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Stage Lighting: A basic technology manual.

Basic Dimmer Types

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Control of Intensity

Apart from the repertoire of luminaires, the most important tool at the disposal of the lighting designer is  The Dimmer

This has a number of functions:
•     To create moods and effects
•     To balance individual lights against one another to create the desired picture
•     To move smoothly from one scenic picture to another
•     To fade up and down from blackout

The dimmer is a device which is either :

•     inserted in series with the load.
•     Added to the optics of an individual luminaire 

The First dimmers:

This is not intended as a historical treatise, but lighting has been controlled for theatre purposes since time immemoriable. For details of lighting control in pre-electric theatres, please see the book "Theatre Lighting in the Age of Gas" by Terence Rees read review

The Savoy theatre, London was the very first theatre anywhere to be equipped with electric light in 1881. The battens and perchlights used were dimmed by means of controlling the DC generators which powered them. In due course I will post an article about this "The White Heat of Technology" written for Cue Magazine in 1981- as soon as I get round to scanning and OCR-ing it!

Series Dimmers:

A very early form of dimmer, the liquid dimmer consisted of a vertical earthenware pipe full of acid-loaded water.


                  

 

Much more likely, a dimmer would consist of a manually-operated resistance chosen to match a particular load so that it would dim smoothly from full to out.

                       resistance dimmer

This might consist of a large iron frame fitted with coils of heavy wire and a lever to move a wiper from one end of the coil to the other. They could be operated singly or numbers of these (In a large installation there might be 96) were fitted to a board and linked together with a complicated system of drive shafts, wheels and clutches. The load had to be matched to the dimmer, so you would typically have a dimmer labeled as “750 watts plus or minus 25%”. If you over or underloaded a dimmer, it  either got hot and buckled or the curve would be unusable, or it simply didn’t dim.

Typically up to the mid sixties, this would be mounted on a perch or at stage level on one side of the stage, usually with no view whatsoever. At Brighton Theatre Royal it was mounted in a kind of Portakabin at the side of the stage absolutely isolated from everything. This gave a great feeling of decoupling from the show and made anything resembling a feeling of contribution to the art virtually impossible. In the authors experience the lighting operators of certain No 1 touring theatres where the control had no view of the stage, usually casual part timers, paid by the show, were indifferent to the point of negligence, using their time between cues to drink heavily smoke illegally and in one particular case, hold fart-lighting contests. On the other hand there were some absolutely wonderful, skilled, talented, committed people as well.

This was difficult and manpower-heavy to operate and not particularly accurate in reproduction. Dimmer levels were usually referred to as ‘half’ “quarter’ etc., and many hands assisted by all sorts of tools like pieces of stick and lengthe of string were needed to perform cues. Cues were typically written on cards hanging by a piece of string from a convenient dimmer handle, or written on blackboards at the end of the perch. Subtle fades were performed by administering a series of light taps to the dimmer handles. The metalwork frame was very useful for opening beer bottles, as the scar on the authors thumb will testify after 30 years.

Grand Master and Sunset boards are still to be found. They are very reliable and virtually foolproof. In the hands of a skilled and fit operator or better still an octopus, they are capable of performing well and can produce fast moving shows and fades of great beauty. No presetting is of course possible.

Later, dimmer banks were motorised, and a complex system of electromagnetic clutches was employed to knock dimmers off the shaft when they has reached their predetermined position. A Compton Organ keyboard  the Light Console was employed as the controller for this, complete with keyboard (Go, up, down, stop etc.,) and stop tabs (dimmer selection).

The big advantage of this system was that the control could be kept separate from the dimmer racks, and many installations made their way to the front of house in various positions, including the orchestra pit, the stage boxes, or specially created lighting boxes. This in time was replaced by the more familiar preset console.

Dimmers could also be made from Auto transformers operated by tracker wire by remote control, or magnetic amplifiers, or from saturable reactors, a form of choke. This was a particularly favoured method on the continent.

Thyratrons made a brief appearance as dimmers. These were the first electronic dimmers based on thermionic valves, very temperamental, and had to be treated with loving tenderness. The temperature in the dimmer room had to be just right, and when they “stuck” as they often did, the preferred method of curing them was to gently tap the anode cap with an old rubber slipper.

The first Thyristor dimmers,  made their debut at Glyndbourne in the early sixties, complete with four presets and a stalls control. They were not perfect, and some problems were yet to be solved. But it was the way forward and is the precursor of all our modern dimmers.

