Introduction

This notes guide is intended to give a basic, jargon-light overview of the main issues around stage lighting and energy efficiency. Stage lighting is often seen as the visible front end of energy use within theatre and the performing arts, although research for the Mayor of London's Green Theatre Plan carried out in 2008 revealed that in larger venues at least, only 9% of a theatre's energy use could be attributed to stage lighting.

As you can imagine, some types of light generation are more efficient than others.  At the time of writing, there are three main kinds of lamp source in use on stage - tungsten (including tungsten-halogen), discharge (including metal halide) and light-emitting diode (LED).

Tungsten

Tungsten sources (now generally referred to usually as 'conventionals') are the least energy efficient in terms of lumens per watt (amount of light out for amount of power in) - but it is unlikely that all fixtures will be on at full for a whole show - the main reason stage lighting uses less energy than most people think. Still, only about 10% of energy in results in light out – the rest is the heat used to get the filament to glow. There really isn’t much more to the technology than that.  

Discharge

Discharge sources are usually found in moving lights and followspots - and produce a huge quantity of light output for relativelylittle power input (relative to tungsten, that is). The main drawback here is that since the arc needs to be kept stable in order not to drop out, discharge lamps cannot be dimmed like tungsten - i.e. by varying the power input by means of a dimmer. The only way to achieve suitable dimming for stage purposes is to keep the arc struck, and use a manual or motor-controlled shutter to physically interrupt the beam. In the not too distant past, repeated re-strikes were likely to wear out both the lamp and its control gear, and combined with ballasts unreliable enough to have the potential to not restrike at all, it was almost unheard of to douse a moving light or followspot between the rig-check and the show, meaning that vast quantities of energy (and therefore money and carbon) were expended lighting, well, nothing. Not only do they therefore cost a lot to run, their end-of-life point is reached much earlier.

Thankfully, lamp and fixture manufacturers have stepped up to the plate, as it were, and have issued statements in writing, no less, to reassure users that you can douse between rig-check and the half. So do it. Even the National Theatre do it, and they have a lot at stake. 6 days a week, at an electricity cost of 11 pence per kWh, 10 x1200w movers rig-checked at 5 for a 7.30 show will set you back about £70 and 340kg CO₂e emissions a week in the period between first strike and the half.

An aside about cooling

It is useful at this point bear in mind where the waste energy from your luminaire is going at any given moment. As we established earlier, in a tungsten lamp, only 10% of the energy transformed is transformed into light – the rest is heat. In a discharge source, the lamp is ‘on’, even when it’s ‘off’, which is also creating a huge amount of waste heat. If you are fortunate enough to have air conditioning or air cooling in your venue, then those systems are going to have to be working a whole lot harder to pump the heat your units are creating somewhere else. And that’s costing you money, too.... Anyway, on to LED sources.

LEDs

LED is of course the new kid on the block (ish - it's actually 50 years old this year), but is it any good? Well, yes, some of it now is. It just really depends what you want to do with it – and quite crucially, how much you’re prepared to pay for it. Quality varies hugely, and as with so many things in life you really do get what you pay for. Most units are made by combining one or more LED chips to become the light source. Many are colour-mixing luminaires, and use red, green or blue chips controlled at different intensities to create different colours. To create white light, LED sources either fire up all the red, green and blue chips together, which usually nearly works, or they employ a further white chip to handle the whites single-handed. Some then add further chips to flesh out other colour ranges, such as warm whites and blues – some fixtures have as many 7 chips in.

Cheaper units will have fewer chips, and the chip life will be significantly lower, making low-cost LED units a real false economy. You also need to be using the units a fair bit to achieve any kind of speedy payback on your investment – rep theatres and schools and colleges (where usage hours are likely to be high) are better candidates than, say, a studio theatre with only a handful of performances a month.

If you do end up with a rig full of them, though, despite the fact that they will still have a permanently ‘on’ power supply unit (PSU), this will consume considerably less energy than any other traditional source, both when on, and in blackout. This results not only in lower overall energy consumption, but lower waste heat levels too, meaning a comfier ride for your patrons and your air-con system.

You also need to think about throw - all but the top-end LED fixtures do not have anything like the throw of their conventional and discharge predecessors, and although this is changing quickly, LED is currently more suited to smaller spaces, or shorter throws. An upside is that most LED profiles run so cool you can use acetate gobos in them. 

So should I be changing everything to LED?

No - not necessarily. You need to do some serious thinking and a bit of number crunching to work out the best course of action for you. If your stage lighting is on a lot, then yes, the payback you’ll get from switching to LED will be relatively quick, even if you go for the high-end kit – just make sure you factor in the other investment – more hard power points, more DMX cable). If you are a low to moderate consumer, think about changing over when your next round of equipment ‘dies’, or slowly integrating LED into your rig. And if you’re a low-level consumer, the chances are that you’ll be ok with your tungsten equipment for the foreseeable future...

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