Discharge Lamps 1 Discharge Lamps Chapter 14 part

Discharge Lamps 1 Discharge Lamps Chapter 14 part 2 1020 C

Discharge Lamps 2 Introductory Question n A fluorescent lamp tube is coated with a white powder on its inside surface. If that powder were not there, the lamp would appear A. brighter dimmer about the same overall brightness, but with an unpleasantly bright white line near its center B. C.

Discharge Lamps 3 Observations about Discharge Lamps They often take a few moments to turn on n They come in a variety of colors, including white n They are often whiter than incandescent bulbs n They last longer than incandescent bulbs n They sometimes hum loudly n They flicker before they fail completely n

Discharge Lamps 4 4 Questions about Discharge Lamps Why not stick with incandescent lamps? n How can colored lights mix so we see white? n How can white light be produced without heat? n How do gas discharge lamps produce their light? n

Discharge Lamps 5 Question 1 n Why not stick with incandescent lamps?

Discharge Lamps 6 Shortcomings of Thermal Light Incandescent lamps are reddish and inefficient n Filament temperature is too low, thus too red n The temperature of sunlight is 5800 °C n The temperature of an incandescent lamp is 2500 °C n n An incandescent lamp emits mostly invisible infrared light, n so less than 10% of its thermal power is visible light. n

Discharge Lamps 7 Question 2 n How can colored lights mix so we see white?

Discharge Lamps 8 Seeing in Color n We have three groups of light-sensing cone cells n Primary sensors/colors of light: red, green, and blue When the primaries mix unevenly, we see others colors n When three primaries mix evenly, we see white n

Discharge Lamps 9 Question 3 n How can white light be produced without heat?

Discharge Lamps 10 Fluorescent Lamps (Part 1) n Fluorescent tubes contain low density gas and metal electrodes, n which inject free electric charges into the gas n to form a plasma—a gas of charged particles n so that electric fields cause current to flow in plasma. n

Discharge Lamps 11 Fluorescent Lamps (Part 2) n Collisions in the plasma cause electronic excitation in the gas atoms n and some ionization of the gas atoms, n which help to sustain the plasma. n Excited atoms emit light through fluorescence n Fluorescence is part of quantum physics n

Discharge Lamps 12 Quantum Physics of Atoms n In an atom, n n An electron in a specific orbital has a total energy, n n the negative electrons “orbit” the positive nucleus and form standing waves known as orbitals. Each orbital can have at most two electrons in it that is the sum of its kinetic and potential energies. An atom’s electrons n n are normally in lowest energy orbitals – the ground state but can shift to higher energy orbitals – excited states.

Discharge Lamps 13 Atoms and Light n Electron orbitals are standing waves, which do not change measurably with time n and therefore do not involve charge motion n and do not emit (or absorb) light. n n While an electron is changing orbitals, there is charge motion and acceleration, n so the electron can emit (or absorb) light. n n Such orbital changes are call radiative transitions

Discharge Lamps 14 Light from Atoms n The wave/particle duality applies to light, so light travels as a wave (diffuse rippling fields) n but is emitted or absorbed as a particle (a photon). n n An atom’s orbitals differ by specific energies and these energy differences set the photon energies, n so an atom has a specific spectrum of photons. n

Discharge Lamps 15 Photons, Energy, and Color Photon’s frequency is proportional to its energy Photon energy = Planck constant· frequency n and frequency· wavelength = speed of light. n Each photon emitted by an atom has n a specific energy, n a specific frequency, n a specific wavelength (in vacuum), n and a specific color when we detect it with our eyes. n

Discharge Lamps 16 Atomic Fluorescence n n n Excited atoms lose energy via radiative transitions During a transition, electrons shift to lower orbitals Photon energy is the difference in orbital energies n n n Small energy differences infrared photons Moderate energy differences red photons Big energy differences blue photons Very Big differences ultraviolet photons Each atom typically has a bright “resonance line” Mercury’s resonance line is at 254 nm, in the UV

Discharge Lamps 17 Phosphors A mercury discharge emits mostly UV light n A phosphor can convert UV light to visible n by absorbing a UV photon n and emitting a less-energetic visible photon. n The missing energy usually becomes thermal energy. n Fluorescent lamps use white-emitting phosphors n Specialty lamps use colored light-emitters n n Blue, green, yellow, orange, red, violet, etc.

Discharge Lamps 18 Introductory Question (revisited) n A fluorescent lamp tube is coated with a white powder on its inside surface. If that powder were not there, the lamp would appear A. brighter dimmer about the same overall brightness, but with an unpleasantly bright white line near its center B. C.

Discharge Lamps 19 Fluorescent Lamps (Part 3) Starting a discharge requires electrons in the gas n Heated filaments can provide those electrons n Manual preheat lamps (initial filament heating) n Automatic preheat lamps (initial filament heating) n Rapid start lamps (constant filament heating) n n Rapid start lamps can be dimmed, unlike the others n High voltages can provide electrons n Instant start lamps (high voltage pulse start)

Discharge Lamps 20 Fluorescent Lamps (Part 4) n Gas discharges are unstable Gas is initially insulating n Once discharge is started, gas become a conductor n The more current it carries, the better it conducts n Current tends to skyrocket out of control n n Stabilizing discharge requires ballast Inductor ballast (old, 60 Hz, tend to hum) n Electronic ballast (new, high-frequency, silent) n

Discharge Lamps 21 Question 4 n How do gas discharge lamps produce their light?

Discharge Lamps 22 Low-Pressure Discharge Lamps n Mercury gas has its resonance line in the UV n Low-pressure mercury lamps emit mostly UV light Some gases have resonance lines in the visible n Low-pressure sodium vapor discharge lamps n emit sodium’s yellow-orange resonance light, n so they are highly energy efficient n but extremely monochromatic and hard on the eyes. n

Discharge Lamps 23 Pressure Broadening n High pressures broaden each spectral line Collisions occur during photon emissions, n so frequency and wavelength become smeared out. n Collision energy shifts the photon energy n

Discharge Lamps 24 Radiation Trapping n Radiation trapping occurs at high atom densities Atoms emit resonance radiation very efficiently n Atoms also absorb resonance radiation efficiently n Resonance radiation photons are trapped in the gas n Energy must escape discharge via other transitions n

Discharge Lamps 25 High-Pressure Discharge Lamps At higher pressures, new spectral lines appear n High-pressure sodium vapor discharge lamps n emit a richer spectrum of yellow-orange colors, n are still quite energy efficient, n but are less monochromatic and easier on the eyes. n n High-pressure mercury discharge lamps emit a rich, bluish-white spectrum, n with good energy efficiency. n Adding metal-halides adds red to improve whiteness. n

Discharge Lamps 26 Summary about Discharge Lamps Thermal light sources are energy inefficient n Discharge lamps produce more light, less heat n They carefully assemble their visible spectra n They use atomic fluorescence to create light n Some include phosphors to alter colors n