Versions of this question also include: Why don't LED bulbs get hot? How are LED lights different than standard mini lights?
The answer to these questions covers a very broad range of super-technical information. I’ve tried to summarize the major points as best as I can without geeking out too badly.
How is LED light generated?
LED stands for Light Emitting Diodes and unlike incandescent bulbs that operate by using electricity to heat a filament that produces light, they are semiconductor-based.
LEDs generate light when free electrons are excited - or energized - by an application of voltage - and then fall to lower energy levels. Manufactured with the correct materials, this lowering of the energy level of free electrons produces photons of light. (Who knew photons were so pretty?!) The metals used in the semiconductor substrate determine the color of the light. Since materials vary in cost, this makes the final price of LED light strings vary based on color. (For example, materials to create blue, purple, pink and teal lights are pricier than yellow and red.)
Random note: Since LED lights use technology similar to that used in your home computer, it’s always a good idea to plug them into a surge protector to protect them from surges and noise on your electrical lines. This is in addition to plugging your outdoor lights into a GFCI protected outlet.
Well then, how do incandescent bulbs produce light? or How do incandescent bulbs work?
Traditional incandescent Christmas lights generate light by heating a tungsten wire which glows in response to increased electrical current in the filament.
So, right off the bat, you can see one advantage of LED lights. They light up your Christmas tree without generating heat.
On the other hand, one big advantage of a traditional mini light bulb is the ambiance of the light and consistent color of the light they produce.
From bulb to bulb the color of incandescent lights is consistent since most bulbs are manufactured using the same filament material (usually tungsten) and the bulbs are filled most of the time with argon. From year to year, these lights will basically be consistent no matter who the manufacturer is.
In contrast, LED bulbs have a wide swing in color produced. White can be warm, really warm, pure, cool, super-cool, and almost blue. Staying with the same manufacturer and color profile each season becomes a factor in Christmas display planning.
More thoughts on white LED Christmas lights....
In the mid-2000's, pure white lights were called "winter" or "cool" white and used to have quite a bit of blue in them but have become much more neutral or “pure” over the last few years. Their manufacturing technology has improved and the light wavelength “drift” which caused their color to swing within a range of whiteness has significantly improved.
You'll see the results of this improvement if you've been adding to your display over the years and have seen a color change in your pure white bulbs both between strings and between bulbs on the same string.
LED Christmas lights that have color (ie., red, green, blue) have the advantage of using lenses that are manufactured with pigment distributed through the polycarbonate material which makes their color more resistant to sun fading. The lenses make color variations much less noticeable.
More thoughts on LED light strings....
LED lights also experience ‘burn in’. After a few weeks of operation, the color will change very slightly. This effect isn’t noticeable unless you mix the lights in operation with new ones. The difference will be really subtle but it’s good to know what to expect and to purchase most of your white LED lights in a single purchase, if possible.
Hopefully, this quick article gives you a quick understanding of the differences between the different way LED and incandescent bulbs produce light as you plan your Christmas lighting purchases.
Let us know if you have any questions!
Originally published as part of a magazine article response on Nov 19, 2014 8:30 am