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FAQ: True Ws / Effective Ws / Lumenseconds
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PostPosted: Mon Mar 12, 2007 2:12 pm 
True Wattseconds vs. Effective Wattseconds vs. Lumenseconds:
How Do You Determine The Output?

True / Real Wattseconds
Quantity of electrical energy is measured in Wattseconds (Ws), also known as Joules. This rating defines the amount of electrical power discharged with each flash. While the actual amount of light produced for a given number of Wattseconds varies, depending on the unit's design, the term provides a reasonable guide to comparative light output, as long as real or true Wattseconds are specified.

Lumenseconds
A Lumen is a unit of measurement of light intensity falling on a surface. A Lumensecond (Ls) refers to a light of 1 Lumen intensity for a duration of one second, or the equivalent, such as 2 Lumens for half a second. The absolute amount of light emitted each time a flash system is fired is correctly specified in Lumenseconds. The number of Lumenseconds produced by a particular flash system depends on the efficacy - how effectively the system turns electrical energy into light energy, or Wattseconds into Lumenseconds. The efficacies of commercial photoflash systems typically fall within the range of from 15 to 50 lumenseconds per wattsecond. What this implies is quite simple: a highly efficient 300 Ws system may produce as much actual light energy as an inefficient system rated at 1000 Ws.

"Effective" Wattseconds
In the conversion of Watts to Lumens, or Wattseconds to Lumenseconds, the efficacy of the system determines how much light will result from a given number of wattseconds. The poor efficiency in this conversion by manufactures has given rise to the term Effective Wattseconds. If one flash system converts 400 Wattseconds of energy into 16,000 Lumenseconds of light, and another flash system converts 800 Wattseconds of energy into 16,000 Lumenseconds of light, then the first system could claim to have "800 Effective Wattseconds" as it is effectively producing the same amount of light as a system starting with 800 True Wattseconds. This terminology was originally used in 1985 by Inverse Square Systems in conjunction with their "Stroblox" series of high-efficiency, self-contained flash units. The term was created because most manufacturers of flash equipment (as well as the press) insisted on the incorrect use of the term "Wattsecond" as a definition of light output (in such wrong statements as "This system puts out 800 Ws of light"). Since the Stroblox system produced on the order of twice the amount of light per Wattsecond as did the average "box-and-cable" system at the time, Inverse Square Systems chose to employ the rating "2400 effective Wattseconds" as a means of saying "our system puts out an amount of light equal to the average 2400 Ws system,” even though the actual stored energy of the Stroblox 2400 was only 1200 Joules / Wattseconds. Indeed, this terminology gave the user a more clear idea of what to expect from the unit than he would have gotten had they simply stated that it was a 1200 Ws system.

We publish Wattseconds, Lumenseconds and Effective Wattseconds for our flash units, with Lumenseconds being the most valuable method of power comparison. The Effective Wattseconds rating, however, is rather arbitrary and cannot be easily proven true or untrue, as it is merely used as a basis for inflated comparison of different flash systems. For more "real world" application, we also publish expected output readings in f-stops for each of our flash units.

To view the complete specifications (with each of these ratings):

The White Lightning X-Series and UltraZAP Specifications:
http://www.white-lightning.com/specs.html

The AlienBees B400, B800 and B1600 Specifications:
http://www.alienbees.com/specs.html

The AlienBees ABR800 Ringflash Specifications:
http://www.alienbees.com/abrspecs.html


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