July 18, 2013

HMI lights: from cinema industry to a photographic session. A review of Profoto ProDaylight 800 Air

An HMI light is a metal-halide gas discharge lamp with two electrodes of medium arc separation which excite the mercury vapour to emit light. They provides a very high output and better efficiency than common incandescent lamps; they also have a high colour rendering index (CRI), never below  90 value.

An HMI light spectrum is very similar to daylight spectrum, this is why it can be used in exteriors without the need of colour correcting by filters and, because of their high emission, they are used to simulate the sun. They need a ballast to operate and make them flicker free and, depending on the lamp, they need around 5 minutes for all the parameters (tension, intensity, power, temperature) to stabilize and operate correctly. Turning them on and off affects lamps life and never they should be turned off throughout the 5 minutes-switch-on process. Their life time should neither ever be extended more than 25% because risk of explosions may increase.

Colour temperature is another issue of HMI lamps: with age the arc length becomes larger and needs more voltage which makes colour temperature decrease (estimated in less than 1 Kelvin per working hour).

HMI lights are most used in cinema industry and the most known manufacturer is  ARRI; because of weight and high heat emitted their use wasn't suitable for photography, unless in very specific mise en scène. But lately Profoto launched on the market a new HMI light with characteristics that make its use in a common photographic shooting possible.

It only weights about 2,3 Kg and its low heat emission allows the photographer to use traditional light shaping tools, including umbrellas and softboxes (only the HR series thou). The 5600 K colour temperature makes it perfect to work with balanced day light film or along with flash light (the combination between these 2 light creates a wonderful peachy skin tones) or sunlight and, unlike normal HMI, it can be held in zenith position without any problems.

But the maximum wattage of the Profoto lamp is  800w which can be a bit of a problem with diaphragm aperture: photographers who normally shoot  at ƒ11 or ƒ16 won't be able to achieve these apertures if they don't use some reflectors that allow to gain 1-1 and 1/2 stop along with a faster ISO, solution that is not always an happy one.  For this reason I think that, unless you are comfortable with shooting at ƒ5,6 or ƒ8, Profoto HMI light are far better used as fill, rim or back light instead of as a key light.

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