Lighting models II: LEDs

Welcome to my article on model lighting! I cover different technologies that are useful for people who want to illuminate scale models of various kinds – cars, spaceships, dollhouses, etc.

And I don’t mean shining external lights on the model, of course. I’m referring to the technology to make tiny light sources that appear to-scale with the model itself.

CHAPTER CONTENTS

LEDs

LEDs have, since the 1990s, transformed lighting for model making. 

They're very energy efficient, they're available in just about any colour you want, they mostly run cool, they're quite cheap, they're easy to control by a computer for animation and colour control, they have incredibly long lifespans given the right power levels, they're available in all kinds of sizes including unbelievably tiny devices... the list goes on. It's a real change from the 1970s when LEDs were these dim lights in red, orange, and green from your local electronic shop.

What’s an LED?

It’s an acronym for "light-emitting diode," and normally pronounced as three separate letters. LEDs are small plastic cylinders or blocks containing tiny little pieces of silicon that have been treated with certain materials.

The “light-emitting” part is straightforward - feed electricity to an LED and it glows brightly. The light is of a specific colour, and depends on the materials used in its manufacture. A "diode" is an electronic component that lets electricity flow in one direction only. Think of it as a one-way valve that works with electricity and not water.

This all relies on mysterious quantum principles that few people truly understand. Fortunately, while the physics are arcane, the LEDs themselves are reliable and easy to use once you understand the basics, which is what this article is about.

LEDs are not perfect light sources by any means. They're a bit counter-intuitive to wire up since they follow different and less obvious rules from light bulbs, they only operate within a limited power range and promptly burn out if fed too much electricity, they can generate heat and suck power if used in large numbers, they're polarized and so must be hooked up to DC power sources with the polarity (positive and negative) correct, they're point light sources and not so good for making flat glowy surfaces, and so on. But on the whole they're the best solution for most model makers.

LED colour

Incandescent light bulbs always produce yellow-white light and thus require tinted glass to emit light of a different colour. But LEDs operate under very different principles.

Single-colour LEDs are designed to produce pure light of a narrow and specific wavelength. And the wavelength is a physical property that determines what colour that light appears to the human eye. A red LED will always emit red light. A green LED will always be green. You can't get a red LED to produce green light, by changing the voltage or frequency of power or anything like that, since the red output is inherent to the specific combination of chemical elements that were used when the silicon chip was manufactured.

You also can't change a non-white LED's colour by dyeing the plastic, sticking a filter in front of it, or putting transparent paint on the end. In fact, since the light output is determined by properties of the chip, it isn't necessary to colour the plastic of the LED. Some red LEDs, for instance, are encased in red plastic; some in clear.

For many years, LEDs were only available in red, yellow/orange, and green. But in the 90s blue and white LEDs were invented by Shuji Nakamura at Nichia in Japan. This revolutionized electrical lighting, and low-consumption household lighting suddenly became possible. It also ushered in an age of far more sophisticated model lighting, especially when devices containing three or more LED colours were invented.

White LEDs

White LEDs come in two basic flavours. Some are actually tricolour devices - they contain three separate red, green, and blue LEDs which, when lit together, produce an approximation of white light. LEDs which use the RGB three-diode approach have three separate visible components when viewed under a magnifier. Some of them also leave weird hints of coloured trails when moved rapidly, because of the way they pulse the diodes. These tricolour diodes typically don't produce perfectly white light, however, as it's difficult to get the balance of each of the three constituent LEDs right.

Therefore most white LEDs these days are actually blue LED chips coated with a thin layer of material which glows yellow when hit by blue light (a phenomenon known as Stokes shift fluorescence, for those interested). And this combination of blue and yellow light looks sort of white to the human eye. The fluorescent compounds are why some white LEDs can look lemon yellow in colour when they’re powered off.

These diodes are commonly available as "cool white" (pale blue cast to the light) LEDs and "warm white" LEDs (there's a slightly yellow cast or tone to the light, to simulate tungsten incandescent). Cool LEDs produce a really fake-looking light because this mixture of blue and yellow looks nothing like light from the sun or even a tungsten bulb. These LEDs generate very little green or red light, and so non-white objects can look very unnatural when illuminated by them.

Many warm white LEDs produce higher quality light than cool white since they contain different mixtures of fluorescent material which emit more red. This reduces their efficiency slightly but also makes their light slightly closer to full spectrum (ie: midday sunlight). However some “warm” white LEDs can actually generate rather sickly yellow or greenish light - it all depends on what the manufacturer has used to make them.

