9 - Production of Light

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? Fice-alcohol mixhuman body Cwater freezeswater boils (sea level!) Kmotion stops(same scale as C)

Laws of Blackbody Radiation Wien's Law Wilhelm Wien, in 1893

Stefan-Boltzmann Law Josef Stefan and Ludwig Boltzmann

Examples: Wien's Law

Stefan-Boltzmann Law

Color Temperature For non-BBs, can measure I 1 and I 2 and ask: What BB has that ratio? Assign the BBs T to the object as the T color

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, T color 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 T color has any real physical meaning…

Tungsten Incandescent Lamps Higher T higher filament evaporation, coating glass, filament destruction This leads to shorter lifetime…. Lower emissivity at longer T color > T. Ex. T=2814 K bulb has T color ~2865 K Higher T bulb emits more light where eye is sensitive - more luminous efficiency Halogen (Tungsten) Lamps Higher 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 and 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- Light Amplification 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, N 2, CO 2, 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 O 2 and N 2 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