Forget the viewfinder debates, we've got new HMI lights in the studio.

K5600 Lighting. Alpha 200.
This is a beautiful and compact HMI light 
that is also a focusable spot. It's a wonderful 
tool for finely crafted lighting. 

If you read the blog on a regular basis you've got to know that I have a soft spot for continuous lighting. I like using tungsten lights, LEDs, fluorescent fixtures and now, HMIs. But that doesn't mean for a second that I think you should abandon the ubiquitous photographer love affair with flash of all varieties. It's just that continuous lights provide such immediate results. It's so much easier to see the effect your lighting is having on a subject if the light hangs around long enough for your eyes to register what you are actually seeing. I'll go out on a sturdy limb here and say that continuous lights are the best learning tools for people who want to really, really learn how to light. Not just how to bounce some photons off a white ceiling but to be able to see definitive changes in outcome from seemingly small adjustments. Feathering an parabolic reflector is a good example. Finding the penumbra of light is harder when you use a modeling light or depend on trial and error...

If you are an event photographer then by all means, grab that SB-900 or that Canon Speedlight and go to town. You need to be untethered and mobile. And you probably want to freeze the action as your subjects freeze their smiles onto their faces. But, if you want to try your hand at still life or even portrait photography you might be pleasantly surprised at how flexible and satisfying it can be to work with light that sticks around. 

I love tungsten lighting because it comes in all shapes and sizes and some fixtures can project razor sharp beams of light while others, when used through big diffusion, can give the soft effect of northern light coming through high, thin clouds. The downsides of tungsten are that it consumes a lot of electrical energy which it converts mainly into heat and infra-red radiation. Tungsten also has a color temperature that is very different from daylight which makes balancing this light source with ambient daylight a bit more difficult than with most other light sources. You can easily filter it accurately but you'll need to worry about heat. A powerful tungsten fixture can melt a filter used too closely in minutes....

That's why a lot of people who work with continuous lighting have embraced LEDs and fluorescents. They are much more efficient with electrical energy and can be filtered or engineered to work in tandem with daylight. The weak spot for both of theses sources is both the lower light output you get with them and the discontinuous spectrum you'll get with all but the most expensive fixtures. (If you are using either source as your dominate light in a space with no daylight infiltration the color balance is not important as custom white balancing takes care of most spectral mismatches...).

The optimum continuous light would be nicely and accurately daylight balanced, strong enough to use in any ambient lighting situation and agile enough to provide many looks. While LEDs are encroaching into the film making space at a rapid clip the long time gold standard for most cinematographers has been the HMI light. 

So, what are they, really?  Here's what the Wikipedia says:

Hydrargyrum medium-arc iodide, or HMI, is the brand name of Osram brand for a metal-halide gas discharge medium arc-length lamp manufactured for film and entertainment applications. Hydrargyrum is Latin for mercury (Hg).
An HMI lamp uses mercury vapour mixed with metal halides in a quartz-glass envelope, with two tungsten electrodes of medium arc separation. Unlike traditional lighting units using incandescent light bulbs, HMIs need electrical ballasts, which are separated from the head via a header cable, to limit current and supply the proper voltage. The lamp operates by creating an electrical arc between two electrodes within the bulb that excites the pressurized mercury vapour and metal halides, and provides very high light output with greater efficacy than incandescent lighting units. The efficiency advantage is near fourfold, with approximately 85–108 lumens per watt of electricity. Unlike tungsten-halogen lamps where the halide gas is used to regenerate the filament and keep the evaporated tungsten from darkening the glass, the mercury vapour and the metal halides in HMI lamps are what emit the light. The high CRI and color temperature are due to the specific lamp chemistries.