Abstract

OK, you signed on to be a light tech at the auditorium, what now. We will go through all the basic information for how to fix, run and design the lights.


1.  What does a light tech need to know?

In this presentation I will attempt to explain some theory with a lot of practical knowledge concerning theatre lights. The aim here is to lay out the basics of light and color, discuss how it is used in a theatre situation, then get our hands dirty with the nuts and bolts of lighting.

If this sounds like a lot to cover, it is. But I want you to learn more than just how to run the equipment. I would like each and everyone of you to have an understanding of what lighting does for a performance. We will go through electricity and how a dimmer system works. We will learn what wire colors mean, how to wire a twist lock or a stage plug. How to hang, cable, gel and focus the lights available here in the theatre. How to create a light plot, and a dimmer list. How to light angles, color and brightness combine to set a mood or focus attention during a production.

2.  Electricity, Dimmers, and Wires

This section will discuss how electricity is distributed by the power companies, what wire colors mean, and how to connect a lamp to a circuit. We will then discuss dimmers, how they work and how we control them.

2.1.  Comparing Electricity to Water

 

Electricity seems very difficult for most people to understand. You can’t properly see it, you can’t properly feel it and you can’t taste it or smell it so what is it? It travels quite well through metals and generally poorly through non-metals. It doesn’t behave quite like anything else.

But this is not quite true: fluid flowing through a system of pipes can behave in many respects like electricity flowing in a circuit. The ‘obvious’ exception to this is that if you pierce a pipe full of water, the water comes out.

Try piercing the insulation and touching the conductor: you will soon find the electricity coming out! The main difference is that water will fill any space whereas electricity will ‘fill’ only a conductor. Yes - the analogy is not perfect, but it is helpful and understanding will come not only from the similarities but also from the differences.

Pressure.

In a water system we measure water pressure in feet (foot head). In an electrical system pressure is measured in volts. There is quite a close analogy between feet and volts in this context.

Flow.

Water flow can be measured in many units but we are using feet, so let’s keep to cubic feet per second to measure flow rate. The electrical equivalent is Amperes (amps).

Volume.

In our water system volume is in cubic feet. Electrical equivalent is Coulombs, which is not that commonly used. So one Coulomb per second is one amp. Flow and volume correlate nicely - cubic feet per second is clearly a flow rate.

Resistance and Ohm’s law

In an electrical system resistance is measured in Ohms. There is no name for the water equivalent. So what is it? In electricity, if a pressure of one volt will drive a flow of one amp then the resistance is one Ohm. Two volts would drive 2 amps through 1 ohm: pressure divided by current equals resistance.

In the water system resistance doesn’t equate to anything useful as an aid to understanding. However ‘resistance’ is just that: the amount that the pipe ‘resists’ the flow of water. Double the water pressure and the flow will double. Double the length of the pipe and, if we didn’t change the pressure, we would expect the flow to halve. This, in common sense terms, is exactly what Ohm’s law says! Increase the pressure and the flow must increase. Increase the resistance and the flow must decrease. So Ohm’s law is Pressure/Resistance = flow. Or Volts/Ohms = amps.

 

 

Understanding Electricity - an analogy with water

2.2.  How Electric Currents are created

Although it is not strictly necessary, I thought it might be interesting to understand how electricity is generated. Since we now understand that electricity is basically a movement of electrons through a wire, what caused these electrons to move in the first place?

The answer is that a wire passing through a magnetic field causes the electrons to move. Rather than try to explain it in words, have a look at this picture.

The basic idea is that the flow of electrons is caused by the wire moving through the magnetic lines of force. Now instead of a single wire, picture a loop of wire moving through the field. It then looks something like this.

So now that you can see how it is generated, lets take a look at how the power is brought into a house.

The only thing wrong with this drawing is that it only shows a single phase of electricity entering the building. In most homes, there are 2 phases, or 2 legs they are called. Typically the electric generators produce 3 phase. As an example, have a look at this demonstration using Java Wave Simulation .

In case the wave simulation above is not viewable, here is a static picture of a sine wave.

2.3.  Power in Watts

The size of electric lights is often expressed in terms of watts. So here is an explanation of watts, and how to remember what a watt is.

 

Power in Electrical Resistors

Electrical power is converted into heat when a current flows through a resistor. We can calculate this power by multiplying the current (Amps) by the voltage (Volts)across the resistor (Ohms). For the readers with a love of algebra, we have a formula; well, three actually.

