Figure it out yet? Any idea what’s going on in the image above? Here’s a hint: that’s a Philips LED bulb in action.

There’s no two ways around it: many potential buyers have been turned off by the yellow cap pieces on some LED bulbs. These bulbs might be efficient, increasingly affordable, and last for upwards of 25,000 hours, but what the heck is with that day glow yellow?

Those three yellow pieces on the top of certain LED bulbs are the remote phosphor… and they are not there just because some misguided designer thought they looked cool. These are a crucial part of the design for one reason, which the image above makes abundantly clear: the bulb uses blue LEDs. What the remote phosphor does is convert that blue light to a shade that is more acceptable to what we expect from indoor lighting. In this case it’s 2700K, or warm yellow.

So, as you probably guessed by now, the image shows a bulb that is missing one of its remote phosphor panels. The phosphors that are in place are doing exactly what phosphors do — emit light through the process of luminescence. What this means, functionally, is that the yellow pieces get the blue light and convert it into exactly the shade of light that Philips is looking for.

As for the “remote” part, that just means that the phosphor is not connected to or built into the LEDs. This is done for a number of design reasons, including the placement of the highly directional LEDs and to prevent multiple shadows from forming.

When one of the yellow pieces is removed — be careful of your eyes if you try it at home — you can clearly see that the light from that third of the bulb is a sort of royal blue. The rest of the light goes through the phosphor and is emitted as a shade of white.

And those neon yellow plastic pieces? When the bulb is on you can’t see them at all, which is why the remote phosphor isn’t as big of a problem as you might think when seeing a bulb on the shelf.

As you can see in the smaller image, Philips’ AmbientLED uses three sets of six blue LEDs, plus the remote phosphor, to generate its light. The company’s LED Prize bulb, in order to take efficiency to the next level, uses a different mix. That bulb uses three red LEDs, two blue, and then another shade of blue. It’s a more complex operation but it helps the L Prize winner get to an excellent 94 lumens per watt, where the normal model gets about 65.

And why use blue LEDs at all? Why not just use LEDs that produce a color that we like? This gets pretty complex, but basically it comes down to something predictable: blue LEDs are more efficient than other colors.

If you were interested in this post, you might want to check out my book, LED Lighting: A Primer to Lighting the Future. It’s available from O’Reilly (DRM-free), Amazon, iBooks, and others.