In short, not all white LEDs are created equal, and there can be a huge variation between the light produced by one LED and another. Some more closely approximate full spectrum white light than others, owing to the specific kinds of fluorescent material used. Some may also distribute fluorescent material more evenly, to avoid the common white LED problem - yellow light surrounded by a blue ring of light. The only way to know what an LED is like, frankly, is to buy an LED and test it.

Incidentally, you can tint the output of a white LED somewhat by using transparent paint or a coloured filter if you need to. In this respect they work a bit like normal light bulbs, though they don't work as well since they mostly aren't full spectrum.

LED colour temperature

Colour temperature is a complex concept beyond the scope of this short article. But at its simplest it’s a way of describing either the yellowness or blueness of white light, using either descriptive language ("warm" or "cool") or numeric values (2700 Kelvin or 5500K, say). It's not necessarily the actual physical hotness or coldness of the light source.

The terminology was developed for light sources like incandescent bulbs, but has been extended for LEDs and other devices as well, since emulating traditional lighting technology is very important for the newer tech. And it's key to remember that white light is very much a subjective concept. In fact, our sensation of light is as much governed by the way our brains interpret incoming photons as it is the way those bits of energy are recorded by our eyes.

There's quite a variation in colour output of different white LEDs, depending on the manufacturer's choices. The most common white LEDs are of the "cool" or blue-ish variety. Typically this means a colour temperature of about 5000-5500 K/Kelvin. The problem with “cool” blue light, even though it can accurately simulate daylight at noon, is that it looks pretty unnatural when used in a dark space.

Take a look around you at night - you'll see quite a range of colour, but a lot of "white" light is actually pretty yellow in tone. This is partly for historical reasons - tungsten light was the only artificial light around for many years - and partly for aesthetic reasons. Warmer indoor light is much more flattering to people’s skin, for example. Bluer light is kind of harsh and can even make people look a bit sickly. It's cool white LEDs, which were the earliest white LEDs, which helped give LED lighting a bad name, I think.

So, as noted in the previous section, LED makers have been selling "warm" white LEDs (usually around 2700K) which are designed to approximate the light output of old-fashioned tungsten bulbs. Some of these warm LEDs look excellent, and produce light that’s surprisingly close to an old-school light bulb. But as mentioned earlier, others look awful, and are nasty greenish-yellow in tone. Unfortunately the only way to know what works is to buy and try. There are also “neutral” white LEDs, which tend to be around 4000K.

Colour temperature for model makers

Many model makers like to use cool white LEDs, especially for things like rocket engines. But personally I'm not crazy about cool white – it just looks like cheap consumer electronics to me. The colour tone looks fake – the lack of red in the light in particular makes it look unnatural.

And lots of model situations call for warm lighting. Movie spaceship models from the 1970s and 80s were often lit with tungsten bulbs. A doll house probably won’t call for anything other than very warm white LEDs. You’re going to want cosy yellow lights in your model bedrooms and living rooms, not harsh cool white sources. A fridge interior is possibly one of the few exceptions here! A model railway village or town, with its windows glowing at night, will need yellower tungsten light for realism. A contemporary highway scene for a model city would look best with a mixture of warm and cool lights for vehicle headlights, depending on whether it's an old or a new car.

But having said that, always use whatever seems more realistic, and what floats your proverbial boat!

Longevity

One of the great things about LEDs is that they can be really long lasting. Our old friends incandescent bulbs are pretty fragile. They burn out after a while. They get quite hot and can burn or yellow other parts of a model. The glass cracks if something drops on them. If you bang or shake a model the filament might break. They get dimmer and yellower as they're used. And so on.

By contrast, LEDs are solid electronic devices that can produce light for huge numbers of hours. Smaller LEDs used in models don't get hot, and are quite energy efficient. They're pretty resistant to vibration, making them ideal for things like cars. The main drawback involves white LEDs which use fluorescent/phosphor materials. Just like fluorescent tubes, these materials will fade with use.

Unexpected LED Behaviour

We're all used to basic things about how incandescent light bulbs function. You decrease the voltage to a bulb, and it glows more dimly. More voltage, it's brighter. You chain a bunch of bulbs in series, and they all glow at a reduced level. And so on.

