The State of White LEDs
LED technology has advanced significantly in the last ten years. Twenty years ago, you could choose between red, green, and yellow LEDs, with the green and yellow significantly more expensive than the red. A decade ago, blue LEDs started becoming popular, though they were many times the price of the other colours, which had by then gotten cheaper. In the tail end of the nineties, white LEDs made their debut, but it's only today that we are seeing decent pricing, volume, and creative uses for them. This little essay will summarize what today's options are for white LEDs, and how they compare to incandescents.
To start, small traditional incandescent bulbs, often called wheat bulbs for their colour, have existed for a very long time, and have been used in scale hobbies throughout that time. They are great except that they produce significant heat and do burn out eventually. However, they produce a brilliant spectrum of light, which is very soft at the center and picks up a deep orange-brown tone at its fringes. Producing this same effect with LEDs is not possible, as their operating principle relies on a single frequency of light. Nonetheless, options are expanding...
The original white LED (far left) is an offshoot from the technology used to make blue LEDs. This is why a typical white LED has a bluish tint, particularly in the halo, or otherwise lower-intensity region. These lights are good for internal lighting in models, mimicking the custom-made fluorescent tubes that Industrial Light and Magic typically used for interior model illumination.
The next LED is the same white unit painted with Tamiya Clear Orange paint. It doesn't look quite right from the side, but the light reflecting from the paper it is shining on is of a very similar temperature to the colour from the mid-range of a traditional wheat bulb diffusion gradient. The reason it is weaker is that the paint affects the lens of the LED; through a fiber optic strand it still makes for decent scale light. For some time, this was the only way to soften the light from white LEDs. Some manufacturers have even produced LEDs with a clear orange resin to achieve the same effect. The advantage of paint is that the level of orange can be controlled by the thickness of the coating.
A few years back, breakthroughs occurred that made it possible to produce softer light. The first "warm white" LEDs are actually closer to a cold yellow everywhere except at their brightest point. Really, they are just white LEDs with a tiny yellow lense over the silicon. These are not very useful, but started the right kind of thinking to get us to where we are today.
Finally, recent advances make "soft white" LEDs a reality. These LEDs produce a light that is very similar to what is produced by soft white compact fluorescent tubes. Unfortunately, as with CCFLs, there is a very confined spectrum, leading to a much less interesting diffusion colour than what emanates from classic wheat bulb. This photographs with a very "sepia-tone" quality. However, the colour at the source is virtually identical to that of the bulb's filament.
With the colour of modern soft white LEDs, it is finally practical to use these in lieu of wheat bulbs in scale models. This has always been difficult, as many studio models have used a combination of technologies to achieve the right look. For example, the refit Enterprise from Star Trek: The Motion Picture included white fluorescent lighting for the windows, while using wheat-bulb spotlights on the exterior. Using a combination of classic white LEDs and the new soft ones, it is finally possible to get the right effect, or at least come close to it.
Why am I bringing this up at Christmas? The stores are already having clearance sales to get rid of Christmas decorations. More to the point, every $8 chain of soft white LED lights contains 35 of the latest in soft white LED technology. The string of interior lights produced by Noma is actually even more interesting, because their LEDs are the 3mm variety, normally quite difficult to obtain. At less than 25 cents per unit, the price is right!
Update: I have been informed by several people that "grain of wheat" bulbs are so called because of their size and resemblance to a grain rather than their wheaty colour. Learn something new every day.