Yes, HMIs are expensive (really, really expensive). That’s why a majority of pros who use them simply rent as needed, and even that can run over $1,000 a day if you want a three-light setup with a 2.5K ARRI Fresnel and two 4Ks. But it’s money well spent (especially if you get to pass it along to the client).
What’s So Special About HMI?
At 5600 Kelvin, the bulbs are daylight-balanced, which is a big advantage if you’re shooting outside and want to match the ambient light or if you’re shooting indoors and want to replicate sunlight. You won’t run into white-balance issues as you would when using tungsten lights with a noticeably warmer color temperature of 3200K to 3400K, which can wreak havoc with skin tones.
Because of their origins in the film industry, HMIs are flicker-free, so you can use a greater range of aperture and shutter speeds and experiment with a little motion blur if you prefer. Flicker is caused by the alternating current powering a lamp, so if this wasn’t corrected by the HMI’s ballast, you’d only get half your frame at certain shutter speeds.
HMI bulbs are very efficient as well, producing four to five times the output per watt compared to standard filament bulbs, so a lot less power is needed and a lot less heat is generated. With a 2000-watt HMI, you’d need an 8000- to 10,000-watt bulb in quartz or tungsten to have the same output.
What makes this possible is how HMI light is actually made. As in arc welding, an electrical current is created between two electrodes, which gives the light some unique characteristics besides output-per-watt and color temperature. There’s a crispness of the arc source, which photographers often desire. It gives a sharp shadow because the light is being created from a very small point source.