Wolfgg
Hello!
The question keeps cropping up: continuous light versus flash – how many watts do I need to achieve the same aperture? Anyone unfamiliar with electricity and related topics will struggle to find the right answer. In the online discussions I’ve come across, the facts are annoyingly scattered across numerous posts, so here’s a concise explanation of the topic.
First, a brief insight into electronic flash units. They all work on the same principle: there is a capacitor which is charged to a high voltage by the voltage transformer, usually 300–500 volts. Connected in parallel to this capacitor is the flash tube, which, however, does not fire at 300–500 volts, but must first be ionised by a high-voltage pulse of, say, 4000 volts, causing the capacitor to discharge through the flash tube and produce the desired flash of light. The amount of light emitted by the flash tube is determined solely by the energy stored in the capacitor (assuming the flash is not cut short by a special electronic circuit). Manufacturers then state, for example: the flash delivers 1000 Ws. What does that mean? It simply means that this device has been designed (capacitor and charging voltage) in such a way that, upon ignition, 1000 Ws of electrical energy is converted into light from the capacitor via the flash tube.
But what exactly does this Ws mean? Anyone who is at loggerheads with electrical engineering usually has difficulty distinguishing between the terms power and energy. Everyone is familiar with the unit watt for power (abbreviated to W); after all, it’s printed on every light bulb. However, to work out how much energy a light bulb consumes (what the electricity meter measures), one must consider not only its power but also the duration it is switched on. For example, if you switch on a 100W light bulb for 10 seconds, it has consumed the product of power and operating time, i.e. 100W × 10s = 1000Ws (= watt-seconds). Or a 1000W lamp for 1 second also gives 1000Ws. If, for the sake of simplicity, we assume that all types of lamps have the same efficiency, we can already see how a flash unit should be converted to continuous light:
if I want to use continuous light to achieve the same amount of light as 1000Ws on the subject whilst the shutter is open, then I can
- expose for 10 seconds with a total lamp power of 100 watts (10s × 100W = 1000Ws)
- or expose for 1 second with a total of 1000 watts (1s*1000W=1000Ws)
- or expose for 1/10 of a second with a whopping 10,000 watts, i.e. 10 kilowatts (1/10s*10000W=1000Ws)
- or, for live subjects, 1/30 of a second, which results in a hefty 30 kilowatts of lamp power (1/30s*30000W=1000Ws).
In all examples, the energy amount of 1000 Ws flows through the lamps during the shutter’s open time (lamp power multiplied by shutter speed). A key relationship is evident here: the required power of the lighting setup depends directly on the chosen shutter speed; the longer the shutter speed, the smaller the lighting setup can be.
If we now also take into account the different efficiency levels of the lamp types (xenon flash lamp approx. 50 lm/W, fluorescent lamp also approx. 50 lm/W, halogen approx. 20 lm/W), the following lamp power ratings could replace a 1000Ws flash unit, depending on the shutter speed:
When using fluorescent lamps:
Shutter speed -- Lamp power for 1000Ws
1s 1kW
1/4s 4kW
1/8s 8kW
1/15s 15kW
1/30s 30kW
etc.
When using halogen lamps:
Shutter speed -- Lamp power for 1000Ws
1s 2.5kW
1/4s 10kW
1/8s 20kW
1/15s 37.5kW
1/30s 75kW
etc.
These figures are approximate, but they give you an idea of what to expect under roughly similar conditions (reflector type, etc.). The figures can also be easily converted for other flash units; for 500Ws, the lamp power should be halved, for 2000Ws doubled, and so on.
As you can see, a flash unit delivers an extreme amount of light in a short time; even an amateur flash unit with 100Ws still corresponds to a 7.5kW halogen lamp array at a shutter speed of 1/30s! The difference is simply that a flash unit draws the energy released during the exposure ‘at rest’ from the mains before the exposure (1000Ws charged in 10s results in a mains load of only 125W at 80% converter efficiency), whereas ‘continuous lights’, lacking the ability to store energy, are inevitably forced to draw the energy required for the exposure from the mains during the (short) shutter opening time.
I hope this makes the situation clear to everyone.
Regards, Wolfgang
Wolfgg
Or to put it even more simply: if 1000 Ws of energy is released in 1/1000 s, the power output of the capacitor and flash tube during that 1/1000 s amounts to a staggering 1 million watts ( = 1000 kilowatts = 1 megawatt)!
Another interesting aside:
The watt-second (Ws) and the unit of measurement used by electricity meters, the kilowatt-hour (kWh), are related; the following applies (with the abbreviated notation in brackets where available):
1 kilowatt-hour (kWh) = 1000 watt-hours (Wh) = 60,000 watt-minutes = 3.6 million watt-seconds (Ws).
This means, for example, that even a fairly powerful flash of 1000 Ws would need to be fired approximately 3,000 times to consume 1 kWh (assuming a converter efficiency of 80%).
Regards, Wolfgang
cfb_de
It’s amazing how you can impress photographers with applied science.
Electronics for beginners: Hey there, ohm, what are you up to?
Best regards,
Franz (a scientist by trade)
