LIGHT SOURCES - NOTES

 

 

Light Sources

 

 

6 types of sources:

1). Incandescence: light given off by a glowing filament

2). Arc Light: electrons jumping a gap and ionizing air

3). Gas Discharge: gas excited by high voltage

4). Short arc: combination of short arc and gas discharge

5) LED / OLED

6) Electroluminesence

INCANDESCENCE - most common type of illumination in use in theatres

-flames are incandescent gas. Red hot steel is incandescent.

Incandescent lamps have 3 parts:

Bulb ( the glass envelope )

Base – makes electrical contact and correctly positions lamp

Filament – typically tungsten, high temp/high resistance

2 types: regular – w/tungsten filament and inert gas

Halogen – w/ tungsten filament , halogen gas and high temp glass bulb

Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than visible light.

Incandescent light bulbs are being phased out for newer, more efficient light sources. 

Already gone in Brazil, gone by 2014 in the US currently being phased out in EU.

 

The Filament Incandescent Lamp

-1879 Thomas Edison invented first practical incandescent lamp. It had a carbon filament made by charring a strand of bamboo fiber – enclosed in an evacuated glass bulb. It produced 1.4 lumens of light per watt of electricity. Had an efficiency of 4%. Edison chose carbon because it had a high melting temperature. 

Tungsten introduced in 1907 – melting point of 6120 degrees F,

1913 vacuum replace with inert gas – now up to 14 lument/watt 10X Edison’s lamp.

1910-1920 technology invented to create tungsten wire – more concentrated filaments.

 

Lumen Maintanence 

Lumen output of a lamp tends to drop off over time – so may be reported as “ average Lumens”

Sublimation: Tungsten converts from solid directly to a gas ( skipping a state of matter – or sublimating) this gas condenses on the bulb of the lamp clouding it over time. High temperature and low pressure accelerate this process. Burning out is actually a misnomer – lamps don’t “burn” because the oxygen is missing.

Filaments:

Tungsten filament wants to be as compact as possible for greater efficientcy. 

 

FILAMENTS

 come in different configurations depending on intended use.

These forms are: Barrel 

Corona

Monoplane

Bi-plane

coiled coil.

 

BULB – regular lamps are just glass.

Tungsten halogen lamps have a high temperature and pressure resistant glass based on quartz – like pyrex

Different shapes for different uses: A+ Arbitrary ( typical house hold  light bulb)

PS= Pear shape, G=Globe shaped, T= Tubular, PAR, R=reflector etc….

The glass can be ordered in frosted or clear finish color – or lamp dip- can be applied in only lower wattage lamps.

 

Diameter of bulb is expressed by how many 1/8th’s of an inch it is at the largest diameter.

Ex. PAR 64

 

BASE:

Bayonet ( flashlight lamp)

Screw

Bi-pin

Pre-focus- all come in different sizes (mini, candelabra, regular and mogul)

 

Light center length or LCL.

The length measured from the center of the filament to the bottom of the base. This information is important in order to make sure the filament is aligned properly for maximum light output.

 

Using LCL, it is possible to get tungsten halogen lamps for older fixtures- these lamps are called RETRO-FITS.

quartz lamps w/ std incandescent bases matching original LCL. Used to upgrade old instruments to more efficient lamps

PARs and R lamps are like small self –contained lighting instruments - they have lamp, reflector and in some cases a lens contained in the lamp.

 

Tungsten –Halogen or Quartz lamps:

-more compact filament – closer to a point source = more efficient.

-same intensity thoughout lifespan – no coating sublimated on the bulb

-longer life

very hot source – required for sublimation cycle to be broken

has weaker – less supported filament

- instruments with tungsten halogen lamps must be handled gently.

- NEVER touch the glass of the lamp its self.

- Universal burning position

 

Lamp Life-

Things that contribute to shortened lamp life.

-too much heat

-rough handling

too high voltage – low voltage increases life

wrong burning position

ANSI developed a 3 letter identification code for all lamps. example EHG (on hand out)

 

Color Temperature

-blackbody – a theoretical device ( spectrophotometer ) which emits light which changes wavelength as optical temperature  increases. Expressed in degrees Kelvin.

Ex. Stage lights = 3000 – 3200degrees K

Arc light = about 6000degrees K

A color difference of around 200 degrees K is imperceptible

Dimming a incandescent source decreases color temp. significantly – this is called color shift.

 

LOW VOLTAGE LAMPS

-more intense light, closer to a point source

-smaller filament

requires low voltage power source - NOTE: a modern SCR dimmer does not vary voltage.

As decreases,amperage increases - the load on the circuit must be carefully calculated - use the power formula.

 

 

ARC LIGHT

- concept of 1st arch lamp demonstrated by Sir Humphrey Davies in the early 19th cent.

-essentially electrons jumping a gap and emitting high energy photons.

Carbon Arc is the most common type of arc light used in the theatre.

WARNING the by-product of a carbon arc besides visible light, UV light and heat is Carbon Monoxide.

 

GAS DISCHARGE

-current passing through pressurized gas – excites gas and causes light to be emitted. Common type is Mercury vapor (fluorescent lights) which emits mainly in the UV range and requires a phosphor to emit visible light.

By comparison, gas discharge happens very fast

-another common type is neon light. = glass tube with pressurized neon gas, an electode on each end and a high voltage current. The gas becomes a conductor once the voltage is high enough, completing the circuit. 

Dependent on: 

gas pressure

temperature

kind of gas

these factors also determine the color of the light which is in line spectrums - to a continuos collection of wavelengths.

Gas discharge lamps are rarely used in the theatre as they require a ballast(transformer) warm-up time and can’t be used in a lensed system or easily dimmed.

-does not depend on the production of heat for illumination.

-very efficient power consumption.

 

SHORT ARC

-cross between Arc and Gas Discharge.

-examples mercury vapor and Xenon

- operate by passing a relatively high electrical current through a tiny gap between 2 electrodes – ionizes gas and emits a brilliant light in the form of a line spectrum.

-nothing is consumed so no need to adjust electrodes

-have very heavy quartz glass envelopes, electrodes of tungsten ( high operating temp)

- operate under very high temperature and pressure ( 40 atmospheres )

-very high efficiency = 40 lumen/watt

-color can vary with type of gas

- is a dc device

-is NOT self limiting

-cannot be dimmed on but once on can be dimmed down to 10% - no color shift

emits both IR and UV

- 10 atmospheres at room temp – may explode for no apparent reason increases to -40 atmospheres at operating temp.

LED/ OLED

-electronic light sources

-new emerging technology. 

-low power consumption

-colors generated in very pure wavelengths

allows for excellent color mixing

no heat generated

extreme long life (25, 100,000 hours continous use.

use electronic means for intensity control

like all electronic devices are heat sensitive.

 

ELECTRO_LUMINESCENT

a phenomenon of electrons moving through certain materials.

common example is night lights and watch faces

brightness of the surface appears the same from all angles of view.

 

 

SAFETY

If you don’t know what you are doing, don’t do it. Get help. Ask your supervisor.

ALWAYS unplug an instrument before working on it

lamps are expensive - treat them with care

un-shielded ACR or Short Arc light can blind you.

fingers off quartz lamps

the bulb or envelope of a burning lamp is too hot to handle - even with the best gloves.

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