The output of all studio electronic flash units is typically measured in Watt-seconds (Ws,) a unit of electrical energy equal to the work done when a current of one amp passes through a resistance of one ohm for one second. Sometimes you’ll hear it called a Joule but it all boils down as a way to measure the power and discharge capacity of an electronic flash’s power supply.
You can think of it in terms like automobile horsepower but because Watt-seconds doesn’t consider reflector design it’s not a perfect indication of the total amount of light that can be produced by an electronic flash. Because of that, you’ll occasionally encounter Effective Watt-seconds used as a way give you some idea of what you can actually expect to get from the flash.
Another method of measurement is lumensecond, a unit of measurement of light intensity falling on a surface. A lumensecond 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 energy. Most electronic flash units produce between 15 to 50 lumenseconds per Watt-second, so sometimes an efficient 300 Watt-second system can produce as much real light as an inefficient system rated at 1000 Watt-seconds.
Then there’s the old standby from the speedlight world: Guide Number is a measurement of flash output that considers the entire lighting package. 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 above image was made with 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 measured with a Gossen Luna Star F. Main light was a 320 Ws Buff DigiBee DB800 with an Alien Bee B800 as side/backlight. Background was a 5×7 Savage Infinity Photo Grey vinyl backdrop.
If you’re interested in shooting portraits and how I use cameras, lenses and lighting, in my in-home studio, please pick up a copy of “Studio Lighting Anywhere” which is available from Amazon.com, with used copies selling for less than $12.