  • The first one tells us the power if we know the Current and the Voltage - we simply multiply them.

  • The second one enables us to work out the Current, if we already know the Power and Voltage.

  • The third one allows us to work out the Voltage if we know the Power and Current.

Remembering three formula is a real drag, so we use the power triangle

 

 

Power Explained

One thing to remember, the filament of a lamp is a resistor which glows white hot to produce light.

2.4.  Standard wire and color codes

So we now understand that typical wiring consists of three wires. They are know as Load or Hot, Neutral, and Ground. These terms relate to wiring shown above.

Table 1. Wire Color and Connections

Designation Wire Color Connector Type Connector Material/color
Hot or Load Black typically Narrow Tab Brass
Neutral White always Wide Tab Silver
Ground Green always Round Tab Green

One more item I want to touch upon is wire size. Wire is rated using a standard sizing called AWG (American Wire Gauge). The normal sizes we will be dealing with is 12, 14, and 16 gauge. Here is a simple table showing the current and power capabilities we will use

Table 2. Wire Current Capacity

Wire Size in AWG Current Capacity Power Capacity at 120 Volts
10 Gauge 30 Amps 3600 Watts
12 Gauge 20 Amps 2400 Watts
14 Gauge 15 Amps 1800 Watts
16 Gauge 10 Amps 1200 Watts
18 Gauge 5 Amps 600 Watts

The reason to know the wire size is so that you do not try to use too small a wire to carry power. Now for the most part we do not need to worry about this because we will be using 14 gauge for most all of the auditorium.

2.5.  Circuit Breakers

One more piece of electrical equipment you need to be familiar with is the Circuit Breaker. This is a device whose main purpose is to prevent fires. Any electrical wire which caries power generates heat. The circuit breakers are designed to prevent the wires from getting too hot and causing a fire. In addition they protect us from getting killed by the circuit.

Here is a picture of how a circuit breaker is wired in a circuit.

2.6.  How to use a Meter on AC

I was going to explain how to use a Multi-Meter for testing a circuit or a lamp. But instead I found a good explanation on the internet. Have a look at Safe Meter Usage, which I got from Lessons In Electric Circuits , an on-line book on electricity which is quite good at explaining the concepts behind electricity, and it is FREE on the internet.

2.7.  How to Wire a Stage connector

Now it is time to have a look at the typical stage cable we will be using and making. The auditorium already has electric distribution on the downstage strip light boom. The circuits are already brought out to twist lock receptacle. To connect lights on the stage, we will be building cables which go from a twist lock plug to a stage receptacle.

Here is a picture of a twist lock plug of the type we will be using.

The way to wire this plug is Green wire, ground, to the G tab, the White wire, neutral, to the X tab, and the Black wire, hot, to the Y tab.

Since we will be using wire type 14/3 type SO Oil resistant P-123-MESA wire, I will describe how to use this wire. First with a mat knife, cut back one inch of the outside insulation. Slip if off the other wires and cut off the reinforcing threads. Next strip back 1/4 inch of insulation from each of the three wires. Now twist the strands together to make them easier to work with. Now slip the plug shell over the wire, then insert each of the wires into the appropriate holes in the plug. Tighten the associated screw until it is quite tight. We will leave the shell open of now and do the other end of the wire.

On the end of the wire where the stage plug will go strip back one inch of the outside insulation with a mat knife, and cut back the reinforcing threads. Strip 1/4 inch of insulation from each wire, and twist the strands together. Now following the diagram above, loosen the screw for the Hot and wrap the black wire around the screw in a clockwise direction. Tighten the screw to secure the wire. Repeat this with the Neutral screw and the white wire. Finally attach the green wire to the ground screw. We now have connections on both ends of the wire.

Before putting on the covers, I need to examine your connections. This is a safety issue and may not be skipped.

Now that the connections have been inspected we can put on the covers and strain reliefs. Starting at the stage plug, wrap the insulation where it leaves the connector with 2 layers of electrical tape. This is to make a better strain relief. Now place the cover over the opening and make sure it grabs the electrical tape securely. If so, tighten the screws down snug.

On the twist lock end of the wire, slide the handle over the connector, line up the alignment pin with the notch and tighten the three screws in the connector. Next you need to place cable clamp on the handle and align the screws to the holes. If they do not align right, turn the clamp around. Once it aligns correctly tighten the clamp down tight.

