Lighting equipment specifications

Specifications are extremely important to audio and music equipment. What is the power of that amplifier? The sensitivity of that microphone? The number of frets on that guitar? We tend to take it on trust that where specifications are published then they are reliable. Well, they have to be, otherwise it would only take one guy with an oscilloscope and meter to find fault, then lawsuits would be flying and so would product returns.

For a PA amplifier, for example, you would want to know the output power, frequency response, noise and distortion figures. You would also want to know what is the lowest impedance load it can drive, measured in ohms. If you are of a practical nature, then you will want to know its dimensions and how much it weighs. This raises the question, therefore, of what are the kinds of parameters that are measurable in lighting equipment? What are the figures that you need to know before you commit your hard-earned money, and how can one piece of equipment be compared with another?

Let’s dive straight in with lighting instruments themselves. The first thing you would want to know is the nature of the source of illumination: the lamp. Lamps can be of various types, each with its own characteristic subjective quality of light. Subjective quality of light isn’t something that can be measured and one single figure put on it. However, there are measurements that can be taken that will give clues to the subjectively perceived performance.

The longest-established light source for stage performance is the tungsten filament lamp. I think we can ignore flame-based illumination, as this is only currently used for special effects, although prior to tungsten that’s all there was, of course. One specification about the tungsten filament lamp — or ‘incandescent lamp’ as it is often known — that is always a constant is that the filament is always made of tungsten! The reason for this is that it has a high melting point and is structurally very stable. It would be interesting to experience lamps made with filaments other than tungsten, just to see whether the quality of light is different in any way, but I for one don’t think I ever have. What’s good about the tungsten filament lamp compared to other methods of illumination is that the light is very similar to the spectrum of the sun, which is, of course, what our eyes are designed for. I’ll come back to this in a moment, but first the parameter that is going to capture your attention first: the lamp’s power (say ‘power’ the way Jeremy Clarkson would!).

Incandescent lamps historically have been available in all sorts of power ratings, all the way from 100W (and less for domestic use) up to 50,000W, although you won’t see very many of the latter around these days. The reason why large incandescent lamps are not used so much now is that other types of lamp are very much more efficient at these power ratings, and when you are burning 10kW or more in each of possibly several lighting instruments, the current is really starting to flow (and the electricity bill mounting). In practice, you would probably find yourself using lamps with 300W to 1000W rating. A thousand Watts is enough to have a nice bright followspot. And think of the performer centre stage: the beam is concentrated into a narrow angle and it’s going to get pretty warm out there. Tungsten lamps would be most often found at the lower end of the scale.

If you are an ultra-purist, then you would probably prefer a traditional tungsten filament incandescent lamp. But if you are not quite so pure, then you are very likely to prefer tungsten-halogen lamps. These lamps have all the good spectral qualities of common-or-garden tungsten, but they have the amazing ability to deposit evaporated tungsten back onto the filament, therefore enabling them to be run hotter, thus making them brighter, and potentially have a longer burn time too. Halogen lamps are more efficient than the conventional type, resulting in more useful light and less useless heat.

Power rating is clearly an important specification for lamps. It is also very important for the fixture that you put the lamp in. Since incandescent lamps in particular are not very efficient, then the disposal of waste heat is an important matter. Heat will not only make equipment hot to the touch, perhaps to the point of being hazardous, it will also contribute to the degradation of the fixture as the metal parts expand and contract performance after performance. This is therefore an important choice that you will make. If you have decided that a particular style of fixture is ideal for your application, then your next decision will be whether to buy a smaller one or a bigger one, in terms of the maximum lamp power it can accommodate safely. Of course, a bigger unit can always be dimmed to the same level as a smaller one, where the reverse is clearly not true.

Heat can be disposed of easily in three ways. The first is to ignore it and let nature take its course. The metal of the fixture will conduct heat reasonably effectively, and many designs have no, or hardly any, features that appear to be deliberately put in place to cope with heat. Drilling a couple of holes in the back of a PAR can is about the minimum standard. The second is to provide pathways for natural ventilation, aided by convection currents; finned heat sinks increase the area over which heat can be dissipated into the atmosphere. The ultimate solution to heat is to incorporate a fan into the design. This is a specification point that you should consider carefully; some lighting instruments that have fans are rather noisy and can be very irritating to the audience. You won’t commonly find a noise level specification for lighting instruments, and since acoustic noise level measurements are very variable according to the method used, it would be difficult to compare different products without actually listening to them.