LEDs don't work like this. And much of the confusion that people experience when working with LEDs stems from this basic misunderstanding – LEDs are not incandescent bulbs! There are many weird things about LEDs that seem counterintuitive when you get started, but the big three are LED polarity, LED failure under high current conditions, and problems with dimming LEDs.

Polarity

This one is really simple. Unlike light bulbs, which have two leads that can be wired up either way round, an LED must have its positive lead attached to a positive electrical connection, and its negative lead attached to the negative. If you wire a normal LED backwards it won't light up. Pretty straightforward.

Current

However, a more complicated difference between LEDs and light bulbs is the former's use of current. Basically, LEDs have their Kryptonite or an Achilles heel: they can only work when given power of a certain number of milliamperes. If you exceed this amount they'll eventually stop working! Poof. Depending on how far you’ve gone over, this reduced lifespan could be could be microseconds or days.

And note that this is current, not volts. That's a different issue.

This is actually both an important and complex issue, so for more information, have a look at chapter III, on powering LEDs.

Dimming an LED

Another important difference between incandescent bulbs and LEDs involves dimming. Incandescents simply get dimmer when you turn down the voltage. There are some practical complications - human light perception is basically logarithmic and not linear, household lights that run on AC are dimmed using different techniques, and finally light bulbs produce yellower light as they get dimmer. But on the whole, dimming a tiny low-voltage incandescent bulb is relatively straightforward.

LEDs, on the other hand, can’t dim gradually like this. They get a bit dimmer and may colour-shift very slightly as you drop the voltage, and then they simply stop emitting light. So you have to simulate a dimming effect by turning the LED on and off at an extremely high rate. You then vary the amount of time the LED is on compared to the time it’s off - a technique known as pulse width modulation (PWM). An LED that’s on 75% of the time, for instance, will seem brighter than one that’s on 25% of the time. The colour won’t shift using this method.

The key to making this technique work is the frequency of the blinking. Low-frequency PWM produces noticeable flicker and strobing, especially if the LED moves or your eye moves. But if you pulse the LED fast enough the human eye can’t see the blinking and simply sees a reduced light output.

The point of all this is that dimming an LED properly is complicated and requires more hardware than just sticking a rheostat into a circuit. You can use a micro controller (see section IV) to do this.

LED plastics

The body of an LED is made of a light-transparent epoxy plastic. Sometimes LEDs use transparent material that's tinted the same colour as the LED's light output – red or green or whatever. Other LEDs are made of totally clear untinted or "water clear" plastic. And others are "diffused:" cloudy or milky to soften the light somewhat. Infrared LEDs, which mostly produce an invisible form of near-light energy, are often dark purple or nearly black.

LED sizes

Small LEDs are mostly differentiated by their physical size and shape, and their colour. They're all internally built around the same design - tiny rectangular slivers of silicon, with a pair of metal wires or contacts linked to them. It's the type of plastic housing that really differs. Here are some useful diodes commonly used by model makers.

5mm LEDs.

These are about the oldest type and most common. They're plastic cylinders 5mm in diameter with rounded lens-like tips, usually to focus the light. Two wires extend out the other end, and the longer wire is the positive terminal. For obscure historical reasons these were sometimes called T-1 3/4 LEDs*.

3mm LEDs.

Same as the 5mm, only smaller. These used to be called T-1 LEDs.

1.8mm/2mm LEDs.

Sort of like the base of a 3mm LED, with a protruding plastic lens or cylinder. This protruding bit contains no circuitry, and simply extends the light out to a smaller point.

The ones with longer cylinder ends, usually 2mm, are sometimes called “tower” or “lighthouse” LEDs.

* if you want to know, T-1 is part of an antiquated system for describing light bulbs. T apparently meant that the bulb was tube shaped, and the system was divided into units where 1 was a maximum diameter of 1/8".  Fortunately this idiotic and non-scalable system has been largely supplemented by the logical approach of providing the LED diameter in millimetres.

SMDs

I put surface mount devices (SMDs) in their own category, because they're particularly noteworthy. These are small electronic components, sometimes called SMT devices for surface-mount technology, which lack external wires.

Most traditional discrete (individual) components have two or more thin metal wires coming out of them. These wires go through tiny holes drilled into a circuit board, are soldered in place, and serve both to conduct electricity but also to keep the component fastened to the board.

SMDs, however, simply have tiny metal tabs or conductive areas on the edge. These are used to solder the component down to the circuit board. They are tiny and therefore much more fiddly to solder in place by hand. The big advantage from our point of view is that LEDs with an SMD design are really really small. You can light up models you'd never think were lightable before! And despite their tiny size they're not dim at all.

