Why do LED lightbulbs have a bulb?

January 29, 2013 4:41 PM

While changing a bulb in one of our rooms, replacing a LED with a CFL because of a need for brighter lighting in this work area, it suddenly hit me: Why does an LED light have a bulb? In both incandescent and CFLs, the bulbs trap gas (or vacuum as the case may be.) But why an LED? It doesn't use a gas and it doesn't need to operate in a near-vacuum for efficiency. Closer examination showed that the bulb was plastic and had no lensing effect. This question is almost answered here. Because of the question regarding heat sinks (which seem much reduced in modern LED lights, I'm guessing the answer is because they get hot. So I'm guessing that's what it is, though I also think that having a bulb makes it easier to screw into the socket. Taking these points together, I can imagine getting a burn if there was not bulb by brushing up agains an LED stalk that was on until a few seconds ago when you decided to unscrew it. So...have I answered my own question or is there more behind the bulb?
posted by BillW to technology (9 answers total)

There are a lot of fixtures you can see the "bulb" or at least the shape of it, and they'd look stupid if it was a stalk of LEDs.

Also, people are familiar with the bulb shape. Selling something new and different is hard enough as it is. If it's easy to make it look somewhat like something familiar, it eliminates at least some of that.
posted by primethyme at 4:44 PM on January 29 [1 favorite]

They also act as diffusers. LEDs are uncomfortably bright tiny points of light, and the plastic globe helps make the light less harsh.
posted by scruss at 4:51 PM on January 29

It can be a diffuser, or for example, in the Phillips bulb, it's a phosphor. The LED emits blue light and the phosphor absorbs some of that and reemits yellow light. Together, the light appears white.
posted by pombe at 5:05 PM on January 29 [2 favorites]

Yeah, diffuser. LEDs are point-source lights, which cast very harsh shadows. A cluster of LEDs, especially when they are all different colors (I believe that most LED lightbulbs use several colors of LEDs in order to achieve a fuller spectrum) give weird multi-colored shadows with multiple edges. A bit of translucent plastic helps to smooth all that out and make it much more pleasant. Also it can be used to adjust the color temperature of the light, by tinting the plastic.
posted by Scientist at 5:07 PM on January 29

Also there are a lot of things built with the standard incandescent bulb shape in mind, like lamp shades that clip onto the bulb.
posted by xedrik at 5:09 PM on January 29 [3 favorites]

Light bulbs (the OED insists that there's a space between the words) are one of the truly iconic shapes of the industrial era, much like the dial-fronted telephone and the envelope. The manufacturers are introducing something new but they want it to look and feel like a light bulb. The base needs to fit standard sockets, and the top needs to be graspable even when it's in a recessed fitting. The light needs to radiate from a point above the base above the base, otherwise it'll be obscured by many existing shades. And as other people have said, existing LEDs require diffusion to look nice.

If we were starting from scratch I suspect light fixtures would be very different; given the longevity of LEDs they'd probably just be built into the fixture itself and the whole idea of "changing a bulb" would disappear. We'll probably see this in a decade or so, once LED lights or their replacements are good enough.
posted by Joe in Australia at 5:36 PM on January 29 [1 favorite]

I agree with those who suggest that this is often for aesthetics and marketing rather than any technical reason. On the LED bulbs I bought recently the casing was pretty flimsy and seemed to serve little in the way of either protection, heat protection or diffusion of point sources.

LEDs have become the chameleon of light fittings in recent years. It seems there is now an LED version of most common fittings: spots, incandescent bulbs, tubes and so on. What is odd is that there seems to be a dearth of bulb designs (other than Christmas tree lights) which are adapted especially for LEDs. Such a bulb would be a flat circle or a hexagon that could tessellate perhaps.
posted by rongorongo at 5:40 PM on January 29

The more expensive led bulbs its to change the color , the cheaper ones its just for looks.
posted by majortom1981 at 10:57 AM on January 30

Note that not all LEDs come in a bulb form factor - I have LEDs directly mounted on plastic tape about 2cm wide stuck to the underside of my kitchen cupboards.
posted by Dr Dracator at 9:29 PM on January 30