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Effective watt-seconds - a nice idea done badly
There have recently been discussions on the matter of manufacturers stating some property of their photoflash systems using the unit effective watt-second. I thought I would talk a little about this.
In the illumination of a subject with a flash unit, we are ultimately, from an exposure standpoint, interested in the illuminance-time product on the subject surface.
If the light source has small dimensions, that is the joint result of:
• The luminous intensity-time product of the "beam" from the flash rig in the direction toward the subject.
• The distance from the flash rig to the subject
If the light source has substantial dimensions, it is more complicated than that, but that will not affect the issues here.
In any event, for any given "light modifier" (reflector, diffuser, etc.), and a subject at a certain distance from the lighting rig, the illuminance-time product at the subject will be proportional to the total luminous output of the flash tube, a quantity denominated (in the SI system) in the unit lumen-second.
Ultimately, if we are considering the ability of a flash unit (assuming a certain "setup") to deliver the illuminance-time product on the subject we desire from an exposure standpoint, we are interested in the total luminous flux output (again, best denominated in the unit lumen-second).
Stored energy and total luminous output
Most photoflash units operate by charging a capacitor to a certain voltage, thus storing a certain amount of energy in the capacitor.
That energy is, in SI terminology, denominated in the unit joule. This unit is identical to the unit watt-second (but joule is preferred in modern scientific and technical use). Since photographers seem to feel a necessity to show daily that they are not, God forbid, scientists, in their realm the use of the unit watt-second is still most common for this quantity, and we will use it here.
When the flash tube is "fired", a large fraction of that energy is quickly consumed by the electrical discharge in the tube. Much of that energy is converted to light (our real objective here). The remainder is converted into invisible radiation (ultra-violet and infra-red) and into conducted heat.
For any given design of flash unit, the ratio of total luminous output to stored energy is fairly constant, and at any given era, fairly consistent across different designs, from different manufacturers.
It is common (but not really precise) to refer to that ratio of total luminous output to stored energy as the "conversion efficiency" of the unit. (I will not bore you with why this term is not really apt; it will serve our purposes here.) Typical conversation efficiencies for modern studio-type flash units are on the general order of 40 lumen-seconds per joule.The amount of stored energy in a flash unit is easily determined, and not surprisingly, it became common to describe the "potency" of flash units in terms of their (maximum) stored energy (again, denominated in watt-seconds, even after that unit was replaced by the joule in engineering and scientific work).
But we note that while this number may give a good approximation of the total luminous output (for flash units of any given era and general design style), it does not really describe it in a precise way.
Nevertheless, photographers learned what stored energy rating would produce what effect in use, and chose units for purchase, and then for use in a particular task, on that basis. This was not said to be the stored energy rating (the typical photographer had no idea that was what it was); rather, it was named from its unit, as the "watt-second rating".
Actually knowing the total luminous output
There were some who thought it would be really best to know the actual total luminous output of a flash unit, and learn to plan acquisition and deployment on that basis. (Imagine such a thing - must be infiltration of the photo clubs by soulless engineers!)
We might have thought that manufacturers would respond to that interest by publishing, in the specification of their flash units, their (maximum) total luminous output (which properly would have been denominated in the unit lumen-second. And some manufacturers did.
But of course acceptance of this obvious tool would have required photographers to embrace a new, and to most, rarely previously heard, unit, the lumen. (How scary!)
So an industry group devised a new way to actually state the total luminous output of a flash unit without introducing this horror. In this system, the total luminous output of a flash unit was given by stating the energy storage of a hypothetical flash unit whose conversion efficiency was 17.5 lumen-seconds per joule. That value was said to be the "equivalent watt-second rating" of the unit.
The choice of the constant 17.5 lumen-seconds per joule was arbitrary (although there may have been a hidden agenda to its choice).
The thought was that manufacturers would soon begin to state the total luminous output of their various models in terms of effective watt-seconds, and that shortly photographers would come to plan acquisition and use of flash units on that more meaningful basis.
At the time the "effective watt-second" scheme was introduced, the typical conversion efficiency of a studio flash unit was perhaps in the range of 35 lumen-seconds per joule.
As a result, a unit with an energy storage of 200 watt-seconds might have its total luminous output stated as 400 effective watt-seconds.
