Pique the Geek 20101212: LED Christmas (and Other) Lights

One of the more revolutionary innovations in lighting is the Light Emitting Diode, or LED.  This cutting edge technology was first discovered 103 years ago!  However, only comparatively recently have LEDs been either efficient or cheap enough for wide use.

LEDs operate just like any other diode, allowing an electric current to pass, for the most part, in only one direction.  They are built by placing into contact a P-type (positive) and an N-type (negative) semiconductor and passing a current from the N to the P materials.  In the case of LEDs, when the electrons and holes recombine, light is emitted.  In most diodes, heat is emitted.  Actually, LEDs do produce some heat as well and this becomes important for reasons to be discussed later.

The first commercially useful LEDs came out around 1968, mostly used by Hewlett-Packard for its higher end handheld calculators.  Before that, the only feasible displays were fluorescent.  Does anyone remember the old “red dot” HP displays, or the blue fluorescent ones?  Interestingly, very few calculators still use LED displays because of relatively high power consumption when compared to liquid crystal displays.  Whilst LEDs require a current to illuminate them, LCDs require only an electric field and are actually illuminated by ambient light, so battery requirements are much lower.  I have a little Casio that works well with only light from a 23 watt CFL near the ceiling.  It has an LCD.

LEDs do many, many things very, very well.  However, currently room lighting is not one of them, and until around 1995 when the blue LED was developed, not even feasible.  Part of the problem is that to get enough lumens to illuminate a room, high power LEDs are required, and they give off enough heat to damage the LED materials themselves unless good heat conductors (heat sinks) are provided, thus driving up cost.

Another disadvantage in room lighting with current LEDs is that color rendition is not very good yet.  We will get to that in a while, after we examine color perception.

Let us explore this just a bit.  In light produced by blackbody radiator, a continuum of wavelengths is produced.  Common blackbody radiators include the sun, incandescent light bulbs, and heating elements in electric ranges.  These are called blackbody radiators because the wavelengths of light produced depend only on the temperature of the body.  The heating element in a range gets only to around 1000 Kelvins (water boils at 373 K), while an incandescent bulb filament is around 3000 K.  The photosphere of the sun at midday is around 6000 K, making sunlight bluer than incandescent light.  The important point is that very many wavelengths are produced at the same time.

When such a continuum of wavelengths impinges of a colored object, some wavelengths are absorbed and others reflected.  For example, if an object appears to be a pure yellow, it is because green light is preferentially absorbed, and white light minus the green wavelengths are reflected and eventually focused onto our retinae, where the cone cells absorb either red, green, or blue wavelengths.  Because of extremely elegant mechanisms of perception of color, the object appears to be yellow because our brains interpret this deficiency of green to look yellow.  Our entire visual evolution has been developed with blackbody radiators, so the way we perceive color is unlikely to change in the near future.

LEDs operate completely differently.  In an LED, the wavelengths produced are very nearly monochromatic (approaching a single wavelength).  Thus, a LED looks, for example, yellow not because it is white minus green, but because ONLY yellow wavelengths are produced.  Going back to our example of a yellow object, if we illuminated it with a green LED, it would appear to be black, since all of the green wavelengths are absorbed by yellow objects, so there is no light remaining to be reflected to our cone cells.  Thus, LEDs are fine for street lamps where color perception is not critical, but poor for home lighting.

This same problem exists with fluorescent lamps.  The low pressure mercury are produces mainly light at 253.7 nanometers, far into the ultraviolet and invisible to the human eye.  However, materials called phosphors are used to coat the interior of the lamp, and these materials absorb the 253.7 line and reemit it in the visible, a phenomenon known as fluorescence.  By using a mixture of different phosphors, it is possible to produce a continuum of light that resembles that from a blackbody, although never quite the same.  However, it is much better at color rendering than the currently available LEDs.

Now, it is possible to use ultraviolet LEDs and phosphors to do the same thing that is done in conventional fluorescent lamps, but it is not quite as simple.  The most common way to approximate white light in LEDs is to use a blue LED constructed in such a way that some of the blue light is converted by a phosphor to yellow light.  The combination of blue and yellow is perceived as white light by the human eye when shining on white objects, but because of the complex interaction of colored objects, the wavelengths of light impinging on them, and the mechanism of human color perception they reproduce colors rather poorly, as described above.

