A World of Color Session 2 OLLI at Illinois Spring 2018 D. H. Tracy
Course Outline 1. Overview, History and Spectra 2. Nature and Sources of Light 3. Eyes and Color Vision 4. Origins of Colors of Things 5. Propagation and Modification of Light 6. Visual Perception: Neurons and the Brain 7. Color Recording and Reproduction, Media 8. Color in Arts and Culture, Emotive Aspects 2/1/2018 World of Color 2 2
Session 2 Outline Photons How Light is Produced Light Sources Hot bodies Electric Discharges Electroluminescence Photon Impact (Fluorescence) Particle Impact Chemical 2/1/2018 World of Color 2 3
EM Waves Wavelength λ Institute of Sound and Vibration Research, U. of Southampton, UK 2/1/2018 World of Color 2 4
The Broad ElectroMagnetic Spectrum 2/1/2018 World of Color 2 5
Hot Things Glow 2/1/2018 World of Color 2 6
The Solar Spectrum in Detail O 3 Yellow areas lost on the way down 2/1/2018 World of Color 2 7
Triumph of 19 th Century Physics Lord Kelvin, 1900 There is nothing new to be discovered in physics now. All that remains is more and more precise measurement. 2/1/2018 World of Color 2 8
Ultraviolet Catastrophe Solar Spectrum 2/1/2018 World of Color 2 9
Ultraviolet Catastrophe Huge Discrepancy between Measurements and 19 th Century Theory 2/1/2018 World of Color 2 10
Solution was Quantum Theory Quantum Theory Max Planck Albert Einstein 1900 1905 2/1/2018 World of Color 2 11
The answer was. Duality: Part Wave Part Particle 2/1/2018 World of Color 2 12
Wave Picture is Fine for Optical Instruments But For Creating Light we need the Photon Picture Likewise Detection of light needs Photon Model 2/1/2018 World of Color 2 13
Color Perception Train Source Brain Optic Nerve Object Medium Eye 2/1/2018 World of Color 2 14
Color Perception Train: Sources Source Brain Optic Nerve Object Medium Eye 2/1/2018 World of Color 2 15
Logically, if Light is an EM Wave We should wave some electric charges up and down to generate light at the right (high) frequency of course Wavelength λ Institute of Sound and Vibration Research, U. of Southampton, UK 2/1/2018 World of Color 2 16
That s how radio transmitters work AM Radio Station 650 khz Cell Phone Radio Transmitter 800 2100 MHz Antenna Radio Frequency Generator Chips 2/1/2018 World of Color 2 17
But these wavelengths are still far too big We need 500,000 x shorter λ Electronics are not remotely capable of this frequency* * with one exception to be mentioned We need a different way to generate visible light one by one as photons 2/1/2018 World of Color 2 18
*Exception: Synchrotron Radiation Electrons whirling around at 99.9% of the speed of light sling off photons of all frequencies up to x-rays RIKEN Storage Ring, Japan 2/1/2018 World of Color 2 19
We ll Generate Photons via Electronic Transitions in Matter Atoms Molecules Solids Plasmas 2/1/2018 World of Color 2 20
Example: Hydrogen, the simplest case Photon Emission Balmer Lines Photon Absorption 2/1/2018 World of Color 2 21
How do we get electrons excited? Heat (thermal) Electric discharge Photon Impact (Fluorescence, Phosphorescence) Particle Impact Electroluminescence LEDs, Lasers, etc. Chemical Reactions 2/1/2018 World of Color 2 22
Thermal Excitation (Heat) 2/1/2018 World of Color 2 23
Thermally Excited Radiation 2/1/2018 World of Color 2 24
Thermally Excited Radiation Key concept is the Black Body An ideal Black Body: Absorbs all wavelengths of light perfectly It also emits all wavelengths according to the Plank law Temperature determines the intensity and spectrum shape Problem: How do you actually make a BB? 2/1/2018 World of Color 2 25
Pie in oven eventually reaches thermal equilibrium with oven 2/1/2018 World of Color 2 26
Cavity Black Body Glass Blowing Glory Hole Uniform Temperature T Radiation inside cavity reaches Blackbody Equilibrium Accurate Black Body Spectrum even if cavity walls are not black! Peephole 2/1/2018 World of Color 2 27
Plank Blackbody Spectra 8500 F 500 1000 1500 2000 2500 (nm) 2/1/2018 World of Color 2 28
Emissivity: Blackness If a hot emitter is not black, reduced BlackBody emission is obtained Emissivity Factor ε measures the blackness ε = 100% means perfectly black ε = 0 means perfectly reflecting no emission Examples: Tungsten metal: ε 35-45% Silver metal: ε 3% Fire Brick: ε 75% Lamp Black ε 95% 2/1/2018 World of Color 2 29