Heat dissipation is actually a measurable parameter. For instance, the Martin Professional MAC 575 Krypton moving head is specified at “2800 BTU/hr”. Hmm that would be 2800 British Thermal Units per hour. It would be easier in Watts, wouldn’t it? That will be 820.6W, thank you Wolfram Alpha. And considering that the lamp is rated at 575W, it does seem to be very adequate. I actually find this rather comforting that the manufacturer goes to the trouble of giving this specification item. It shows that they care, and that is an important specification in its own right.

Although the power rating of a lamp is significant, and some designs of lamp are more efficient than others, there is also the important criterion of the directionality of the light. In a lighting instrument with movable optics, clearly the directionality is a property of the instrument rather than the lamp. But commonly used PAR lamps have directional characteristics all of their own, simply because they incorporate a reflector. PAR lamps are available with beam angles from narrow spot to wide flood. The angle of coverage could be precisely measured and given as a specification item. However, since the beam is not precisely defined, it is a subjective decision to say where cutoff occurs, hence approximate descriptions of the angle of coverage are really all that is necessary.

One point worth consideration, though, is the perceived light output of the lamp. This isn’t just a function of power rating, because you also have to take into account the directionality. A lamp with a narrower beam angle will seem brighter than an equivalent lamp with a wider beam angle. Each lamp is putting out the same amount of power, but the wider-beam lamp spreads it more thinly, hence at any point within its coverage it doesn’t seem as bright.

The actual light output of a lamp, as opposed to its power rating, which is the electrical input power, can be expressed in lumens. The lumen, like the Watt, is a measure of the rate of transfer of energy, which is the formal definition of power. However, it takes into account only the light we can see and not any other wasted energy. So when a lamp is given a rating in lumens, that will accurately put a value on its total light output. Lamps of varying efficiencies and beam angles can therefore be compared directly, which gives a much better impression of real-life results than comparing them by power. Remember that if you take a given amount of lumens and beam them more widely, then at any one point on the stage the illuminance will be less.

Going back to colour temperature, since the filament is clearly not as hot as the sun, then the light output will be biased towards the red end of the spectrum. And here we get our next measurable parameter: the colour temperature. Colour temperature is based on the concept of a ‘black body’ — a piece of something or other that is so black that it will absorb all the light that falls on it. You then put it on a very powerful Bunsen burner so that it gets hotter and hotter. As it heats, it will first glow in the infrared, then barely visible red, then bright red, then yellow, then increasingly blue-white. The temperature of the body gives a measure of the spectral bias of the light. The colour temperature of sunlight is in the region of 5000K (K being the unit of kelvin, which scientists use to measure temperature rather than Celsius. To convert from kelvin to Celsius, subtract 273.15). An incandescent lamp would typically have a colour temperature rather lower than this, somewhere in the 2800K to 3000K range.

In stage lighting terms, the precise colour temperature is nowhere near as important as it is for photography or video. The reason for this is that for your rock show you will be using loads of colour, which throws the concept of colour temperature almost out of the window. Not quite though, because different types of lamps can differ considerably in their colour output from incandescents, and this could be of some significance to the discerning lighting designer.

Discharge lamps represent a very significant increase in efficiency compared to incandescent lamps, which is why we are being encouraged to use them in our homes in the form of compact fluorescent lamps. In stage lighting, however, the reduction in waste heat is an important factor that has led to their widespread adoption. Discharge lamps work by passing an electric current through a low-pressure gas in the form of an arc. In a fluorescent lamp, the electromagnetic radiation from this arc is converted to visible light by a fluorescent coating on the inside of the glass. In a metal halide lamp, the mixture of gases inside the tube is selected to give the desired quality of light output. Intriguingly clever though this may be, it wouldn’t be entirely reasonable to expect a discharge lamp to produce a sun-like light, because it works in an entirely different way. (OK, so the sun uses nuclear fusion, but it’s the black-body radiation that is significant.) And so the struggle has forever been to produce a spectrum that is as similar to black-body radiation as possible, or at least fools the human eye into believing it is so. And there is a specification for this, with the end result of Colour Rendering Index, or CRI.

Colour rendering is a subjective experience, so the conversion of subjective experience into an objective testing method is complex. To summarise, however, a lamp that is to be tested for CRI is directed at a selection of colour samples to see how much light is reflected from each. That was a short sentence to write, but the theory and practice of CRI testing is hugely complex, which a) shows how seriously people take it, and b) therefore how important it is. You could, for interest, take a CRI reading of a candle, or an incandescent lamp. You would get a reading of around 100, allowing for experimental error. And the reason for that is because the candle flame and incandescent lamp are both black bodies, and a measurement of 100 CRI is defined as natural colour rendering. Typical discharge lamps may achieve somewhere from 60 to 85. Taking colour temperature into account too is an additional complication, but I may look at this topic in more detail in future, if I can summon up the courage to tackle it, because it’s certainly a tough one to explain in a readable way!