Some SMD LEDs are sold with thin wires presoldered to them, which makes things much easier for those of us who lack the skills of neurosurgeons. These are sometimes marketed as "nano" or "litz" or "microlitz" LEDs, though these terms have no strict meanings or definitions here. (true Litz wire is fine wire of a certain type, used for radio-frequency applications) Another way to wire them up is to use electrically conductive paint – see the next chapter. 

SMD products are described by numerical codes, which are basically the width and length of the device. Unfortunately there are two systems - one metric and the other mediaeval. So you need to know which one is being used when looking up sizes.

SMD sizes.

Some common SMD sizes, which are usually described by their imperial numbers, are:

Imperial name   Metric name   Imperial size Metric size

  1206 3216 0.12 x 0.06 inches 3.2 x 1.6  x 1.1 mm

  0805 2012 0.08 x 0.05 inches 2.0 x 1.25 x 0.8mm

  0603 1608 0.06 x 0.03 inches 1.6 x 0.8 x 0.6mm

  0402 1005 0.04 x 0.02 inches 1.0 x 0.5 x 0.45mm

  0201 0603 0.02 x 0.01 inches 0.65 x 0.35 x 0.2mm

Yes, the 0201 LEDs are mindblowingly tiny! And it is indeed confusing that the 0201 LED coincidentally has the same name as the 0603 when you mix measurement systems. It’s a shame that the decimal imperial measures are so common, because they’re really inconvenient for people who aren’t engineers.

LEDs in a strip

Surface mount LEDs are sometimes sold soldered to plastic strips. It’s best to think of these as bendable rather than flexible, in that you can curve the strip to fit the shape you need, and then leave it. They can’t be flexed repeatedly without breaking.

Generally these are sold for convenience - they’re evenly spaced and, because they use SMDs, are fairly flat. They also save you the bother of trying to solder wires onto tiny diodes, and they often have resistors already built in. Those with resistors are rated at specific voltages – 5 volts, for example, or 12.

COB LEDs

A more recent innovation is the “COB”, or “chip on board” LED. This is a way of fastening the actual LED chip to a sturdy substrate, which allows for densely packed LED arrays. Basically, instead of soldering a bunch of individually packaged LEDs to your circuit board the manufacturer groups them together in a COB design.

COBs are popular because you can have a high brightness LED setup in a small device. They can also be compact, more reliable (in theory), and less hot to operate.

COB LEDs are used to manufacture rubbery silicone strips that look like glowing ribbons – you can't see the gaps between the lights, the way other LED strip technologies usually do. These are sometimes called "dotless" strips. COB LEDs can also be used to make very bright flat panels.

Note that COB LEDs have nothing to do with the so-called "corncob" lamps, which are basically a bunch of SMD LEDs soldered to a cylinder, like an ear of corn.

Trimming LEDs

A useful thing to remember about LEDs is that most of the transparent plastic body of the LED isn't needed by the chip itself. The plastic has to stay intact around the wires and chip for structural integrity, but the rest is just there to make a convenient shape. In the case of cylindrical LEDs, the rounded end is also used to help focus light from the chip. This is why LEDs are available in such a wide range of physical sizes - the chip is always tiny, but the plastic around it can be any size or shape a manufacturer wants.

What this means is you can cut or file down the plastic around an LED, and as long as you don't touch the wires, the thing will still work. In the case of models, filing the end flat can be a very useful thing. You can trim a diode to fit an extremely tight space without using an SMD LED. You'll diffuse the light and lose a bit of it, but the light spread will be wider than with a lens.

Low-current LEDs

Most LEDs are officially rated at 15 to 20 mA (milliamps - see the next chapter), which is around a third to a quarter of the energy used by a small incandescent grain of wheat bulb. Even physically small LEDs draw that much power. But in the past few years very efficient chips have been developed, making 5mA and even 1-2 mA LEDs possible. Many of these are dimmer than standard LEDs, admittedly, but that's actually desirable for a lot of model situations.

Low-current LEDs are awesome and I wish they weren't so difficult to find. Even sales catalogues rarely flag them up. Which seems crazy to me since a model with 1mA LEDs will run for fifteen times as long as one with the same number of 15-20 mA LEDs, all things being equal!

