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 There is a great deal of mystery surrounding HMI lighting for many a still photographer and HDSLR motion shooter.  Ask about them and you are likely to hear:  “They are expensive;”  “They are daylight balanced;” “I’ve seen them used on movie sets;” “Did I say that they are expensive;” “They get hot;” and “My stylist pulls clothes from there.” (Tell the person who gives you the last answer that you said “HMI” not “H&M!”)  You may also hear some claiming to have found budget-priced “HMI” lighting, leaving you to wonder. 

HMI stands for Hydrargyrum (Mercury) medium-arc iodide and is one of several types of metal halide, High Intensity Discharge (HID) lights. HIDs are “arc” lamps; and as such, they rely on the combination of gas and metal salts and an electric arc between two electrodes to produce light when ignited by current.  Incandescent lamps, on the other hand, rely on a tungsten filament to produce light.  HMIs require a ballast (today’s are electronic rather than magnetic) to ignite and regulate the arc.  The upshot is that the arc lamps, and HMIs in particular, produce greater light output and are more energy efficient than incandescent lamps.  By way of example, the output of a 200 watt HMI lamp (depending on the type of reflector used) is similar to that of a tungsten bulb in the 650-800 watt range.  With cameras capable of producing stunning results at higher ISOs, Peter Bradshaw who represents HMI lighting manufacturer  K5600® Lighting points out that  “At ISO 800, an 800 watt HMI puts as much light on the subject as a 6,000 watt Fresnel does at ISO 100 — in a smaller, less expensive package, and can be run on standard 15 amp lines instead of tying in to the main (220V) breakers.” 

Image Courtesy of K5600® Lighting

HMI is actually a product name which is a trademark of Osram-Sylvania, but is used generically to describe metal halide lamps much in the same way that Xerox is used as a catchall for photocopying.  In addition to being more efficient than incandescent lamps, HMIs are daylight balanced which means that no filters are required for outdoor use and there is no resulting reduction in light transmission.   HMI lighting is a hard light source which is available in sufficiently large enough wattages to be able to control lighting in outdoor environments including overpowering the sun. HMIs can also, however, be very versatile lights. Bradshaw points out that   “The K5600 Jokers offer a point-source daylight source — the lamp — in a head which can shape light from narrow-beam hard (our Focal Spot) to big source soft (our Multi-Bug Chimera Adapter & Soft Tube), pausing in the middle with the Beamer par reflector and a set of Fresnel lenses.  Additionally, K5600 has introduced the Crossover Adapter, allowing still photo light modifiers to be fitted onto our 200, 400 & 800 watt heads.

 Between energy efficiency, power/projection, and color temperature, HMI lighting has enjoyed popularity in film and broadcast circles. Bradshaw is quick to add that as far digital imaging goes, whether talking about stills or motion, “We all know that digital loves daylight.”

The response to the issue of whether HMIs are truly expensive is not as cut and dry as it might seem. A single 200 Watt HMI kit averages in the neighborhood of $2600.  The lamps can set you back a couple of hundred dollars as well.  For the amateur or enthusiast photographer/videographer for whom ownership is important, they may be viewed as expensive, especially when compared to flash lighting for which the price of admission can run as low as a couple of hundred dollars, or when compared to the initial outlay for tungsten, some florescent and even some LED units. But for photographers/videographers whose work demands a combination of continuous light with reach and flexibility, daylight balance and efficiency or for production and broadcast companies, the expense is justified by the “tool.”  If ownership of HMIs is not an option, whether you are an enthusiast or a working “imageer,” they can be rented when needed.

Now we need to go back to being technical again.  You may hear the term “hot re-strike” or “hot restart” used when HMIs are being discussed.  What this means is that the lights can be restarted immediately after being turned off.    I bring this up now because there are HID metal halide lamps available which are marketed by some as “low cost HMIs” for video and still applications, which are actually CDMs or Ceramic Discharge Metal Halide lamps.  The major differences between CDMs and HMIs are:  CDMs are “cold re-strike” or “cold restart” which means that you have to wait five minutes for warm-up and five to ten minutes once they are extinguished before relighting them; cold re-strike technology results in CDM bulbs being less expensive than HMI bulbs and a having longer life; and CDM power options are not as robust as the HMI options.  CMDs have been around for sometime; they are commonly used for architectural applications where powerful but low heat-generating and efficient lighting that can be run for long periods are required. CDM ellipsoidal and Fresnel fixtures are used overhead in theatrical productions as well.

 Richard Andrewski of Cool Lights™ which offers HMI and CDM Fresnel fixtures makes the distinction between the two types of metal halide options and is very candid about the differences on his Web site and in the various Internet forums he monitors. “You have to keep in mind that HMI lights were originally intended for film and broadcasting whereas CDMs were intended for a host of commercial applications from architecture to grow lights which simulate sunlight.” While HMI lights used for film and broadcast are generally in the 6000 Kelvin range in terms of color temperature and have a high CRI (color rendering index,) Andrewski points out that there are a broad range of CDM bulbs available from 3000 Kelvin to about 8000 Kelvin. “We [at Cool Lights] have gone with the 5400K for our CDM products because it is in the standard range that video and film professionals are looking for.  5400K isn’t a standard color temp they manufacture in so we had to get them custom made.  We have been experimenting with the 6000K ones to match HMIs but they are often lower in CRI than the other color temps.”

Image Courtesy of CoolLights™

Andrewski cautions those who are considering CDM products to not only look at the color temperature of the bulb but the CRI as well.  Why is the CRI important?  The CRI is a rating (0-100 scale) of the lights ability to reproduce color accurately, without distortion:  The higher the CRI rating, the greater the color fidelity. Andrewski also points out that some of the CDM products being marketed for film and video usage use frosted bulbs and are intended for use in modifiers such as soft boxes.   “We chose the smaller single ended bulbs [for our CDM products] because they are very good point light sources, can easily change from tungsten color to daylight color with just change of bulb with no loss from gelling in other words, and they would readily go in today’s compact Fresnel and par fixtures which are the form factor and point source that many video people prefer to work with.”

This is our first installment in a series of articles on HID lighting. I hope that you have found the discussion informative and of value.  Over the summer we will be exploring HMI and CDM lighting further along with other topics of interest. I’d like to thank Peter Bradshaw who represents the Crossover program for K5600 here in the North America and Richard Andrewski of Cool Lights for their willingness to share their insights. 

I would be remiss if I did not stress the need for care and safety in operating HID lights.  Whether you rent or buy, please follow the manufacturer’s instructions for safe operation, and transport of the fixtures, ballasts and lamps.  Many HID lamps emit ultraviolet radiation while lit so do not look directly into an unshielded lamp and take necessary precautions if the fixture you are using them in does not have UV protection. Do not handle the bulbs as the oils from your fingers can shorten lamp life. (Peter suggests that you add a pair of leather gloves to your kit.) Do not touch the lamps when they are hot, and remember that if you need to dispose of them, they do contain mercury and /or other materials that may require special disposal procedures.  If you have any doubts, consult the lamp manufacturer’s Website, and/or contact your local sanitation department or environmental oversight agency.

Disclosure: No consideration has been received in connection with this blog entry, nor has any manufacturer and/or retailer offered any consideration. 

 All images in this entry and in this blog are copyrighted and used with permission.