The thyristors were replaced with simpler Triacs which allowed dimmers to be smaller and simpler

dimmercircuit This is a typical SCR based light dimmer which dims the light through phase angle control. This unit is wired in series with the load. Diodes (D2, D3, D4 and D5) form a bridge which generates DC with lots of ripple. R and C form a circuit with a time constant, as the voltage increases from zero (at the start of every halfwave) C will charge up, when C is able to make ZD conduct and inject current into the SCR the SCR will fire. When the SCR conducts then D1 will discharge C via the SCR. The SCR will shut off when the current falls to zero when the supply voltage drops at the end of the half cycle, ready for the circuit to start work on the next half cycle. (reproduced from Wikkapaedia)

Controls were made in a wide range of sizes and with various facilities, including dipless crossfade and chasers. However systems were getting to be impossibly large and difficult to cope with. Large or touring shows often brought in desks to supplement the house desk, and sometimes three of four desks!
The scene was now set for the emergence of the memory systems, computer based, with the ability to control large numbers of channels and memorise huge amounts of information.

Today’s systems have both softpatching and output patching to simplify fast-rigging fitup techniques, and some are capable of control of a wide variety of equipment, such as scrollers, moving lights etc.
They can also be provided with proportional patching to enable a single channel to control more than one dimmer module, each dimmer proportionally set at a different level, for example to retain the relative relationship between colours on a cyclorama.

 

MODERN DIMMERS

As we know AC voltage forms a sine wave. At any given point on the cycle the voltage is somewhere between 0V and 230 Volts
Modern dimmers are essentially high speed switches which chop into the mains waveform at different points in their cycle, allowing all or part of it to pass with a consequent effect on voltage.


Dimmer circuitry consists of either a pair of back-to back thyristors or a triac . Both are a simple solid state device equivalent to an on-off switch controlled by an external signal.  This signal is produced by a timer chip, which itself is controlled by a signal sent by the chosen control desk.

Because the chopped waveform can cause considerable Radio Frequency Interference, (RFI) a large choke is added as a suppressor. It can also cause harmonic distotion which can result in the filaments of lamps singing or buzzing loudly while being dimmed. This can be very distracting and dimmers have been developed to minimise this.

Sine Wave Dimming employs high frequency Insulated Bipolar Gate Transistor circuitry to ensure that the input, output or both current waveforms of a dimmer are sinusoidal. A conventional thyristor dimmer chops the output waveform to regulate the power to the lamp and draws a current from the supply that is rich in harmonics because of the chopping. A sine wave dimmer delivers a sine wave output current to the lamp and/or draws a sine wave input current. The benefits of a sinusoidal output include elimination of the ringing of a lamp filament while the benefits of a sinusoidal input current include no distortion of the supply voltage.

Domestic dimmers or small commercial dimmers should be avoided for theatre use, as they simply are not fit for the job.

Well made dimmers can be used for the control of switch-only items such as motors, relays, strobes, heaters etc. It is possible to configure these to be set to various thresholds, i.e. to switch as soon as a dimmer lever is raised, or to switch at, say 50%. Alternatively sockets should be provided which are switched either manually or switched from relays controlled from the lighting box. If you want to integrate the switching of these into a memory plot there is available from various manufacturers switching packs which can be substituted for a dimmer module and controlled in the same way as a dimmer, with a control voltage, a DMX or MIDI  signal.. It is also possible with some units to configure these to generate strobe firing pulses directly.

Dimmer controls for architectural lighting control, for example for restaurant or retail situations, are available which store a number of ‘scenes’ or ‘looks’  on a small rack-mounted or wall box mounted dimmer, which can be encoded with a secret access code to prevent unauthorised modification. Staff can select the scene by means of a small push button panel or by infra-red control.

DIMMER PROFILES

The relationship between the movement of a dimmer lever and the output of the dimmer is called the Dimmer Curve, Profile or Law.
In a simple linear system,  a 10 per cent movement in the dimmer lever would produce a ten per cent change in the electrical output of the dimmer. However, this is not exactly an ideal system, because lamp filaments themselves do not behave in a linear manner. A better plan all round would be to create a curve which gives sensitivity where you need it, i.e. down at the bottom end of the travel, and a more or less linear response at the centre and a little more sensitivity at the top.
This forms a sort of italic “s” shape and is in fact known as the S-Curve.

Many desks have the ability to set the curve to your personal requirements.

If for example you are fading up a scene consisting of
1) a 100 Watt practical lamp, say, a wall bracket or small chandelier
2) 3 x 1200 Watt halogen lamps  for covering light, and..
3) a 5Kw scene projector

you will inevitably find that the practical will be the first light that you see, followed by the covering lamps and then, a long way behind, the scene projector will come lumbering up. This is all to do with filament response.

Judicious manipulation of dimmer levers in a manual system will overcome this problem, but in a memory system you will have to either spilt the cue over two or more playbacks or alter the dimmer profiles.

DIGITAL DIMMERS
Digital dimmers are the state of the art answer to dimming requirements for the foreseeable future.

Although the “business end” of the dimmer still relies on the thyristor or triac, the front end is fully digital. There are several advantages to this.

