The MR16 halogen/incandescent lamp has been around for 50 years. I want to touch on a few points not normally mentioned about its history and about its new LED brethren.
Before 1965, it was determined that if a lamp were designed to operate on no more than about 12V, its filament could be optimized for a somewhat longer life but also for a smaller size. The smaller filament, a smaller point-source of light, facilitated the use of a smaller reflector than traditional spot lights. Filling with a halogen gas allowed the lamp to be run hotter without compromising life expectancy. Furthermore, certain versions could, if desired, operate at a higher color temperature, lessening the "yellowish" cast of a typical incandescent lamp. This fact sheet from the Lighting Research Center covers some of these aspects.
By 1971 this MR16 halogen lamp had evolved to become a mainstay, not in residential lighting, but rather in slide and movie projectors. With high CRI and higher color temperature capability -- typically 3,500K -- it could render colors better.
This technology has enabled, to this day, MR16 lamps with efficacies of 15-20 lm/W for commodity applications and up to 30 lm/W for shorter-life projector lamps. The trade-off of a life expectancy under 200 hours has not been a big deal with slide and movie projectors, because operating time is so minimal.
So these lamps cruised along for years mainly as projector bulbs until the 1980s, when lower-efficacy but longer-life versions started to increase in popularity for "designer" applications, especially track lighting.
To operate this 12V lamp, we obviously could not just plug it into 120VAC or it would explode. So the industry needed a simple version of a regular old wire-wound magnetic transformer, having a 120VAC primary and a 12VAC secondary, inside an appropriate can so it would fit into or onto a light fixture. (Sometimes an MR16 track fixture will have one monster transformer to drive multiple halogen lamps.)
Connect the 120VAC through almost any cheap triac dimmer and everything dims just fine. So we move along another 20 years, to the early part of this century, and MR16s are still on nobody's technology radar -- but now it starts to get interesting. A typical transformer for a 50W halogen MR16 can be the weight and size of a pound of butter.
Engineers determined that they could do a similar 120V-to-12V (AC RMS) conversion using a stripped-down version of a switching power supply, eliminating any fancy regulation circuits, feedback loops, ripple filtering, etc. Patent 6,518,711 is just one of hundreds of examples of how to do it. For about the last 10 years almost all track-lighting systems meant for halogen MR16s have used ultra-cheap, China-made, so-called "electronic" transformers. These are just bare-bones switching power supply circuits, which have been known for 50 years, and which are a fraction of the size and weight of magnetic transformers.
This dramatic size/weight reduction allowed all kinds of new stylistic possibilities and ratcheted up the popularity of the MR16 as we arrived at the 2002-2003 time frame. Then along comes the LED, and folks say, "Hey, we're making progress with all our A19 and PAR-type LED lamps, so let's just make an MR16 version to replace that inefficient halogen lamp."
Indeed, from 2003 to 2010, a blizzard of imported LED MR16s appeared on the market. Putting aside all the ridiculous and typically misleading specs from manufacturers, buyers of these lamps were in for any number of surprises. The more such lamps were sold, the greater the range of surprises.
In Part 2 we'll walk through a few of them.