Some Blackbodies (nearly) Sun 5,800 K (9,940 F) ~1500 K (2200F) cooler Black Soot Particle ~1500 K (2200F) 2/1/2018 World of Color 2 30
Some Blackbodies (nearly) Sun 5,800 K (9,940 F) ~1500 K (2200F) cooler Black Soot Particle ~1500 K (2200F) The blue flame is NOT Blackbody Radiation. It is Molecular Band emission, thermally excited 2/1/2018 World of Color 2 31
Some Blackbodies (nearly) Sun 5,800 K (9,940 F) ~1500 K Gas Burner (2200F) Similar cooler Black Soot Particle ~1500 K (2200F) The blue flame is NOT Blackbody Radiation. It is Molecular Band emission, thermally excited 2/1/2018 World of Color 2 32
Some Blackbodies (nearly) Sun 5,800 K (9,940 F) ~1500 K Foil Slit (2200F) View Band Spectrum at home cooler Black Soot Particle ~1500 K (2200F) The blue flame is NOT Blackbody Radiation. It is Molecular Band emission, thermally excited 2/1/2018 World of Color 2 33
Incandescent Lamps Tungsten Filaments ε 35-45% Efficiency ~ 2-5% 2/1/2018 World of Color 2 34
How do we get electrons excited? Heat (thermal) Electric discharge Photon Impact (Fluorescence, Phosphorescence) Particle Impact Electroluminescence LEDs, Lasers, etc. Chemical Reactions 2/1/2018 World of Color 2 35
Lightning T ~ 30,000 K 2/1/2018 World of Color 2 36
Xenon Arc Electric Discharge Lamps T 6-7K Xenon Arc Plasma Graybody T eff 6000-7000K Hotter than the Sun 15 kw IMAX Projector Lamp HID Headlamps 2/1/2018 World of Color 2 37
High Pressure Sodium Note dip in spectrum at 589 nm 2/1/2018 World of Color 2 38
High Pressure Sodium Coolish Sodium atoms near the glass envelope absorb the yellow 589 nm line. 589 nm 2/1/2018 World of Color 2 39
Low Pressure Electric Discharge: Mercury 405 365 436 546 577-579 254 546 nm 2/1/2018 World of Color 2 40
Low Pressure Electric Discharge: Mercury Compact Fluorescent (CFL) Black Lights 405 365 436 546 Germicidal Lamps 254 577-579 DNA 546 nm 2/1/2018 World of Color 2 41
Low Pressure Electric Discharge: Neon 540 640 678 585 Rare Gas Line Emission 2/1/2018 World of Color 2 42
Blackbody Sources Let s take a look Gas Discharge Lamp 2/1/2018 World of Color 2 43
How do we get electrons excited? Heat (thermal) Electric discharge Electroluminescence LEDs, Lasers, etc. Photon Impact (Fluorescence, Phosphorescence) Gentle excitation by electric currents: Particle Impact Chemical Reactions Little Heat 2/1/2018 World of Color 2 46
Electroluminescent Panels 2/1/2018 World of Color 2 47
Light Emitting Diodes First visible red LEDs 1961 (GE) First blue LEDs 1994 (Nichia) Light emitted at PN junction in certain semiconductor crystals Opposite of a Solar Cell Band Gap determines color 2/1/2018 World of Color 2 48
LED Materials vs. color Typical LED Peaks ~ 20 nm 400 500 600 700 800 Wavelength (nm) 3 LED Dies on a board 2/1/2018 World of Color 2 49 from Wikipedia Light-emitting diode
Made as Wafers LED Features High Efficiency Long Life if not Overheated Typically 25-50% (or more) Electricity to Photons Christopher Jobson 2012 2/1/2018 World of Color 2 50
Organic LEDs (OLEDs) Can be printed via dot-matrix techniques to make very small LEDs individual pixels in display 2/1/2018 World of Color 2 51
Lasers Strong Excitation Light Emitting Medium (Gas Discharge, LED, etc.) 2/1/2018 World of Color 2 52
Lasers Strong Excitation 2/1/2018 World of Color 2 53
But Photons are Bosons They come in bunches Satyendra Bose Calcutta (1894-1974) 2/1/2018 World of Color 2 54
But Photons are Bosons They stick with the herd Satyendra Bose Calcutta (1894-1974) 2/1/2018 World of Color 2 55
But Photons are Bosons Satyendra Bose Calcutta (1894-1974) They follow the leader 2/1/2018 World of Color 2 56
Lasers Strong Excitation When this is a semiconductor diode, we basically have a Glorified LED (Diode Laser) All the Photons emerge as clones in lockstep Same direction Same wavelength Same polarization Same phase 2/1/2018 World of Color 2 57
Inside a Diode Laser Wikipedia Laser diode 2/1/2018 World of Color 2 58
Key Features of Laser Illumination Monochromatic Single Wavelength Very Bright Coherent Wave nature of light very apparent Speckle 2/1/2018 World of Color 2 59
Coherence isn t only a Laser problem LED Lamps can be somewhat coherent 2/1/2018 World of Color 2 60
Let s take a look Colored LEDs Diode Lasers 2/1/2018 World of Color 2 61
How do we get electrons excited? Heat (thermal) Electric discharge Electroluminescence LEDs, Lasers, etc. Photon Impact (Fluorescence, Phosphorescence) Particle Impact Chemical Reactions 2/1/2018 World of Color 2 63