2.7.1.  Checking your work.

One principle I want to instill is being careful. Lighting equipment is safe enough if used carefully. BUT that assumes you followed the instructions correctly.

For the adapter built above we should do the following checks.

  • Check the strain reliefs to see that they actually work. Grab the connector and give the wire a tug. If it comes off in your hand, it needs to be fixed.

  • Using the multi-meter set to resistance, buzz out the cable. That is connect one probe to the ground on the twist lock and check that the ground on the stage plug is connected. Next with one probe on the twist lock ground, check the other 2 connections on the stage plug. They should show as open. Finally, check the other two pins on the twist lock connector. That is what I mean by buzzing out the cable.

  • Now buzz out the hot and the neutral on the cable.

When ever we make wiring changes or repairs, we should check the work with the meter. This might seem obvious, but being careful back stage can save a LOT of nasty, and unnecessary problems.

2.8.  How SCR dimmers work

Although this is not strictly necessary here, I think it is fun. You have seen what an AC sine wave looks like above. Now lets have a look at what happens when it runs through an electronic dimmer.

The dimmer works as a fast switch on the wave form. What I mean is, the SCR (Silicon Controlled Rectifier), waits for part of the wave to go by before switching on and allowing the rest of the wave to pass through it. Clear? Didn’t think so, but a picture will make it clearer.

This is how a single SCR works. Normally dimmers have a pair of them to cover both halves of the wave. Since the lights are incandescent type, they do not care what the wave looks like. The longer the SCR waits to turn on, the less power it sends to the light.

OH as a foot note, I should mention that most Multi-meters do not measure the power from a Dimmer correctly. You need a meeter which is rated for True RMS readings. Normally you do not need to measure the power, since you are not repairing these dimmers.

3.  Theatre Light fixtures

Now that we understand basic electricity and how to create the cables, it is time to start reviewing the different types of lights used in this auditorium. There are many more types of lights than I will cover, but these are the basic types. We will put off for now how they affect the stage.

3.1.  Pars

The par is the simplest light in the system. I consists mostly of the lamp and the shell. The type of light is determined by the bulb, which can be seen here.

 

PAR is an acronym referring to “Parabolic Aluminized Reflector” which describes the mirror configuration. PAR lights resemble car headlights. They possess a lens, but the lens is an integral part of the lamp and its position relative to the filament cannot be altered.

The lamp produces an intense oval pool of light with soft edges. The only adjustment is a knob that allows the lamp/lens unit to be rotated within its casing, thus changing the orientation of the oval. The type of lens may be changed, options include extra-wide flood (XWFL), wide flood (WFL), medium flood (MFL), narrow spot (NSP), and very narrow spot (VNSP).

 

 

Wikipedia: Stage Lighting

These lights tend to be very rugged in the field, but simple to use. The front of the shell contains a set of clips to hold a gel frame.

The light from the par is broad and is often used to fill in general areas. It does not focus, or shutter so it can be used as it. It’s a good general lamp for fills, but not specific lighting. It does not focus of cast shadows.

3.2. Ellipsoidals

The ellipsoidal lamp is so named for its reflector which puts the filament of the bulb at the center of the ellipsoid. Another name for the ellipsoidal lamp is the leko The light is then focused with the lenses

 

Characteristics of an ellipsoidal lighting unit include:

  • An adjustable barrel, allowing a lighting designer to change the size and focus of the light thrown by the unit by changing the distance between the mirror and the plano-convex lens or lenses

  • A set of brackets on the end of the barrel for the insertion of gel frames or a color changer unit to alter the col-our of the light

  • A series of shutters at the base of the barrel for shaping and narrowing the light beam

  • A slot in the body of the unit for the insertion of metal gobos to change the pattern of the light

  • A further slot for the addition of an iris to narrow the beam whilst keeping its circular nature.

 

 

Wikipedia: Ellipsoidal

This lamp is focused using the lenses and the focus knobs so you can create a sharp or soft focus. The sharp focus is useful if you wish use the shutters to restrict the area covered by the lamp. When the sharp focus is used the actor can walk into and out the pool of light. This is valuable when working with dramatic lighting or with dancers who move through pools of light.

The follow spot is a larger leko type of lamp. The main advantage of the follow spot over the leko, is the ability to move during the show.

