Illuminati Creative Technology, Colchester UK

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Stage Lighting: A basic technology manual.

Technolgies for the production of light

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Light is produced by one of ten basic technologies or by a mixture of these technologies:

1) Incandescence
2) Discharge glow
3) Cold Cathode lighting
4) Electric arc light
5)Electroluminescence
6) Light Emitting diodes
7) Lasing
8)Ultra violet excitation
8)Luminosity
8)Bioluminescence

Which Lamp for what purpose??

The design of any lamp will take into account:

•     Operating voltage
•     Operating current
•     Total brightness
•     Life of lamp
•     Operating temperature
•     Range of permissible burning angle
•     compactness of light emitting area
•     shape and size of envelope
•     current handling capacity of the connections.
•     Whether clear glass or frosted
•     spectrum of light emitted
•     Colour appearance
•     Colour rendering ability
•     Correlated colour temperature required
•     Overall ruggedness.

1) Incandescence
The Principle:
If a controlled current is passed through a suitable resistance it will heat up and glow incandescently. The filament of a lamp is such a resistance chosen to make the best balance between long life and brightness. The filament is in a glass envelope containing nitrogen to help slow down the burning process. In Tungsten Halogen lamps a little iodine gas is placed in the envelope to  preserve filament life and prevent lamp blackening by setting up a chemical cycle which re-deposits burned tungsten on the filament.This is called the halogen cycle.

The tungsten filament, (invented more of less simultaneously by several people including Swan in England and Edison in the USA) is heated to white heat by passing a current through it. The filament has a limited life, as by definition it burns away in use. Its life depends on finding the correct balance between the thickness of the filament and the temperature at which it burns.
The filament has to be supported. This is a weak point and makes a difference at what angle you can hold the bulb before the filament sags, touches and blows. In addition, the pinch seal can overheat.

2) Discharge glow
The principle:
If a current is passed as an arc between two electrodes in an enclosed space in a gas-filled lamp  the gas will become ionized and glow.

The colour appearance, brightness and the colour rendering ability of these lamps varies according to the type of gas enclosed, the pressure of the gas, any fluorescent lining on the inside of the tube, (which will glow brightly in the presence of ultra violet light produced), any added metal such as Mercury or Sodium which is vaporised,  and the current passed.
Lamps in this category include neon, fluorescent tubes, Stroboscopes, Mercury Vapour, Low and High Pressure Sodium, Xenon, Metal Halides including HMI and CSI / CIDtypes.

They require the use of

•   A device for starting the discharge:
•   A device for regulating the discharge

Different lamps have different actual requirements: the manufacturer of the lamps will advise, or a fitting specific to your needs will be supplied complete with the necessary gear.

The regulator is usually in the form of a choke (see earlier). Some discharge lamps. however are supplied with their own regulator in the form of an integral  tungsten filament. This supplies some incandescent light, and the filament acts as a resistive current limiter These are called Blended lamps and require no other gear and can fit in any standard E27 lampholder provided that it will take the heat and the extra weight.

The starter is either
a) a device for providing a pulse or limited series of pulses of high frequency high voltage. or
b) An auxiliary electrode close to one of the other electrodes to provide a starting arc.

The envelope is filled with an inert gas, usually a mixture of Argon and Nitrogen at low pressure, plus a measured amount of the appropriate metallic. Mercury ,or Sodium, or in the case of Xenon, CSI, CID etc., a gas. When striking occurs, the metal vaporises and the pressure in the envelope rises.

Most of the light produced  by the arc a mercury lamp is in the U.V. range, both long and short wave. These lamps have a phosphor coating on the inside of the lamp to convert this into visible light, and to correct the spectral distribution. A special range, known as Blacklight lamps or Woods lamps have a coating on the exterior to filter out all the dangerous short wave U.V. and visible light. This is used to fluoresce sensitive materials for special effects.

