What are the different Arri HMI heads all about ? Which one is best for my shoot?

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    What are the different Arri HMI heads all about? Which one is best for my shoot?, PAR’s, Compacts, D Series, M Series what’s the difference? In the good old days it was simple, you either took a Fresnel or a PAR. This week we give an overview of the different HMI heads and why you might want them on your next production. 

    The Fresnel bends the light in the same way as a plano-convex lens (used in PAR’s) but without the weight and heat retention. Moving the lamp to the rear will spot the beam, moving to the front will flood the beam-angle. At full flood, it’s a very even circular light with no obvious hotspot.

    The Fresnel creates a more ‘attractive’ quality of light that you can actually shine directly onto the talent for a very filmic look.

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    Par Fixtures (Parabolic Aluminized Reflector) however, give a much harder (elliptical shape) light but have the advantage of more output at any given power. PAR’s come with sets of lenses added to the fixture to spread the beam-angle of the head, they are often put through diffusions such as Silk’s or Trace frames, alternatively bounced of Polys or Ultrabounce textiles.

    PARs have basically been superseded by the Arri M series, these heads don’t have a fresnel but a very clever (multi-faceted, paraboloidal-ellipsoidal) reflector (told you it was fancy!) allowing them to punch above their weight, whilst giving you a nice shadow quality (closer to a Fresnel than an open face lamp).

    An M18 is the same size as a 1200W PAR but with 70% more output! In fact, the M18 will give slightly more output than a 2.5k Fresnel HMI. Similarly, the M90 gives the same output as an 18K Fresnel!

    The M series can be configured with different Bubbles - So the M18 can be 1.2k or 1.8k, the M40 2.5k or 4k and the M90 6k or 9k.

    Compacts and D Series. These two fixtures look very similar, but the True Blue D series (Arri will tell you have over 30 improvements from the Barn doors to the new tilt lock) so what features are actually of any use to you? Well Cross cooling allows (as the name suggests) better cooling meaning lamps last longer, meaning less time wasted on set.

    Most importantly the D series can be used at 90 degrees (the older compacts are restricted to 42 degrees to vertical) this makes the D’s much more versatile when bouncing or lighting from overhead (see below diagram on head symbols)

    Tip: When turning a fixture on, keep the bulb at the rear position of the focus (Spotted) to avoid thermal shock (A cold Fresnal can crack if the bulb is turned on when in flood position due to rapid temperature change.) This also applies to the glass on M series heads.

    Scrims

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    Single scrim is Green and reduces light by around ½ a stop, a double scrim is red and reduces light by 1 stop.

    Often overlooked scrims are a very useful accessory to the HMI fixture.

    Half scrims affect just one-half of the beam. Very useful for reducing the light falling on near subjects, bringing the light level down to match objects farther from the light. Or if the talent is moving closer to the light!

    Tip: Allow around 7 minutes for a HMI to ‘warm-up’ and get to correct color temperature

    When you turn off an HMI (maybe to move it to another room) wait at least 5mins before re-striking it (they don’t like hot-restriking)

    Tip: If using HMI’s on domestic power in conjunction with other fixtures (say a 750W Redhead) it’s better to turn the HMI on first and allow it to come up to full power, before turning on the Redhead. HMIs’ have a heavy initial load when starting up (Spike).

    What are those funny symbols on the side of the Fixture?

    HMI’s get hot! Not only the fixture itself (use Leather gloves when handling) but the focussed light can burn if the safety limits of the head are not met!

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    These symbols will tell you:

    1. Maximum Surface Temp (HOT)
    2. Safe distance for flammable objects around the head. (A piece of wood four feet from a Focussed 2.5K will begin to smoke in a matter of minutes.
    3. Minimum distance to people in the main angle of reflected beam. (The two figures above refer to the ArriSun and M series)
    4. Maximum tilt angle (remember difference between Compact and D series fixtures)

    We Have a wide range of HMI’s at jj’s, including Compacts, D Series, M Series, Jokerbugs, Briese & Dedo’s.  Most of our Ballasts are High speed 1000hz allowing frame rates up to 1000fps.

    We measure the colour temp of all the fixtures before they go out so we can make sure they match when using multiple heads. It’s always good to have small strength CTB, CTO, and Plus Green, Minus Green filters on hand to further tweak CT.

    Let us know if you would like to try any of them out, or do a side by side test…..