James Watt was, by any measure, an accomplished man. He created the first efficient steam engines and in doing so helped the birth of the industrial revolution and the creation of the modern railway. But Back in the 18th Century electrical light sources were unknown to all but a few physicists and the WATT as a measurement of energy was unfamiliar to all.
Fast forward to the present and steam power has come and gone. The WATT however remains.
1 Watt equalling the energy of 1 Volt at 1 Ampere. W=V*I. Simple enough. Clear to all. It is a unit of energy.
So What? Pun very much intended.
We now use the “Wattage” of many things to give a supposedly accurate impression of their power and therefore performance. Except this magical number is beginning to mean less and less.
Let us look at a little history just inside our own industry, with the Patt. 45 and Patt. 123 fresnel fixtures. Both use the almost extinct T1 lamp, 500W of raw tungsten power. Yet any user of both luminaires will tell you that the resultant output is no where near equivalent. The Patt. 123 with its larger lens and reflector behind the lamp is far brighter. The Wattage is the same, the output is not.
This is a fine example of “optical efficiency”. One system out performing the other.
Then again becoming steadily more modern let us consider the Vari*Lite VL1000. The AS and TS models are well known to many theatre practitioners. The TS is a tungsten unit of 1000W, the AS an arc lamped unit of 575W. the optical systems are close to identical. Yet the AS out performs the TS dramatically.
This is a good example of “source efficiency”. One lamp out performing the other.
Presented with these 2 examples we can all guess the next step. Wattage is not really a good measurement of light output. LED fixtures contain both variables inside the light emitting engine itself. Each LED converts energy into light with different efficiency. The rate of development in the field still very fast in comparison with more traditional sources. The LED is then “packaged” with a set of optics to make them useable in our industry. These optical assemblies are wildly different depending on the aim of the fixture designer. And so to is their efficiency.
An inefficient LED chip combined with an inefficient optical package could be 1/3rd the output of an equivalent wattage unit with an efficient LED engine and optic. So 180W LED and 330W LED and 400W LED start to have little relevant use other than to calculate the power your show might need.
All this before the minefield of measuring actual output. Lux vs lumens vs footcandles….Field angle, beam angle, centre beam and a number of other metrics.
So before asking “What’s the wattage?” you might have to instead ask…”Can I look at this alongside my favourite luminaire?” and let your eyes do the measuring.