Discharge lamps usually take a period of time, up to several minutes, to attain full brightness when the whole of the metal is vaporised.
If the current is interrupted, or falls below a certain level, then the lamp will extinguish, and unless it is rated specifically as hot restrike will require a period of time before it will work again
Where capacitors are across the circuit (see Power Factor) a high transient current will occur on startup

.DISCHARGE LAMPS: THE BASIC CODES

THE GENERIC MARKS ARE CUMULATIVE: THEREFORE A LAMP LABELLED MBTF
IS A MERCURY LAMP WITH A HIGH PRESSURE FILLING LESS THAN 100 WATTS PER CM OF GLASS TUBE WITH AN ADDITIONAL TUNGSTEN FILAMENT AND AN INTERNAL FLUORESCENT COATING.

Follow spot or projector discharge lamps:

It is important to treat these lamps with respect. Not only do they cost perhaps hundreds of pounds but can explode spectacularly if mistreated or suffer sudden temperature changes.

Lamps have a limited life. In the case of the high pressure lamps, Xenon, HMI or CSI, it is very important to keep a log of the lamp usage and to replace the lamp before it blows. Many follow spots and discharge luminaires are fitted with an elapsed time counter. The usual practice is to reset this to zero when changing a lamp or if this is not possible, to note the reading on a sticky label on the lamphouse together with the estimated reading when the next lamp change is due.

When handling Xenon or HMI lamps alweays wear a protective face mask and heavy gloves. Hired follow spots will usually include these in the flight case.

For more info see http://en.wikipedia.org/wiki/High-intensity_discharge_lamp

3) Cold Cathode (category includes neon.)
The principle:
A tube containing gas and a pair of simple electrodes glows brightly when a high voltage (approx. 12 - 20,000 Volts) or a low voltage high frequency, is applied.
Colour appearance depends on the gas used: Krypton produces red, Argon blue-green. Other colours are produced by mixing gases and adjusting voltages. For more info see: http://en.wikipedia.org/wiki/Cold_cathode

4) THE ARC
The Principle:
Light is emitted when a continuous spark is induced between electrodes in free air. Two carbon rods connected to 60 volts or so DC (Direct Current) of about 100 Amps capacity are momentarily brought together and a bright spark is established between them when they are parted. The spark gap is increased to about 15mm at which distance, the arc becomes stable, consuming the carbon rods as it continues. The carbon rods are “fed” i.e., kept a uniform distance apart either by continuous hand adjustment or with a motor.

5) Electroluminescence
The Principle:
This is a solid-state device consisting of a substrate coated with a chemical compound.
When excited by  a voltage of 120V 400Hz, the whole area glows fairly brightly. As the substrate is a soft plastic only about  0.6mm thick,  the material can be fitted to curved surfaces, cut easily and is useful fitted to props and costumes. Available in various colours. For more info see http://www.bgsu.edu/departments/chem/faculty/pavel/electrolum.htm

6) Light Emitting Diodes
These are solid state devices which emit small amounts of light when a current of a certain preset value is passed across impurities in a piece of silicon. Can be various colours, including white, green, red or amber. Life expectancy of an LED lamp can be from 10,000 hrs upwards. for more info see; http://en.wikipedia.org/wiki/LED

7) Lasing
The principle:
A laser tube is fitted with special mirrors at each end. A bright flash from a Xenon tube within the tube starts the process whereby the light moves back and forth within the tube until the photons form standing wave of such power that it exits through the Brewster window at the end of the tube as a tightly collimated beam of considerable power.
The strength and colour of the beam is a function of the gas within the tube and the power applied.

8) Ultra Violet Excitation
The principle:
Ultra violet light of approximately 350 nanometer wavelength can be turned into visible light when it is used to excite fluorescent phosphors. Such phosphors can be incorporated in paint or glowing plastics. Useful for highly controllable glow-in-the-dark special effects and props 

9) Luminous Materials
The principle:

Luminous materials ( as opposed to UV excitable materials) have the ability to absorb and store small amounts of light, continuing to glow when the excitation light is removed. Useful for marking the stage for blackout changes, or to show actors where the edge of the stage is.

10) Bio luminescence

I add this for completeness : but it is not excited by electrical technology, merely a natural process. Try throwing a stone into the river at Southwold after dark and you will witness the finest display of biolights you will ever see! http://en.wikipedia.org/wiki/Bioluminescence

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