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How to strike a 5k tungsten fresnel

Bradley Mowell

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I purchased a used Arri True Blue T5 tungsten fresnel wired for 220v w/a new 220v bulb. The lens and wire screen are fully intact, and the unit itself appears to be in very good condition.

Do I simply strike it using the inline switch or should I power it up gradually from a dimmer? 

Thank you!

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Here's the relevant quote from p. 308 of the Set Lighting Technician's Handbook (4th ed.) by Harry Box:


Any metal has less resistance at a lower temperature; as temperature increases, so does resistance.  With tungsten, this effect is very pronounced - when cold it has very little resistance but when heated it has very high resistance.  As a result, from a cold start, tungsten lights have high in-rush current (25 times the nominal current) which lasts a couple milliseconds until the filament temperature increases, bringing the resistance up, and the current down to their nominal values.  This is the reason that we use dimmers to bring up 20k lamps - to avoid slamming the (expensive) lamp filament with high inrush current.  Once at operating temperature, however, resistance is constant.

I can't find any mentions of requiring a dimmer for tungsten fixtures smaller than a 20k.  I've seen a dimmer board op 'sneak' up 10k fresnels out of an abundance of caution in an older installation.  I've never seen anyone bother with this for a 5k, and can't remember ever having a problem striking a 5k right from the switch. 


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The lamp will last much (couple orders of magnitude) longer if powered on with a dimmer. Quartz lamps, no matter the wattage, practically never die from age - it's inrush current and shock/vibrations that break them. Abruptly turning a larger light off also causes arcing in the switch - hence there are no mechanical switches on the 20K. 


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