A new way to produce an approximate blackbody curve with LEDs is to use quantum dots.  Very basically, a quantum dot is a nanomaterial that reradiarates light at different wavelengths depending on the size of the dot.  Thus, by using quantum dots of many different sizes, something like a continuum can be produced.

At present, for lighting purposes LEDs are less efficient than compact fluorescent bulbs.  Affordable LEDs have an output of around 45 lumens per watt (three times better than incandescent bulbs at around 15 lm/w), but only about half as efficient of CFLs at around 100 lm/w.  And still there is the color rendition problem.  I suspect for the near term, CFLs will continue to be favored, although I also suspect that LEDs will gradually get better and better.

Fluorescents have the advantage of being relatively immune to damage from heat.  I mentioned that LEDs are quite heat sensitive, and white ones have the double whammy of the conversion of some of the blue to yellow.  Remember, blue light is more energetic, einstein for einstein, than yellow light.  In converting blue to yellow, that excess energy has to go somewhere, and that somewhere is degradation to heat.  This heat has to go somewhere, and sinking it adds cost.  There is no such of a problem with CFLs.  Also, like many solid state devices, LEDs are more efficient at low temperatures than at high ones, so the light output decreases with increasing temperature.  In fluorescents, the opposite is the case.  Thus, LEDs work better in cold environments than in hot ones.

LEDs are, however, extremely rugged.  Properly designed, they can survive shocks that would shatter either incandescent or fluorescent lamps.  This speaks in their favor for severe environments that have lots of vibration, impact, or other mechanical hazards.  In addition, LEDs are essentially immune from damage caused by cycling them on and off, while both incandescent and fluorescent lamps suffer damage each time that they are cycled.  As a matter of fact, LEDs can be cycled at thousands of on off transitions per second with no damage.  Both of the other kinds of lamps are damaged at each cycle, mostly due to thermal shock to the filaments at each cycle.  Since LEDs have no filaments, there is no thermal shock.

LEDs also, if not damaged by heat, have an extremely long lifetime, measured on the order of 100,000 hours of operation (typical ranges are 25,000 to 100,000, depending on operational conditions).  I am looking at a box that held an 18 w, 1200 lumen CFL (efficiency of 67 lm/w, CFLs are less efficient than traditional, bigger ones) and they have a stated lifetime of 10,000 hours.  The box of 100 w incandescent ones says, with an output of 1690 lumens (efficiency of 16.9 lm/w) the rated lifetime is only 750 hours.  This makes LEDs ideal in situations where replacement is dangerous, costly, or both.

Now, what I have up to now described as a liability in LEDs becomes an asset.  LEDs are unsurpassed for Christmas lights because of the very fact that they emit almost monochromatic light.  This very color purity is decoded by our central nervous system as extremely deep and rich.  The only other similar richness of color of which I can immediately think of is fireworks, about which I have written several times here, and it is for similar reasons.  In addition, because they run so cool, the danger of igniting any combustible object with them is nil.  Since they are brighter in the cold, for the northern hemisphere they look even better!  A string, for example, of 50 6 w incandescent bulbs consumes 300 w, while an equally bright string of LEDs consume only around 100 w (actually less since we are talking colored ones rather than white ones).  All colored incandescent lights utilize filters (usually colored lacquer on the bulbs), and that cuts luminosity significantly, because the undesired wavelengths are absorbed and converted to heat), whilst colored LEDs radiate all of the light.

Finally, and on a completely different note, I must mention that my brother sent be a huge shipment of photographs, slides, original recipes written in our mum’s hand, and other family heirlooms that I received Thursday.  I really appreciate it, and shall call him tomorrow to thank him in person.  I know that he often reads my rants here, so perhaps he will see it tonight.

Well, you have done it again!  You have wasted many einsteins of photons reading the dim post.  And even though John Kyl gives up his dream of nuclear proliferation when he reads me say it, I always learn much more than I could ever hope to teach by writing this series, so keep those comments, questions, corrections, and other correspondence coming!

I have been a bit under the weather recently, but am much better now, so I will hang around here for Comment Time until comments quit coming, and shall return here tomorrow after Keith for Review Time.  I shall also be back Friday with a new installment of Popular Culture.

Warmest regards, and Happy Christmas to all!

Doc

Crossposted at Docudharma.com and at Dailykos.com