form of light as the electrons drop from high energy orbitals to low energy orbitals.

However, there are also a lot of free-flowing electrons that have even more energy than electrons in high-energy orbitals. When they drop down to stable energy levels, the emitted energy overshoots the visible spectrum into the UV regime. Not only is this a drop in efficiency but it also poses health issues [for obvious reasons].

The nice thing about UV radiation is that it CAN be down-converted into visible light . The technical term for this process is Fluorescence. Electromagnetic energy gets absorbed by a specific material and gets re-emitted at a longer wavelength. Two quick technical factors:

  1. To converse energy, the energy difference is absorbed by the material typically in the form of thermal energy (In – Out = Heat). So yes, the bulb does heat up over time, but not as fast as an incandescent bulb.
  2. And since nature prefers disorder,  visible light will not be absorbed and emitted in the form of UV rays. The only extremely rare exception is when you have A LOT of EM energy in a confined space (anything weird like this is typically labeled “non-linear”).

Lots of things fluoresce! Natural things (like the rocks below) fluoresce a variety of colors when exposed to UV light.

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Then again, humans also make a lot of artificial stuff that experiences the same effect (like your favorite energy drinks and alcoholic mixers)!

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So where am I going with this….

A fluorescent light bulb is typically the same as a gas-discharge bulb [typically mercury-based], except they “spray” the inside of the glass tube with “phosphor” that ironically does not contain Phosphorus, the element. Instead of choosing gas mixtures, phosphors utilize a blend of elements to fine tune the final lighting color.

Even the phosphor thickness has its consequences. While the Mercury plasma emits primarily UV/Purple/Blue rays (high energy), the phosphor typically emits Green/Yellow/Red rays (low energy). A yellow/orange glow could be acquired by applying a thicker phosphor coating to the bulb permitting less purple/blue light radiation from escaping the bulb’s interior.

Note: Blacklights are basically the same thing except the bulb is coated in a material that filters out all radiation except everything between blue and near-UV (. And that is why “blacklights” glow purple!

In terms of efficiency, fluorescent light bulbs (~20%) are significantly more efficient than incandescent light bulbs (< 5%). And I hope you know why now!

 

 

Lighting – Gas-Discharge

In the previous blog, I talked about incandescence and how heating a material is utilized to produce light, inefficiently. The broad range of electromagnetic radiation emitted from the source is due to a plethora of different interactions on the atomic level each resulting in a specific amount of energy release.

A very simplified list of interactions => energy is as follows:

Molecular rotation => microwave energy

Molecular vibration => infrared energy (~heat)

Electron orbital shifts => visible energy (~light)

If someone wants to improve lighting efficiency, one has to focus on maximizing electron orbital shifts. And this is where plasma come in.

If you are not familiar with plasma, don’t feel too bad. A plasma occurs when there is enough energy present to strip the electrons from their atoms. This results in this hot, disorderly mess of atom nuclei and run-away electrons. The most naturally occurring forms of plasma in our lives are the sun in the sky and lightning in thunderstorms.

Turns out that since stars are what make up almost all that we see in space, plasma makes up >99% of our visible universe. And as long as I’m the minority [AKA, not a plasma], that’s cool!

To create a gas-discharged plasma, follow these steps!

1) Contain a gas in a glass tube (preferably at low pressure)

2) Stick electrodes at the ends [which don’t melt]

3) Find a high voltage power source and attach to the electrodes

4) Throw back the massive ON switch and laugh menacingly! [Maybe wear UV glasses too, with the cool, dark tint]

The unique thing about plasma is that since the electrons are stripped away, there is no longer any atomic bonds available for molecular rotations and vibrations. Keeping the gas pressure low further reduces the amount of energy released from atom-atom interactions.

Thus, the main form of energy release is in the form of electrons shifting orbitals in the atoms.  If you have taken quantum physics, you also know that these orbitals are “quantized.” And based on these specific orbital levels, the atom can only give off specific colors of light [its Atomic Emission Spectrum].

Hydrogen: Red, Cyan, and Purples

Helium: Dark Red, Yellow, Aquamarine, Blues, and Purples

Etc…

Thus, each atom on the periodic table has its own unique atomic “finger print.” It’s one way we can determine how much Helium/Carbon/Iron/Etc. exists in a star. Or a burning flame. That is also why we can get packets of “magical dust” that turn your campfire green!

By mixing and matching gases [Noble Gas / Mercury Vapor / Metal Halide / Sodium Vapor], one can create varying levels of color, white warmth, and intensity! Also, any metals can also easily vaporize into a gas form, expanding your possibilities (Yay, gaseous Mercury!).

And since you can’t “melt” a gas, you can pump out A LOT of visible energy to illuminate a large area from a single unit. This makes them great for large audience venues (stadiums), movie theater projectors, and car headlamps.

Lighting – Incandescents

 

If you look up incandescence under a dictionary, the term relates to the release of electromagnetic radiation from a source with heat. And with that, everything releases EM energy.

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Once can visually see light bulbs  with your eyes. You can observe animals with the help of thermal cameras. Even the cold depths of space itself at -270 C (~3 degrees above absolute zero) can be detected in the form of microwave radiation [https://xkcd.com/54/].

If you want to have an object visibly glow, all you have to do is heat it up above 900 C. That’s not the problem  with the creation of practical bulbs; just run lots of electrons through it to cause thermal vibrations and heat the material. It’s getting the material to handle the heat without melting/burning/warping/etc.

So when inventors, like Thomas Edison, were inventing long-duration filaments, they were actually playing the role of material scientists. That’s where carbon and tungsten came in as key factors in long-lasting light bulbs; they don’t “melt” until brought above 3000 Celsius. They also contain the filament in an inert gas, so it doesn’t oxide really fast (or in simpler terms, catch on fire).

The radiation coming from a heated source isn’t as simple as shifting what colors an object emits. Mass gives off energy all across the spectrum from radio to gamma, forming a skewed “bell-shaped” curve. The hotter an item, the more energy it gives off at all levels (or wavelengths). Furthermore, the center of the bell curve shifts towards shorter wavelengths (infrared -> red -> green -> blue -> ultraviolet -> and beyond). HyperPhysics has a good image that illustrates this property very well: [http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/wien3.html]

This is why one can classify a star’s temperature by its color. Red stars are “cool” and yellow stars are “warmer” as the brightest color shifts from red to yellow. White stars  hit a sweet spot where the temperature is “hot” enough to radiate equal amounts of red, green, and blue light [that is also why they are not green]. And the most intense stars overshoot the peak into the blue/violet/Ultra-violet (invisible) resulting in a bright blue hue.

Note: Even if an object is hot enough, we will never perceive it as being purple, because the cones in human eyes are so much more sensitive to blue light [the same reason why the sky is blue and not purple despite the strong inverse-wavelength dependence of  Rayleigh scattering].

And that is the problem with incandescent light bulbs. Most of the energy they emit isn’t even noticed (unless you are using the Infrared rays up close as a heat source). Thus, they are very inefficient source of lighting. Efficiency is usually determined by (Visible Light Power Out) / (Electrical Power In).  And the current efficiency values of typical light bulbs is ~2 %.

So if you are too cheap to upgrade your tree with LED lights for your Christmas tree this year, just remind yourself that your old-school bulbs create a dazzling holiday display while assisting in the heating of your house this winter. And don’t forget to purchase the “shimmering” lights, where the filaments warp upon heating, cycling through open and closed circuits, and resulting in a slightly random twinkling effect!

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Happy Holidays!