One LED maker that produces low-current LEDs is Kingbright. I found these LEDs to be particularly useful for model making purposes:

APG0603RWF-TT-5MAV 0.6X0.3mm white SMD LED 5 mA

APG1005RWF-T-5MAV 1.0X0.5mm ultra thin (0.2mm) white SMD LED 5 mA

APHHS1005LQWF/D-V 1.0X0.5mm white SMD LED 2 mA

APT1608LQWF/D 1.6X0.8mm low current white SMD LED 2 mA

WP710A10LVBC/D 3mm low current blue LED 2 mA

WP710A10LYD 3mm low current yellow LED 2 mA

WP710A10LID 3mm low current red LED 2 mA

APF3236LSEEZGKQBKC 3.2X3.6mm low current RGB SMD LED 2 mA

High brightness LEDs

Some LEDs are brighter than others. Some may generate more light when loads more power is applied, but many are more efficient than standard LEDs, and so can produce more light for the same amount of current. Bright LEDs are great for making model rocketship engines or headlights for large model cars. But they can also be useful as lower-current LEDs since they may still light up brightly when fed lower current. Some really bright ones are mounted on large metal heat sinks, and are used in high-output situations such as vehicle lights or household lamps. These are less useful for models, on the whole. Though I suppose you could stick one of these LEDs into a large model spaceship engine or whatever.

Piranha LEDs

“Piranha” or “superflux” LEDs (from tradenames apparently), are a popular high brightness device type. They're flat square LEDs with two pairs of pins on each side for improved ruggedness, and frequently with large protruding lenses to increase their field of coverage. They’re often used in vehicle lighting or signage, and are ideal for putting into model spacecraft engines, etc. Surprisingly many of these chips don’t consume that much more power than standard LEDs.

Special effect LEDs

Many special-purpose LEDs exist these days, with complex internal circuitry that goes beyond a simple light-emitting chip. Here are a few types that are useful for model makers.

Self-flashing LEDs

Some LEDs contain additional microcircuitry that lets them flash on their own, independent of any additional components. Plug 'em in and off they go! These can be very useful for blinking lights on vehicles (turn lights, hazard lights), in dashboards, etc. However, they have some annoyances which restrict their utility.

First, they tend to flash quite fast - typically 1.5 to 2 times a second (1.5-2 Hz or hertz) – and you can't adjust the speed. The slowest I've found is 0.5 Hz, or one flash every two seconds. Which isn't bad for some applications, but still isn't long enough to simulate random flashing behaviour. Strangely, hardly anyone sells flashing LEDs that slow - they're hard to find for some reason. 1 Hz is about the slowest common LED.

Second, you can't change the duty cycle - they're preset to one time unit on, one off. So you can't have brief blinks and long pauses or Morse code or anything like that. For more sophisticated flashing patterns you need more complex external electronics.

Third, they're generally rated at 15-20 mA of power and aren't available in low-current versions. (though to be fair their power consumption is half that of a regular LED since they're off half the time!)

And fourth, they're mostly 3mm or 5mm in size. SMD flashers exist but I’ve never seen a slow flasher in SMD form.

However, all that being said, there's definitely a massive amount of convenience associated with a self-flashing LED. Especially if you don't have the time or the interest in learning how to program a microcontroller for complex light patterns. If you just want to pop a few blinkers inside your spaceship cockpit with zero effort, then these things are the perfect solution. Another really handy use for self-flashing LEDs are turn signals on model cars.

Flickering LEDs

You’ve probably seen fake tealights that contain flickering yellow LEDs, powered by a coin cell. These contain special LEDs which contain onboard circuitry that cause the lights to flicker randomly or pseudo-randomly. They actually look moderately convincing as fake candle flame. The main giveaway is that the light produced by the LEDs changes brightness but never changes direction. Real flames move around, causing shadows to "dance" in different directions.

You can of course buy these LEDs individually, and they come in a variety of colours. They can be useful for making model fireplaces, for example. Some modelmakers also use them for engine lights in model spacecraft. Personally I think the flickering is a bit much for realistic-looking rocket engines - it’d be good if you could tone the flickering down a bit - but it’s all a matter of taste.

One way to make the effect more subtle and believable is to put a single flickering LED into a group of steadily glowing LEDs. Another is to put a whole bunch of flickering LEDs, say half a dozen, all together. That way the randomness of their flickering results in an overall shimmer.

