Quantity of electrical energy is measured in Wattseconds, 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/true wattseconds are specified. 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. 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.

Flash Duration

Flash Unit	t.5 Flash Duration		t.1 Flash Duration
	Full Power	1/32 Power	Full Power	1/32 Power
B400 (160Ws)	1/6000 second	1/3000 sec.	1/2000 sec.	1/1000 sec.
B800 (320Ws)	1/3300 second	1/1650 sec.	1/1100 sec.	1/550 sec.
B1600 (640Ws)	1/1800 second	1/900 sec.	1/600 sec.	1/300 sec.

When a flash system fires, although it may appear so to humans, the light emitted does not come on and go off instantaneously. In reality, the flash tube 'turns on' very quickly and then dims gradually as the flash capacitors discharge. Short flash durations are important to prevent blurring when the subject is in rapid motion such as a pirouetting ballerina or a Michael Jordan slam-dunk.  Flash manufacturers do not rate flash duration as the entire time the flash tube is emitting light however, because the end of the slowly dimming 'tail' does not contribute much to the overall exposure (or to blur).  t.1 is defined as the time during which the flash lamp output is above 0.1 (or 1/10) it's peak intensity. t.3 you may guess is the time the output is greater than 0.3 (about 1/3) peak, and t.5 is how long it's above 0.5 (half) the peak value.

Sync/Trigger Voltage

The output readings have been measured with a Gossen Ultra Pro flashmeter (ISO 100) placed 10-feet from the flashtube for direct reflector specs, 10 feet from the center of the radiating fabric for umbrella specs, and 4 to 6 feet from the center of the radiating fabric for softbox specs. Measurements were taken in the center of 20' x 27' white room with 20' ceilings. Readings may vary when taken in other environments and with other meters, as different brands of flashmeters can vary by as much as 1 whole f-stop in their interpretation of "correct" exposure values. 

The following accessories were used for these measurements:

7AB/R: The Standard 7” Reflector on the AlienBees Flash Units (silver field reflector, 80º beam spread); 11R: The 11” Reflector (silver long throw reflector, 50º beam spread); U48SW: 48” Silver and White Reversible Umbrella (silver side used for silver bounce reading), U48TWB: 48” Shoot-Thru Umbrella (black cover removed for diffused reading); and SB2436: 24” x 36” Medium Softbox (silver lined rectangular softbox, double-diffused with the second internal diffusion panel in place).

The Guide Number (GN) is a standard measurement serving to indicate a flash unit's output by calculating the distance times the f-stop, with respect to the film speed. The guide number relates the film speed to the actual flash output. A Guide Number is determined by placing the flash unit at a desired distance from the subject (usually 10-feet), and metering with an accurate flash meter. The guide number can be calculated from the distance and the f/stop (GN = f/number x flash-to-subject distance).

 We get a lot of questions asking, what is the "guide number" of your flash unit? This term is not wholly reliable because it relates more to the angle of the reflector than to the actual amount of light produced. For example, a flash unit with a given amount of light output will register a much higher guide number (f-stop at 10') if it has a narrow angle reflector (for telephoto lens) than if it has a wide angle reflector. But the actual amount of light is the same in both cases…it just covers more area with the wide angle reflector (at a lower guide number). In comparing flash units, the max GN of a flash unit is usually given for ISO 100 film. When it comes to choosing a flash unit, you have to be careful, as these measurements can be altered. It is common practice within the flash unit industry (for marketing reasons) to provide the max GN at the smallest coverage, as this will provide the longest focal length. Such a focused flash unit is more powerful, which produces a higher Guide Number, implying a more powerful output. Therefore, the max GNs of different flash units are usually not directly comparable. Studio flash units are designed to be used in many different configurations, with different accessories.  The angle of coverage will be different in each case as will be the achieved guide number for a given amount of emitted light. As merely an indication of output, we publish the max guide numbers, noting that they are not true measurements of what you can do with our lights, as the real output will be determined by how you choose to modify the light.

These output readings have been measured with a Gossen Ultra Pro Flashmeter (ISO 100), placed 10-feet from the flashtube for direct reflector specs. The readings represent the output of each unit at its full power setting. Readings were taken in the center of a 20-foot x 27-foot white room, with 20-foot ceilings. Please note that readings may vary when taken in other environments and with other meters, as different brands of flashmeters can vary in their interpretation of correct values. The stated maximum Guide Numbers (GN) were calculated by multiplying the distance (10 feet) times the measured output in f-stops.