2 Introductory Question A fluorescent lamp tube is coated with a white powder on its inside surface. If that powder were not there, the lamp would appearbrighterdimmerabout the same overall brightness, but with an unpleasantly bright white line near its center
3 Observations about Discharge Lamps They often take a few moments to turn onThey come in a variety of colors, including whiteThey are often whiter than incandescent bulbsThey last longer than incandescent bulbsThey sometimes hum loudlyThey flicker before they fail completely
4 4 Questions about Discharge Lamps Why not stick with incandescent lamps?How can colored lights mix so we see white?How can white light be produced without heat?How do gas discharge lamps produce their light?
5 Question 1Why not stick with incandescent lamps?
6 Shortcomings of Thermal Light Incandescent lamps are reddish and inefficientFilament temperature is too low, thus too redThe temperature of sunlight is 5800 °CThe temperature of an incandescent lamp is 2500 °CAn incandescent lampemits mostly invisible infrared light,so less than 10% of its thermal power is visible light.Demonstration: The Spectrum of an Incandescent Filament
7 Question 2How can colored lights mix so we see white?
8 Seeing in Color We have three groups of light-sensing cone cells Primary sensors/colors of light: red, green, and blueWhen the primaries mix unevenly, we see others colorsWhen three primaries mix evenly, we see white
9 Question 3How can white light be produced without heat?
10 Fluorescent Lamps (Part 1) Fluorescent tubescontain low density gas and metal electrodes,which inject free electric charges into the gasto form a plasma—a gas of charged particlesso that electric fields cause current to flow in plasma.
11 Fluorescent Lamps (Part 2) Collisions in the plasma causeelectronic excitation in the gas atomsand some ionization of the gas atoms,which help to sustain the plasma.Excited atoms emit light through fluorescenceFluorescence is part of quantum physics
12 Quantum Physics of Atoms In an atom,the negative electrons “orbit” the positive nucleusand form standing waves known as orbitals.Each orbital can have at most two electrons in itAn electron in a specific orbital has a total energy,that is the sum of its kinetic and potential energies.An atom’s electronsare normally in lowest energy orbitals – the ground statebut can shift to higher energy orbitals – excited states.
13 Atoms and Light Electron orbitals are standing waves, which do not change measurably with timeand therefore do not involve charge motionand do not emit (or absorb) light.While an electron is changing orbitals,there is charge motion and acceleration,so the electron can emit (or absorb) light.Such orbital changes are call radiative transitions
14 Light from Atoms The wave/particle duality applies to light, so light travels as a wave (diffuse rippling fields)but is emitted or absorbed as a particle (a photon).An atom’s orbitals differ by specific energiesand these energy differences set the photon energies,so an atom has a specific spectrum of photons.Demonstration: The Spectrum of a Gas Discharge
15 Photons, Energy, and Color Photon’s frequency is proportional to its energyPhoton energy = Planck constant· frequencyand frequency· wavelength = speed of light.Each photon emitted by an atom hasa specific energy,a specific frequency,a specific wavelength (in vacuum),and a specific color when we detect it with our eyes.
16 Atomic FluorescenceExcited atoms lose energy via radiative transitionsDuring a transition, electrons shift to lower orbitalsPhoton energy is the difference in orbital energiesSmall energy differences infrared photonsModerate energy differences red photonsBig energy differences blue photonsVery Big differences ultraviolet photonsEach atom typically has a bright “resonance line”Mercury’s resonance line is at 254 nm, in the UV
17 Phosphors A mercury discharge emits mostly UV light A phosphor can convert UV light to visibleby absorbing a UV photonand emitting a less-energetic visible photon.The missing energy usually becomes thermal energy.Fluorescent lamps use white-emitting phosphorsSpecialty lamps use colored light-emittersBlue, green, yellow, orange, red, violet, etc.Demonstration: The White Fluorescence of the Phosphors in a Fluorescent Lamp
18 Introductory Question (revisited) A fluorescent lamp tube is coated with a white powder on its inside surface. If that powder were not there, the lamp would appearbrighterdimmerabout the same overall brightness, but with an unpleasantly bright white line near its center
19 Fluorescent Lamps (Part 3) Starting a discharge requires electrons in the gasHeated filaments can provide those electronsManual preheat lamps (initial filament heating)Automatic preheat lamps (initial filament heating)Rapid start lamps (constant filament heating)Rapid start lamps can be dimmed, unlike the othersHigh voltages can provide electronsInstant start lamps (high voltage pulse start)Demonstration: Different Fluorescent Fixtures
20 Fluorescent Lamps (Part 4) Gas discharges are unstableGas is initially insulatingOnce discharge is started, gas become a conductorThe more current it carries, the better it conductsCurrent tends to skyrocket out of controlStabilizing discharge requires ballastInductor ballast (old, 60 Hz, tend to hum)Electronic ballast (new, high-frequency, silent)Demonstration: Point Out Ballasts
21 Question 4How do gas discharge lamps produce their light?
22 Low-Pressure Discharge Lamps Mercury gas has its resonance line in the UVLow-pressure mercury lamps emit mostly UV lightSome gases have resonance lines in the visibleLow-pressure sodium vapor discharge lampsemit sodium’s yellow-orange resonance light,so they are highly energy efficientbut extremely monochromatic and hard on the eyes.
23 Pressure Broadening High pressures broaden each spectral line Collisions occur during photon emissions,so frequency and wavelength become smeared out.Collision energy shifts the photon energy
24 Radiation Trapping Radiation trapping occurs at high atom densities Atoms emit resonance radiation very efficientlyAtoms also absorb resonance radiation efficientlyResonance radiation photons are trapped in the gasEnergy must escape discharge via other transitions
25 High-Pressure Discharge Lamps At higher pressures, new spectral lines appearHigh-pressure sodium vapor discharge lampsemit a richer spectrum of yellow-orange colors,are still quite energy efficient,but are less monochromatic and easier on the eyes.High-pressure mercury discharge lampsemit a rich, bluish-white spectrum,with good energy efficiency.Adding metal-halides adds red to improve whiteness.Demonstration: A High-Pressure Sodium Vapor LampDemonstration: A High-Pressure Mercury Lamp
26 Summary about Discharge Lamps Thermal light sources are energy inefficientDischarge lamps produce more light, less heatThey carefully assemble their visible spectraThey use atomic fluorescence to create lightSome include phosphors to alter colors