3.3.  Fresnels

The Fresnel lamp is used when you want a soft pool of light. Even though there is a focus on the Fresnel this is to change the size of the pool not the sharpness as it is with the ellipsoidal lamp above.

The Fresnel is so named due to it’s front lens. The lens originally was quite thick and heavy. But once engineers realized that the amount of glass was not adding to it’s ability to focus the light, they were able to cut away the amount of glass. If this does not make sense to you, let me show you a picture which makes it clearer.

 

Fresnels are typically 8-, 6- or 4-inch, referring to the diameter of the lens. This light is named after Augustin-Jean Fresnel who invented the distinctive Fresnel lens which has a ‘stepped’ appearance instead of the ‘full’ or ‘smooth’ appearance of other lenses. The stepped nature of the lens causes a corresponding pattern of circles of light, so Fresnel lenses are usually ‘stippled’ on the flat side. This pattern of small bumps helps to break up the light passing into the lens to smooth out its eventual pattern.

Fresnels use a spherical reflector, with the filament of the bulb at the focus. Due to this, the bulb and reflector cannot move independently of one another, and remain a fixed unit inside the housing. It is this unit that is moved back and forth inside the lamp to focus the Fresnel. This is done by a slider on the bottom of the light, or by a worm track.

 

 

Wikipedia: Stage Lighting

The Fresnel is used to create pools of light for the actors/dancers to move through. The main difference between using a leko and a Fresnel is their range. The Fresnel tends to be good for overhead lamps or short range light pools. The leko tends to be better for tighter areas, and longer throws.

3.4.  Strip Lights

The other light on the stage is the strip light. In the auditorium we have one set down stage, and one upstage. Strip light, also sometime know as Cyclorama (Cyc) lights, are used to provide a broad and unfocused overhead light. Their original use was to create a colored back drop for the scene. In this capacity, as cyc lights, they are pointed at a cloth backdrop, which is usually white. The scrip light usually come in primary colors, which when combined can create any color. We will discuss color later.

 

Strip lights, also known as Cyclorama (Cyc) lights, Battens or by the brand name Codas, are long housings typically containing multiple lamps arranged along the length of the instrument and emitting light perpendicular to its length. Lamps are often covered with individual gels of multiple colors (Often Red, Green, and Blue which allows practically any color to be dialed up) with each color controlled by a separate electrical circuit. Many strip lights use round pieces of glass (called roundels) rather than plastic gels for color. Varying the intensity of the different colors enables the lighting designer to establish mood or time of day.

 

 

Wikipedia: Stage Lighting

4.  Hanging, and gelling Theatre lights

Theatre lights are attached to booms or pipes using a clamp known as a C-clamp.

The c-clamp goes around the pipe and the large screw forces the pipe into the Clamp. On the side of the clamp is a small screw which allows you to rotate the lamp pin in the clamp. The bottom of the pin is fastened to the yoke of the lamp with a bolt. When ever you hang a light you need to make sure all the screws are tightened to hold the light securely.

Next you need to secure the light with a safety cable. The cable goes through the yolk of the light and around the pipe. Normally the cable will not do anything except prevent the light from hitting the ground should the c-clamp not hold it. Think of the safety cable as a seat belt for the light.

4.1.  Pipes, side arms, ladders, and trusses

The places around the stage where lights can be mounted are many. We will discuss them here so you can appreciate what the different locations are and how they can be used.

4.1.1.  Pipes or fly pipes

The lengths of pipe which are hanging above the stage can be either fixed, also know as dead hung, or mounted on ropes with counter weights. The ones fixed pipes, are anchored to the ceiling beams above the stage with cables or chains. To access these pipes you need to work on a ladder.

The other pipes, sometimes called flying pipes, are able to be lowered to a working height. These pipes have a weight rack attached which allows you to counter balance the light pipe and fixtures using iron weights.

When using the fly pipes, it is usually a good idea to load some additional weights to the carriage before bring in the pipe. Then, after the pipe is hung with lights, and the cables are attached, you will be able to pull it up for additional weights to balance the weight.

For each light clamped on the pipe, we will need to attach wires for a connection to the electric circuits on the down stage light strip. These wires will need to be tied up with cord so they do not hang down from the pipe.

The number of lights and how they are wired will be discussed in the light plot later.

4.1.2.  Side arms and booms

The boom is a vertical pipe which stands on the stage floor and point the light horizontally across the stage. This type of side lighting is often used for dance lighting since it gives the dancers more depth due to the angle of the light.

