Doug Kerr
Well-known member
My treatise on the effective watt-second system of denoting flash unit output was pretty lengthy. I thought a (slightly) more concise version might be useful.
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The watt-second rating of a flash unit tells us the amount of electrical energy stored in the capacitor which feeds the flash tube when it is triggered.
We treat this rating as an indicator of the luminous output of the flash unit, but it is only approximately that. Units with the same watt second rating may have significantly different luminous outputs (a result of their having differing "conversion efficiencies").
We would be better off if we thought in terms of the actual luminous output, which in technical work is stated in lumen-seconds. But that never became the custom among "working photographers".
The "effective watt-second" system was devised as a way to indicate the true luminous output of a flash unit using familiar words. The effective watt-second rating of a flash unit is in fact a precise, consistent measure of the luminous output of the flash unit. It is exactly the lumen-second measure of luminous output divided by 7.5.
Thus two flash units both (accurately) rated at 1750 effective watt seconds will have the same luminous output.
The scale of the effective watt-second is such that, for typical modern flash unit designs (that is, with typical conversion efficiencies), a unit with a watt-second rating of 800 watt-seconds might have a luminous output rating of perhaps 1700 to 2100 effective watt-seconds (depending on its conversion efficiency).
That difference in numbers (800 vs. 1700 to 2100) is neither "peculiar", "wrong", nor "dishonest". The watt-second and the effective watt-second are two different units, used to describe two different properties, not the same unit used to describe the same property under two different "standards".
That having been said, the fact that the two units involve two of the same words certainly is an invitation to misunderstanding. This is aggravated if a manufacturer is careless in using the effective watt-second system. For example, we might see this in the specification for a flash unit:
Effective watt-seconds: 800 watt-seconds
which would be more properly stated:
Effective watt-seconds: 800
The effective watt-second system is not attractive, especially since there is a much better way to specify the same property (luminous output): in lumen seconds.
But we must be careful not to be sucked into the claim that the effective watt-second system is a scheme to allow manufacturers to inflate the ratings of their flash units. We make it that if we see "effective watt-seconds" and carelessly think "watt-seconds".
We must similarly avoid the claim that the effective watt-second rating is meaningless, not an accurate indicator of anything. That's just not so - quite the opposite. It is in fact the watt-second rating that is not an accurate indicator of anything.
Best regards,
Doug
********
The watt-second rating of a flash unit tells us the amount of electrical energy stored in the capacitor which feeds the flash tube when it is triggered.
We treat this rating as an indicator of the luminous output of the flash unit, but it is only approximately that. Units with the same watt second rating may have significantly different luminous outputs (a result of their having differing "conversion efficiencies").
We would be better off if we thought in terms of the actual luminous output, which in technical work is stated in lumen-seconds. But that never became the custom among "working photographers".
The "effective watt-second" system was devised as a way to indicate the true luminous output of a flash unit using familiar words. The effective watt-second rating of a flash unit is in fact a precise, consistent measure of the luminous output of the flash unit. It is exactly the lumen-second measure of luminous output divided by 7.5.
Thus two flash units both (accurately) rated at 1750 effective watt seconds will have the same luminous output.
The scale of the effective watt-second is such that, for typical modern flash unit designs (that is, with typical conversion efficiencies), a unit with a watt-second rating of 800 watt-seconds might have a luminous output rating of perhaps 1700 to 2100 effective watt-seconds (depending on its conversion efficiency).
That means that its actual luminous output might be somewhere in the range 12,750 to 15,750 lumen-seconds.
That difference in numbers (800 vs. 1700 to 2100) is neither "peculiar", "wrong", nor "dishonest". The watt-second and the effective watt-second are two different units, used to describe two different properties, not the same unit used to describe the same property under two different "standards".
That having been said, the fact that the two units involve two of the same words certainly is an invitation to misunderstanding. This is aggravated if a manufacturer is careless in using the effective watt-second system. For example, we might see this in the specification for a flash unit:
Effective watt-seconds: 800 watt-seconds
which would be more properly stated:
Effective watt-seconds: 800
The effective watt-second system is not attractive, especially since there is a much better way to specify the same property (luminous output): in lumen seconds.
But we must be careful not to be sucked into the claim that the effective watt-second system is a scheme to allow manufacturers to inflate the ratings of their flash units. We make it that if we see "effective watt-seconds" and carelessly think "watt-seconds".
We must similarly avoid the claim that the effective watt-second rating is meaningless, not an accurate indicator of anything. That's just not so - quite the opposite. It is in fact the watt-second rating that is not an accurate indicator of anything.
Best regards,
Doug