What ARE watt-seconds, anyway?

(see subject line). I have no idea what they mean in relationship to

my camera. From what I can tell, my Interfit 350 w/s flash seems to

give me the exact same portrait lighting as the 1600 w/s White

Lightnings used at the studio I'm working at.

Watt-seconds, or joules, are a measure of energy -- watts x seconds. It is calculated from the voltage and capacitance of the power supply -- joules = V^2xC, where V = volts, and C = capacitance in farads.

 

Energy is related to the total amount of light produced by the flash tube. Normally, that light is collected and focused by a reflector. The same power, in joules, can result in different exposure levels, depending on how the light is distributed.

Try a search here on this topic.I know that several knowledgeable shooters here have rightfully concluded WS's equal an energy "potential", and dont really offer much insight into what comes out of the tube at 10 feet.

Check out this info page from the White Lightning web site. It explains the relationship of watt seconds to power delivered.

 

http://www.white-lightning.com/manuals/bluebook.htm

Steve,

 

The point I'm trying to make is that there are too many variables to make generalities about the amount of light at 10 feet or whatever -- the reflector angle, focus point, diffusion, modifiers... The beauty and curse of studio lights is the flexibility to deliver light where you need it. That's being practical, not vague.

 

I am comfortable that a 1000 joule flash has more energy (and light)than a 300 joule flash -- energy that is there when I need it and turned down when I don't. As a scientist, I am comfortable that an arc through a pressurized gas converts a reasonably predictable amount of the electrical energy into light (notice I do not use the terms power and energy interchangeably). As a practical photographer, I know that what counts in the chase is the amount (intensity x duration) of light reaching the subject, which is why I use a meter.

Ahhh.. Whitelightning model numbers are based on "effective watt seconds" which as far as I can tell are complete BS marketing numbers. The X1600 actually has 660 true watt-seconds, so it should be slightly less than one stop brighter than the interfit assuming the effiency of the bulbs are the same as well as the reflectors, distance, etc.

Assume 4 gallons of fuel; how far would you get? - WS seem the same.

So if folks are confused with strobes and watt seconds; you could just use edison lamps; which are rated in watts; and lumens too; at your grocery store :) Note that a bare bulb is less effective than one with a reflector. Notice that a flurescent bulb has more lumens per watt. Notice that an edison base 25 watt bulb has less lumens per watt; than a 100 watt bulb does. Common household lights that are 100 watts; have a slightly higher color temp; than a 25 watt bulb.

watt seconds is the energy stored in the flash capacitor. When the Xenon tube is fired; some is lost in heat. Another heat loss is in the UV filter. The efficiency of the tube can vary with tube temperature; DC voltage across it; way it is triggered. Then the "light" energy released is not all visible. Then there is the UV filter; how good is it? Has it degraded; has it fallen off? Then there is the reflector size; shape; how clean it is; how reflective the "reflector" is. Then there the zoom fresnel "telephoto" deal on some portable strobes; that focuses the beam abit narrower. <BR><BR>Then there is the strobe makers specs; are they fluff; or somewhat OK?<BR><BR>Watt SEConds; not W/S.<BR><BR>watts per second is the rate of change in power; the slope of a power versus time plot; at a certain point of time.

Watt-seconds are joules and they only tell you how much power/energy in stored in the capacitor banks, not much else. It will give you an idea how far you can get, but not completely.

 

There are a ton of factors in the effective light a flash outputs. The cone of light, fall-off, ramp up and down of the flash, whether it is cut off or allows the capacitors to fully discharge, the reflector quality and surfacing, is there a fresnel lens in front, flash tube length, flash tube shape, age of tube...etc.