A side arm, is a mounting for the lamp which attaches to the pipes above, but lowers light below the pipe. These can be used for side lighting when you do not wish to have a boom on the floor. They are also used when you want side lighting that is higher than your booms, but not as high as the pipes.

4.1.3.  Light Ladders

The light ladder is a metal frame designed to hold lights below a pipe. These frames look like a ladder. They are hung via their own C-clamp from a flying or fixed pipe over the stage. You can then mount lights for side lighting on the ladder.

4.1.4.  Light Truss

The truss is a metal frame for hanging lights. In the auditorium, we have a truss in the house over the seats. These frames can come in different sizes and types. Once again it is a structure for hanging the lights.

4.2.  Gels and Color

Stage lights are often colored using a piece of gel to color the light. The gel is normally mounted in a metal frame and placed on the front of the light.

We are not going to get into how to select colors for shows at this time. It will come up when we discuss lighting design. But I thought you might like to have a look at a color chart from Rosco which is a local copy. The original is here .

4.3.  Cabling the lights

Cabling the lights is a matter of connecting the lights themselves to the dimmer circuits which will power them. This operation is done based on the light and dimmer plots. Often the lights have a single wire connecting the light to the circuit on the down stage pipe.

Sometimes the lights will be connected together before being cabled to the circuit. If the plot calls for 2 lights on the same pipe to be connected to the same dimmer, we can use a Y cable to connect the lamps together before running the cable to the circuit. Normally, only two lights will be connected together before running to a circuit. The reason is that it is better to use more dimmers for finer control.

The lights are always connected in parallel in this auditorium. This means that each light sees the same voltage.

Once all the wires are run along the light pipe, we need to take some string and tie up the wires to prevent them from hanging down below the pipe. This dressing is not always necessary, but should be done any time the cables will hang down more than a few inches below the pipe. By tieing up the cables, we make a neater configuration.

4.4.  Focusing the lights

Once the lights are hung and cabled, it is necessary to focus them. The process of focusing is mostly pointing the light at the location specified by the design. Sometime you are called upon to change the reflector or lens settings of the instrument to sharpen or soften it’s light. Again, sometimes you will be called on to push in a shutter on a Leko to mask the light off some specific part of the stage.

Focusing a show is a time consuming process since it is done one light at a time. It also requires at least 3 people. One to run the light board, one to look at the focus on the stage to decide when it is correct. And one person to get up next to the light and do the focus.

5.  Lighting plots, dimmer lists, and running a show

We have looked at how to setup the lights and how they are cabled. Now lets look at the tools used for planning a stage layout. This is how a light designer translates his/her ideas into documents the light crew can setup.

5.1.  Light Plots

The light plot is an overhead drawing of the stage showing the position of the lights. Lets start by looking at a sample light plot supplied by Altman Lighting.

What you see in this plot is the stage and the light pipes. On each pipe is the instruments that will be mounted, what type of instrument they are, where they should be mounted, and approximately where they will point. This plot gives you much of the information you need to put up a show. The other piece is the dimmer list. The light plot above comes from Small Theatre Brochure . There is a lot of good lighting information at this site.

5.2.  Dimmer List

Here is a partial dimmer list I created for the above light plot.

Table 3.  Dimmer List

Position Unit # Channel Dimmer Circuit Inst Type Wattage Color Purpose
HT 1 20 5 3 6 x 12 360Q 750 R60 DSL
HT 2 20 5 3 6 x 12 360Q 750 R60 DSL
HT 3 21 65 45 20o Shakespeare 750 R60 USL Special
HT 4 22 4 4 6 x 12 360Q 750 R60 DSLC
HT 5 22 4 4 6 x 12 360Q 750 R60 DSLC
HT 6 23 66 46 20o Shakespeare 750 R60 USC Special
HT 7 24 67 47 30o Shakespeare 750 R60 USC Special
HT 8 23 3 3 6 x 12 360Q 750 R60 DSC
HT 9 23 3 3 6 x 12 360Q 750 R60 DSC

This Dimmer list is only an example. I hope to expand more on this section once I get a chance. In the mean time, you can do a lot more reading on lights at the Altman site above. Or if you want to know more about how to put together a school theatre crew have a look at A British School Shows web site


Written by John F. Moore

Last Revised: Mon Nov 7 09:42:19 EST 2016

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