Photon Impact: Simple Idea 1. Energetic Photon Excites target Dye molecule or Phosphor Solid 2. Target Re-Emits less Energetic Photon (redder) Re-emission can be immediate (~10 nano-seconds) or After a delay ( say > 1 milli-second up to days) Fluorescence or Scintillation Phosphorescence 2/1/2018 World of Color 2 64
Examples of Fluorescence Black Light (ultraviolet) Fluorescent Dye Blue and UV Sunlight Excites Whitener Dyes in clothing Result: Whiter than White 2/1/2018 World of Color 2 65
Examples of Fluorescence Black Light (ultraviolet) Fluorescent Dye Whitener (Blankophore BCH) Blue and UV Sunlight Excites Whitener Dyes in clothing Result: Whiter than White 2/1/2018 World of Color 2 66
Phosphorescence Glow-in-the-dark toys Charge up solid phosphor using bright light Decays slowly, emitting photons Decay time of minutes or hours Because very slow, not very bright Typical phosphor: Zinc Sulfide with Copper doping as activator (5 ppm) 2/1/2018 World of Color 2 67
Examples of Fluorescence using UV LED Flashlight Fluorescence 2/1/2018 World of Color 2 68
Fluorescent Lamps Scintillation in Lighting UV Photons from Mercury Discharge excite Phosphor coating on inside of glass tube White LED lamps Phosphor coating over powerful blue LED converts part of light to green and red. 2/1/2018 World of Color 2 69
Compact Fluorescent Lamp Spectra 2/1/2018 World of Color 2 70
Cerium: YAG Phosphor Typical White LED Lamp Blue LED Phosphor 400 700 Blue LED Die 2/1/2018 World of Color 2 71
Great Flexibility in Phosphors for White LED (Animation) Fluorescence from Phosphor Mix excited by deep blue LED Blue LED yujintl.com 2/1/2018 World of Color 2 72
Innards of a White Light LED Bulb Power Supply (~ 2v per LED) 2/1/2018 World of Color 2 73
White LEDs Can Also Use RGB Mixing (3 separate colored LEDs) Advantage: Color balance can be adjusted 2/1/2018 World of Color 2 74
Fluorescent Lamps Let s take a look White LEDs 2/1/2018 World of Color 2 75
Comparing White Lamp Spectra HoveyElectric.com 2/1/2018 World of Color 2 77
How do we get electrons excited? Heat (thermal) Electric discharge Electroluminescence LEDs, Lasers, etc. Photon Impact (Fluorescence, Phosphorescence) Particle Impact Chemical Reactions 2/1/2018 World of Color 2 78
Light from Electron Impact Cathode Ray Tube 2/1/2018 World of Color 2 79
More Light from Electron Impact: 100 Miles Up N+ O N 2 2/1/2018 World of Color 2 80 (nm)
How do we get electrons excited? Heat (thermal) Electric discharge Electroluminescence LEDs, Lasers, etc. Photon Impact (Fluorescence, Phosphorescence) Particle Impact Chemical Reactions 2/1/2018 World of Color 2 81
Bioluminescence Biomolecule Luciferin is oxidized with the help of the enzyme Luciferase to generate light. L + O 2 Luciferase > oxy-l + Photon + junk Firefly Luciferin Protein Luciferase 2/1/2018 World of Color 2 82
Bioluminescence Biomolecule Luciferin is oxidized with the help of the enzyme Luciferase to generate light. L + O 2 Luciferase > oxy-l + Photon + junk Firefly Luciferin Protein Luciferase 2/1/2018 World of Color 2 83
Chemiluminescence (Glow Sticks etc.) Mix two chemicals: Luminol or similar in tube break glass vial of hydrogen peroxide Luminol + H 2 O 2 3-APA* 3- APA* 3-APA + Photon + Luminol H 2 O 2 2/1/2018 World of Color 2 84
Session 2 Outline Photons How Light is Produced Light Sources Hot bodies Electric Discharges Electroluminescence Photon Impact (Fluorescence) Particle Impact Chemical 2/1/2018 World of Color 2 85
Extra Content: Fraunhofer Spectra 2/1/2018 World of Color 2 86
The Solar Spectrum in Detail O 3 Yellow areas lost on the way down 2/1/2018 World of Color 2 87
Fraunhofer Lines Joseph von Fraunhofer 1787-1826, Bavarian Invented Spectroscope - 1814 Discovered Hundreds of Absorption Lines in Solar Spectrum 2/1/2018 World of Color 2 88
How Fraunhofer Lines Form BlackBody ~ 5500K to Earth Relatively Cool Atoms in Atmosphere of Sun Absorb Certain Wavelengths 2/1/2018 World of Color 2 89
The Sun s Spectrum In Detail Echelle Spectrogram, very high resolution 2/1/2018 World of Color 2 90
Astronomy 101 1. Determine elements present in stars 2. Measure velocities of stars or galaxies via Doppler Shift of spectra Synthetic Spectrum from a putative Galaxy far, far away 2/1/2018 World of Color 2 91