Bicolour LEDs

These are LEDs containing two different diodes, each of which produces light of a different colour, all built into a single two-wire housing. The diodes are wired in reverse to each other. And since LEDs only work when current is applied one way, these LEDs will light up one colour when current flows in one direction, and the other colour when the current is reversed. They're available in red/blue, red/green and other colour combinations. If you apply high-frequency alternating current they can illuminate both LED colours. So red/green could appear a kind of yellow colour.

You can also get three-wire bicolour LEDs, which have independently controllable diodes. One lead is for one colour, one is for the other colour, and the third lead is a common wire shared between the two.

Sometimes people call these ones "tricolour", since they can produce three different colours if you count the yellowish colour produced with AC. I dislike that name, however, as they can be confused with actual three-colour LEDs, such as the ones below.

Tricolour LEDs

As noted earlier, some LEDs contain three separate diodes  – one red, one green, and one blue – which light simultaneously to produce white light. This type of white LED will have all three diodes linked to a single pair of wires, so they'll all glow with the same relative brightness.

However, some LEDs have separate wires for each of the three diodes so you feed varying levels of power to each one. This allows you to output all kinds of different colours, simply by mixing the relative amounts of red, green, and blue. Such diodes normally have four wires – one for red, one for blue, one for green, and then one common wire shared between the three diodes.

With enough of these LEDs you could create a big video display or whatever. Of course, dozens or hundreds or thousands of four-wire LEDs would get pretty unmanageable in terms of wiring. So this type of LED isn't actually a very useful solution for that sort of thing. You'd want individually addressable LEDs for that – see the "Neopixel" section in part IV.

Tricolour (RGB) colour-cycling LEDs

These contain three diodes in one housing - red, green, and blue. An onboard chip then cycles through the colours without any additional hardware. Such diodes are mostly useless for model making unless you're creating the Starship Discothèque or something.  I don't understand why some LED lighting kits sold for models contain them, as they typically look so stupid.

Tricolours are typically of two types - fast or slow. The fast ones generally blink the primaries at a high speed - perfect for making a toy for a child or a party raver. The slow ones cycle through the colours more gradually, generally crossfading between pairs of colours in a second or so. e.g.: the LED will go from red to purple to blue, etc. You can buy tricolour cycling LEDs in 5mm, 3mm, and even SMD sizes.

However, the slow colour-cyclers aren't entirely useless – I have found one great application for them. Basically, since each of the three diodes is a separate component housed at the bottom of the LED, each one is also supplied by its own hair-thin bond wire. I've discovered you can simply use a really fine drill bit (0.5mm or smaller) and carefully make a hole in the LED housing, severing the wire that powers one of the three colours.

I've used this technique to make a 3mm LED that gradually transitions from red to blue only - perfect for the front dome light on R2-D2! It's not perfect, as two of its phases - transitional purple, and off - don't match the movie prop, but it's still a good approximation and costs nearly nothing! Sadly nobody makes a tricolour LED that contains yellow and green lights, which would be perfect for the back of the dome.

Neopixels/addressable LEDs

Neopixels are a popular brand name for individually addressable LEDs. "Addressable" means you can stick a whole bunch of these special LEDs in a chain, and a computer can command individual LEDs to light up with the specific brightness that you want. Since most Neopixels are either RGB (red-green-blue) or RGBW (RGB with an additional white LED), it's also possible to specify any output colour that you like.

These devices are super useful for sophisticated lighting effects, but require a lot of extra work in terms of programming and so on. I therefore talk about Neopixels in the chapter on automating LEDs.

OLED panels

OLEDs are used for a lot of higher-quality video screens these days. The technology is known as "organic" LED lighting since certain organic compounds are used as a light-emitting electroluminescent layer. Most OLED devices aren't particularly useful for modelmakers, but one type will probably become very handy in the future - OLED light panels. These are very thin (a couple of mm thick) plastic panels which emit a very pure white light. They have excellent colour accuracy (colour rendering or CRI is very good) and don't have the blue or green tinge that white LEDs often do. Unlike electroluminescent panels they produce a lot of light, and not in weird useless colours. Flexible OLED panels are particularly interesting, since they're bendable plastic sheets that can be used for all kinds of interesting applications.

Unfortunately OLED panels require driver hardware to function, and they're currently really expensive. They also can't be trimmed to size - they have to have sealed edges. But I can see small OLED panels, as the tech becomes more affordable, having a lot of potential for energy-efficient flexible lighting solutions for model makers. It'll just be a while before they're cost effective.

The previous section

I: Non-LED lighting.

The next section

III: Powering LEDs.