9 - Production of 9 - Production of Light Light
Mar 31, 2015
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