Measuring Studio Flash Output

Today’s Post by Joe Farace

The output of studio electronic flash units is often measured in Watt-seconds (Ws,) a unit of electrical energy that’s equal to the work done when one amp of current passes through a resistance of one ohm for one second. Sometimes you’ll hear it called a Joule but it all boils down to a method for measuring an electronic flash’s power.

You can think of Watt-seconds much like automobile horsepower but because the measurement doesn’t consider reflector design it’s not a perfect indication of the total amount of light that’s produced by an electronic flash unit. Maybe that’s why you’ll occasionally encounter the term Effective Watt-seconds used to give some idea of what output you can actually expect to get from a flash.

Another method of measurement is lumensecond, that refers to the light of one Lumen for a one second or the equivalent, such as two Lumens for half a second. The number of lumenseconds produced by a flash system depends on how effectively the flash turns electrical energy into light output.

Most electronic flash units produce between 15 to 50 lumenseconds per Watt-second, so sometimes a really efficient 300 Watt-second system can produce as much real output as an inefficient system rated at 1000 Watt-seconds.

Then there’s that old standby from the speedlight world: Guide Number is a measurement of flash output that considers the entire package, including reflector The higher the guide number, the greater the output. Guide Numbers are quoted in feet or meters and are valid for a given ISO setting.

The image of Vivean Marie at left was made using a Panasonic Lumix GH4 and Lumix G Vario 14-45mm f/3.5-5.6 lens with an exposure of 1/125 sec at f/8 and ISO 200 that was measured using my old Gossen Luna Star F light meter. The main light was Paul C Buff’s 320 Ws DigiBee DB800 monolight with the company’s Alien Bee B800 as side/backlight. Background was a 5×7 Savage Infinity Photo Grey vinyl backdrop.

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If you’re interested in shooting portraits and how I use cameras, lenses and lighting in my home studio, please pick up a copy of Studio Lighting Anywhere that’s available from with new copies selling for $29.80 with used copies available for $6.13, as I write this, which is a heckuva deal.