Normal printed circuit board mounted components are designed to certain tolerances of accuracy, usually 5% or 10%, unless expensive high specification components are used. This means that from time to time the performance of a normal dimmer can vary. In the digital domain, repeatability is guaranteed.
A digital dimmer taking its signal via DMX never has to work in the analogue domain at all, thus obviating conversion losses.
It can check the signal continuously at several points in the circuit and correct where necessary.
It can test for loss of load or partial load, for example if a single bulb in a circuit of four blows, the dimmer will tell you.
If a short circuit occurs, the dimmer will shut down safely before damage or injury can occur.
It is capable of checking input and output voltage and condition of control signal,  reporting to a PC computer if required, keeping a log of every dimmers performance.

 

Dimmers, and filament noise

That annoying singing noise that seems to emanate from your entire electrical system when you perform a fade is due to the rate of change in current flowing in the circuit which can cause acoustic noise in filaments and cables and also radiated (electromagnetic) noise and harmonic distortion in the mains supply.

If you can make the rise time longer ( say up to 100 microseconds), the current change is less pronounced, and the whole system becomes quieter, producing not only less filament buzz, but less interference in audio, video and radio communications systems, and less mains-bourne noise

Many manufacturers of digital dimmers let you specify the rise time. 350 microseconds is OK, but 850 is better.

If you have a triac or analogue dimmer system, then the only way to reduce noise is by making sure all the connections are really tight, and that your cable routes, both permanent and temporary are carefully planned to avoid interference with audio systems by not running your cables close to and parallel with audio and video cables for any significant distance - or preferably not at all!

Dimmers, and colour temperature

An incandescent lamp operating at its full design voltage has a collimated colour temperature (q.v.) of about 2700?K. As it is dimmed the filament works at a lower physical temperature and becomes ‘redder’, and very much warmer in colour. This can be used to good effect for warm intimate scenes when using lamps in multiples at low check gives an effect unattainable by other means. However, if its a dim harsh steel that you are after, then this becomes increasingly difficult as the dimmer levels are lowered. The only way to counteract this is to add more and more layers of filter, or neutral density filter or to use lower wattage bulbs in the first place.

Dimmers and lamp life.
Dimming incandescent lamps has a beneficial effect on lamp life. A ten per cent reduction in voltage, i.e. running at 90% on the Grandmaster on the desk increases lamp life by 400%: Conversely a 5% overvoltage decreases lamp life by 50%

Dimmers and the halogen cycle
If tungsten halogen lamps are run for extended periods at a dimmer level of less than 80%, the temperature in the envelope never reaches levels high enough to sustain the halogen cycle (q.v.) and shortening of filament life and blackening of envelopes may occur. This can be seen readily in architectural situations, where apparent failure to meet life predictions of expensive lamps can be a source of aggravation between designer and customer. It is therefore recommended as an economy measure that the lamps are run at full temperature for ten minutes or so every couple of weeks.

Dimmers and loading
Modern dimmers are load independent - up to a point. If however you want to run a tiny load, such as a 100 watt practical, you must ensure that a dummy load of at least 25% of the dimmer capacity in patched in parallel in order for the dimmer to behave in a predictable manner. However, this unpredicability might work to your advantage. I once had a practical gas bracket fitted with a 15watt 240v lamp which when used without a load exhibited an interesting flickering effect at just two points on the fade profile on that particular dimmer - about 15% and about 75 %, which I was able to plan in to create an interesting realistic and not to intrusive gas flicker which helped the plotline immensely. However the effect would not be transferable to another dimmer module which had slightly different characteristic.

Dimmers and Motors You cannot vary the speed of an AC synchronous motor such as the motor in a colour wheel or mirror ball by putting it on a dimmer. All it will do if you try is simply stop the motor and may also damage it or the dimmer. If you want to for example run a cloud effect, the motor should be fed independently and switched as required. If you must, you can put the motor and the luminaire on the same dimmer circuit, but you need to ensure that the dimmer had at least 10% capacity over and above the connected load. As you fade this combo in, the clouds will appear to be still until you reach about level 30% when the they will suddenly start to move.

DC motors and can be controlled with a PWM dimmer see this pdf for more info and a circuit

Optical Dimming Devices

Opto- Mechanical Dimmers
These are dimming devices inserted into the optical path. They are used almost exclusively for those items of apparatus which are inherently non-dimmable by electrical means, such as discharge lamps.

In its simplest form it consists of a number of elements arranged like a louvre which can be set in infinitely variable position between open and shut. They do have the disadvantage that they cut down a significant percentage of the available light even when in the open position. The big advantage is that they do not affect the colour temperature of the emitted light at all, and in a properly ventilated fitting, does not affect lamp life one way or the other. These dimmer units can usually be operated from the control desk, a normal mains dimmer being used to control the mechanical dimmer or it can be controlled at low voltage or DMX level.

Neutral density filters are available which cut light down by a specified percentage without significantly changing colour.. More useful for film and TV work. Available as .5 stop, 1 stop, 2 stop, 3 stop or 4 stop.

Graduated wheels can be made to rotate in front of the lens of various lamps, adding various degrees of density.

Liquid crystal shutters are in the early stage of development but at present (2001) can dim out very crudely and only to 80% extinction .

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