This then gave rise (perhaps on the part of manufacturers whom weren't really anxious for photography to move into actually describing total luminous output) to cries of "Fraud! This new scheme is just a way for manufacturers to inflate the watt-second ratings of their flash units".
In any case, for whatever reasons, statement of total luminous power in terms of "effective watt-seconds" never really caught on. Most manufacturers did not adopt the system, and so there was no shift by photographers to basing their use of flash units on knowledge of their actual total luminous output (notwithstanding that was of course the property that is of importance).
Now today, some manufacturers state for their flash units the total luminous output in the "effective watt-second" scheme. Some also state the total luminous output in the "real" technical unit, the lumen-second.
Now, for one contemporary studio flash unit, the manufacturer states both the stored energy (in watt-seconds), the total luminous output (in effective watt seconds and in lumen-seconds) - but not in such careful language. The unit is described as "producing 330 true wattseconds and 800 effective wattseconds of power, with 14,000 lumenseconds of output."
In this context, "power" is an inaccurate, but common, metaphor for two different quantities, neither of which is anything like power: stored energy and total luminous output.What is "true wattseconds [sic]"? Well, again note that in this statement, the actual quantities are never identified - they are just "the thing we measure in watt-seconds" and "the thing we measure in effective watt-seconds". The term "true watt-seconds" is evidently based on the concept that watt-seconds is an appropriate way to measure energy storage, whereas watt-seconds is not an appropriate way to measure total luminous output. (Of course, "effective watt-seconds" - the actual term - is an appropriate, if not handsome, way to describe total luminous output.)
A concern expressed by some is that, given that photographers have never become acquainted with the peculiar notion behind "effective watt-second" ratings, the manufacturer's mention of "800 effective wattseconds" for that unit could serve to mislead users into thinking that its "watt-second" rating (the term, however imprecise, photographers generally use to describe the "potency" of flash units) was 800 watt-seconds.
I won't weigh in on whether that is actually "misleading" or "unethical". (No one has yet engaged me to give expert testimony on either side of that issue in a legal action.)
What is clear is that the effective watt-second scheme, although (perhaps) created with good intentions, has not fulfilled its promise, and because of lack of understanding (coupled with careless use) can actually be counterproductive. It is perhaps best left in history's dustbin.
I certainly do urge manufacturers to state the total luminous output of their flash units in a direct technical way (denominated in the unit lumen-second).
Then perhaps, gradually, sophisticated photographers will begin thinking in terms of actual total luminous output as the property of interest when planning the acquisition, and deployment, of flash units.
I'm glad you address this subject of watt seconds shown in some form with lights we want to buy. To me at least, it's so mislead when someone then talks about his 1600 W.S. White Lightening and immediately I am confused. Do I have to look up the catalog and determine whether or not the light really has a power of 1600 E.S. as Profoto, Hensel or Lumedyne might imply?
Then, when I think of the brand name. I say who knows and just imagine it's underpowered. In fact, all lights should give a total global light output in units we understand immediately.
So an example of a useful designation in the name of a light would be 400ws-8,000L and then there's no doubt what kind of light we have.
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And I don't think that Profoto, Hensel, or Lumedyne imply anything when they describe a unit as having an output of 1600 watt-seconds. They just probably mean exactly that.
And in fact if Paul Buff described a unit as having an output of 1600 watt-seconds, that would probably mean exactly that, too.
Perhaps what has happened is that your hypothetical someone has, with no justification, spoken of his White Lightning model X1600 as a "1600 watt-second White Lightning".
Its manufacturer has not, so far as I know, ever called it that. He does say it has a power (ugh!) of 1600 effective watt seconds. And it probably does.
We have to be careful not to think that a unit with "1600" in its model number has thereby been called by the manufacturer a 1600 W-s unit.
If we're going to do this, we need to be careful about the units, and their abbreviations, which are well-established.
We need not use hillbilly notation just to prove that we are "not scientists". The abbreviation "NAS" ("not a scientist") after our signature should be sufficient.
le crayon rouge ne dort jamais
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|Effective f-number||Doug Kerr||Imaging Technology: Theory, Alternatives, Practice and Advances.||0||September 22nd, 2010 09:17 AM|