need help re: measuring watt per sec

is there a way to measure the watt per second in a studio flash? i'm planning to purchase a studio light but i want to know first if they are really the same watt per second as they claim their product to be. i have a sekonik l308s ( if this could help in my question) thanks in advance!

best regards
cleng

First off, it's not watts per second, it's watt-seconds, which is the same as a Joule.

Typically, this is a measurement of how much energy can be stored in the capacitor and released to light the bulb (ignore the "effective watt-seconds" ratings of some manufacturers).

It is not possible to back-calculate this measurement by measuring light at a specific distance. Small differences in reflectors, as well as other factors, will change this dramatically.

Take a look at the specifications page on the AlienBees website. Scroll down to the output measurements and notice how different the guide numbers are with different reflectors. There is about a 2 stop difference between the 7 inch and 11 inch reflectors.

Look at the electrical specs, and you will notices comments like

"320 true Ws, 800 effective Ws, " That tells you that the capacitor holds as much electrical energy as another manufacturer's 320 w-s unit. Period. It also lets you know that they CLAIM to output as much light as someone else's 800 w-s unit. Marketing hype.

Now let me reveal how unbelievable the marketing claims can be!...
A decade ago, at a tradeshow, a XYZ 750 was tested against a Photogenic Powerlight 750. The XYZ 750 claimed '750 effective w-s', that it was as powerful as 750 w-s unit, even though the XYZ 750 capacitor was rated smaller. In the test, the Photogenic consistently outperformed, and provided about 1EV more light than the XYZ 750. For all tests, even bare tube with the reflectors removed entirely!
So in that series of tests the XYZ 750 was "1/2 as powerful as the Powerlight 750"...so much for claims that XYZ 750 is as powerful as another manufacturer's 750 w-s unit!

wont the guide No tell you what you want to know the larger the Guide No the more power the unit has.
Rob.

Manufacturers of studio strobes just about never list Guide Number. The GN would be dependent upon which reflector is mounted, truly professional units very often have choice of reflector size and coverage area and even surface reflectivity and color. They are not light consumer speedlights.

Plus the OP wanted a way to test their truthfulness..and GN are often exaggerated, too!

GyRob wrote in post #2796748
wont the guide No tell you what you want to know the larger the Guide No the more power the unit has.

Rob, go to this page and lookup the specs of the 580EX, just as an example. Look at how much the GN changes at different zoom settings. The same thing happens when you mount different reflectors on a studio light. Changing the way the light is distributed changes the effective distance. This is why you don't see GN specifications on studio units, and why it's difficult to compare them with a flash meter unless they have identical reflectors.

With all it's faults, the best way to compare flash power from unit to unit, without accomplishing your own side by side shootout, is with the "true" Watt-second rating.

This is valid if :

• You are comparing "true" Watt-second ratings from reputable manufactures of flash gear. If the rating seems high, then it it most likely is an inflated one. I have personally found that comparing the "true" Watt-second ratings on Speedotron, Novatron, Photogenic, Alien Bee "true" ratings (not those inflated "effective" units), Norman, Calumet, etc. is reasonable if not absolutely perfect way to compare available flash power.
  
  
• You must totally dismiss any Watt-second rating that is not a "true" rating, like "effective" Watt- seconds or other such marketing hype.
  

The Chimera soft box company used to publish a product catalog. In it was a chart which compared the output of many strobe units in the same Chimera box. It was the most useful comparison of output I know of. You might still be able to get one from Chimera

thanks for all your inputs guys!