Colour is an issue with LED lighting instruments too. The issue here is rather different because both incandescent lamps and discharge lamps aim to cover the full colour spectrum. True white LEDs don’t exist as yet. The ones we have currently are normal LEDs with a phosphor coating. The phosphor is intended to fill in parts of the spectrum in which the LED itself is lacking, but it is merely the best solution we have at present, not really a good one. LED lighting instruments, therefore, normally use a combination of red, green and blue individual light-emitting diodes, the outputs of which can be combined in varying proportions to produce light in a variety of colours. This is a very workable concept, but the difficulty is in matching the colours of the LEDs to the sensitivities of the colour cells in the retina. When this has been developed to perfection, we will have LED lighting instruments that can produce a clear white light. (Did you notice the ‘when’ in that last sentence? Hopefully, it isn’t an ‘if’.)

Let’s take a break from light sources and look at the lighting instruments themselves. Clearly, there are a number of types of lighting instrument, and you will have to choose among them. However, I am interested in the next level of specification. The several varieties of moving heads by Martin Professional all look pretty similar — apart from the LED one, of course — so how do you choose? The answer is to look at the specifications.

Let’s take the MAC 250 Krypton and MAC 575 Krypton as examples. Their major point of difference is the lamp rating — 250W versus 575W. Bigger is obviously better. Then there are the colour wheels. The MAC 250 has a single wheel with 12 positions, plus open. The MAC 575 trumps that with two eight-position wheels, plus open, allowing for a much wider variety of colours. Gobo-wise, the 250 offers seven indexable gobos, plus open. ‘Indexable’ means that the absolute position of the gobo can be set remotely, so that, for example, if you want a certain gobo to be orientated horizontally, then you can command this from the console. The 575 only has six indexable gobos. What’s going on there then? Oh, it has nine static gobos as well. And it has a four-facet rotating prism, compared to the 250’s three-facet. The 250 boasts ‘quiet operation’. The 575 boasts ‘quieter operation’. Actually, the 575 has a variable fan, so it should be quieter most of the time.

In only one of the published specifications does the MAC 250 beat the MAC 575, and that is in the strobe effect, which goes up to 14Hz, where the 575 can only manage up to 10Hz. Who knows? That 4Hz difference could be vitally important to someone. Deeper in the specs of these items I noticed the colour temperature and colour rendering index of the lamp: 7000K and 65K respectively. Wow, that lamp really is on the blue side! And 65 out of 100 for the CRI is some distance from the holy grail of tungsten. However, this is not a unit you would be using for its accuracy. You would be using it for its wow factor, getting the most out of its colours and gobos. But since you understand specifications and read them, you would know that in advance, and if accurate colour was your requirement, you would look elsewhere in the product range.

When comparing models with greater differences, and comparing across manufacturers, then with moving heads the range of pan and tilt would be significant. The MAC 575 Krypton has a range of pan and tilt of 540 and 246 degrees respectively. Clearly, 540 degrees is more than one circle, but there are good reasons for wanting this. I compared this with a very much cheaper model from a different manufacturer, and actually that had a better range of 540 and 270 degrees. Don’t expect the same build quality though, and bear in mind that sometimes a lower specification coupled with Rolls-Royce engineering results in a better all-round product. Specifications don’t always tell the whole story.

Let’s move on briefly to lighting control consoles. Unlike audio consoles, there are no features that are parallel to frequency response, distortion and noise. The lighting console doesn’t create any light itself, nor does light pass through it in any way. Lighting consoles’ specifications are all about what you get: how many channels, how many faders, how many flash buttons, and so on.

Once you have found a console that has the facilities you need, your ‘commit to buy’ decision would be made on the basis of how usable you find the console to be and whether it seems to have the kind of build quality you expect at the price. Having said that, in both lighting consoles and dimmers (even more specs there) there are certain ‘feel’ factors, and just like audio equipment, judgements are often made on subjective matters that are confoundedly difficult to express and impossible to put numbers to. Still, having an understanding of some of the specification issues of lighting equipment is an excellent foundation for the further acquisition of valuable knowledge and experience. 0

Lighting equipment specifications

A Sound Person’s Guide To Lighting