9 - Production of 9 - Production of LightLight

Types pf Sources:

Continuum (blackbodies, etc.)

Emission line (fluorescent lamps, etc.)

Emission “bands” (LEDs, etc.)

Cavity Radiation, Blackbodies, Planckian Spectra

Blackbody spectrum depends on Temperature - which temperature to use? - Fahrenheit, Celsius, or Kelvin?

0 100

F ice-alcohol mix human body

C water freezes water boils (sea level!)

K motion stops (same scale as C)

Average Speed v =3kTm

AverageEnergyE =32kT

Laws of Blackbody Radiation

Wien's Law

Wilhelm Wien, in 1893

λmax =2.9x10−3

T K( )m =

2900

T K( )μm

Note : λ maxT = 2900 Kμm

and T =2900

λ max μm( )K

For 2 blackbodies :

T1λmax,1 =T2λmax,2 orT1

T2

=λmax,2λmax,1

Stefan-Boltzmann Law - 1878-1884 Josef Stefan and Ludwig Boltzmann

E =σT 4 W /m2

For 2 equal −sizedblackbodies:

E2

E1

=σT1

4

σT24 =T1

4

T24 =

T1

T2

⎛

⎝⎜⎞

⎠⎟

4

Examples:

λmaxT =2900 μmK

If T =1000 K , λmax =29001000

μm=2.9 μm (yellowlight hasλ ≈0.5 μm)

What if T =3000 K ?λmax,2T2 =2900 μmK =λmax,1T1

soλmax,2 =λmax,1T1T2

=13λmax,1 ≈1μm

notethatλmax, 2

λmax,1=T1T2

⎛

⎝⎜

⎞

⎠⎟

Wien's Law

Stefan-Boltzmann Law

E =σT 4Wm−2 whereσ =5.7x10−8Wm−2K −4

If T =T1 =1000 K ,

E =5.7x10−8Wm−2K −4 103K( )4=(5.7x10−8Wm−2K −4 )x(1012K 4 ) =5.7x104Wm−2

What if T =T2 =2000 K ?

E2

E1

=σT2

4

σT14=T2

T1

⎛

⎝⎜⎞

⎠⎟

4

=20001000⎛

⎝⎜⎞

⎠⎟

4

=24 =16

Or E2 =16E1

Color Temperature

For blackbodies :Iλ1

I λ2

= f T( )

For non-BBs, can measure Iλ1 and Iλ2 and ask: What BB has that ratio? Assign the BB’s T to the object as the Tcolor

The SPD of the Sun light:

Before & after passing vertically through the atmosphere

“Air Mass” (AM) - length of path compared to vertical (AM=1)

At larger AM, Tcolor

gets cooler (redder)

SPDs of Standard CIE Sources

SPD of scattered sunlight at different angular distances from Sun

Note: the less planckian the light source, the less Tcolor has any real physical meaning…

Tungsten Incandescent Lamps

Higher Thigher filament evaporation, coating glass, filament destruction

This leads to shorter lifetime….

Lower emissivity at longer λTcolor > T. Ex. T=2814 K bulb has Tcolor~2865 K

Higher Tbulb emits more light where eye is sensitive - more luminous efficiencyHalogen (Tungsten)

LampsHigher T & longer lifetimes if the filament is surrounded by a halogen gas. It binds with the evaporated tungsten & redeposits it on the filament.

Gas Discharge Tubes

•Current of electrons along the inside of a transparent (usually glass) tube collisionally ionize them•Recombination continuous spectrum is produced•Further deexcitation emision lines •While ionized, the gas becomes a good conductor of electric current

Examples: Neon, Argon. Xenon whitish, used in flash & strobe mode

Mercury Arc Lamps

Gaseous mercury produces emission lines over a wide range in wavelength.

A liquid at room temperatures another gas is added and heated by a small “starter” filament, which warms the mercury until it vaporizes. Then the main current flows through the mercury vapor

Takes time to get fully going

Standard mercury has poor color balance for many applications

“Standard” vs. “Improved” mercury lamps

Low Pressure Sodium Lamps (LPS)

“Starter” materials of xenon and mercury, sodium produces two emission lines at 589.1 and 589.6 nm (Fraunhofer “D” lines)

Advantages – high luminous efficacy (lumens/Watt)

Disadvantages – “ugly” to some. Can make it difficult to identify the “true” (i.e. daylight) color of cars on the road

High Pressure Sodium Lamps (HPS)

High pressure broadens spectral lines, allowing a wider range of wavelengths to be emitted. Looks sort of “pink”.

Advantages – better wavelength range than LPS.

Disadvantages – luminous efficacy less than LPS, as some of the light is emitted at wavelengths longer than what the human eye can see. Improved with IR-reflection coating (Indium metal film).

Fluorescent Lamps

•Gas discharge tubes with a coating of a “phosphor” material on the inside surface

•Phosphor converts narrow bands into broader ones

•By choosing the right phosphor, different SPDs can be produced

fluorescence

Example: new fluorescent bulbs

(specifically my desk lamp & end table lamp)

Slit to define the beam

Spectrum of the bulb

Image of lamp - overlapping images made with each wavelength band

Light Emitting Diodes (LEDs)

•Semiconductor diodes emit light when a current passes through them. •Consume little power, can be quite bright, and so have high luminous efficacy

Image of my DSL box LEDs

Viewed through diffraction grating

Closeups of red & green LEDs

Lasers-LightAmplification by Stimulated Emission of Radiation

Spontaneous Absorption

Spontaneous Emission

Stimulated Emission(first predicted by

Einstein)

Lasing in action

Examples:

Ruby

YAG (Yttrium Aluminum Garnet)

Gas (HeNe, N2, CO2, etc.)

Liquid Dye

LEDs!

OTHER LIGHT-EMITTING PROCESSES

Phosphorescence (“glow in the dark”) - slow de-excitation

Chemiluminescence - chemical reaction results in excited state

Bioluminescence - generally chemiluminescence in living system

Triboluminescence - pressure/breakage induced excitation

Aurorae

•Energetic electrons in the Earth’s magnetosphere collide with O2 and N2

•Broken apart and the resultant atoms left in an excited state •Deexcitation will produce emission lines. •Aurora borealis (northern lights)•Aurora australis (southern lights)•Electrons trapped in magnetic field hit mostly near magnetic poles