Optical- Filament Lamps and Radiation Flashcards
What is emittance?
Symbol E. The total energy (summed over all wavelengths) emitted per second per square metre by a blackbody. Measured in joules.
Stefan-Boltzmann equation
E=σT^4 Where σ is 5.67x10-8 T is temperature in K E is emittance This is for a blackbody
What is spectral emittance?
The intensity of radiation at different wavelengths emitted from an ideal blackbody. Equation don’t need to remember on slide 4.
Equation gives energy in 1Å wavelength range
Describe emission curve
Intensity against wavelength. Curves up steep to a peak then goes back to 0 slower. Greater temperature means higher peak and shifted to left.
Wien’s law
λmax=constant/T
Spectral emissivity
Ratio of actual to theoretical emittance (for a real and ideal blackbody) at the same temperature and wavelength.
Emissivity
Symbol e. An integrated value of the spectral emissivity. Used in Kirchoff’s equation E=eσT^4
How does emission curve change for different emissivities and real materials?
Lower emissivity means similar shape to e=1 but lower peak. Real material has irregular shape but can be averaged with constant emissivity.
Colours of hot bodies for different temperatures
Below 450-500C no emission is seen (almost all IR). At 500C emitted visible radiation is red. Higher temperature gives yellow, green, blue and source becomes whiter.
Temperature of sun
Effective surface temperature of 6000C
Visible light wavelength range
400nm-700nm
How does shape of emission curve limit efficiency of incandescent source lamp?
Only a small fraction of the output power is in the visible part of the spectrum. At 3000K a tungsten lamp emits about 8.1% of its energy as visible light.
How to produce a brighter source
Increase the emitting area. Increase its temperature because both the total output and efficiency rise with operating temperature and energy present in visible can rise as steeply as T^10 (not just T^4)
Why are actual efficiencies less than theoretical ones?
Eyes do not respond equally through the visible spectrum
At what temperature is maximum % visible light output?
11000C, hotter than melting point of any known solid
Carbon as a filament
High melting point (3650C) easily oxidises so needs perfect vacuum. Evaporates too quickly
Why is tungsten used as filament?
High melting point (3410C). Low vapour pressure at high temperatures (boil point 5800C, vapour pressure 10-4 torr at 2757C)
What is vapour pressure at boiling point?
1atm. Which is 760torr
How does evaporation work for tungsten?
At high T it slowly evaporates. If filament develops a hot spot: evaporation faster, becomes thinner, resistance increases, temperature rises, evaporation increases, run-away situation and premature failure. Evaporates tungsten condenses on lamp envelope and causes blackening so as bulb ages, optical efficiency falls.
How to reduce evaporation in bulbs
Put gas in envelope to reduce volatilisation because evaporating atoms ‘bounce back’ from gas molecules. Common to use N2 and/or Ar. Heavier inert gas more effective but cost more.
Disadvantages of putting inert gas in envelope
It increases convective heat loss, reducing filament temperatures and lowering efficiencies. Coiled-coil design used to minimise convection.
Describe quartz-halogen lamp
Use vitreous silica (vitreous quartz) as envelope, whose thermal properties allow bulb envelope to operate at higher temperature (good thermal shock resistance (low α) stable at high T). Cl2 is in envelope as gas. W is still filament.
UV emission in standard filament and quartz-halogen lamps
Standard ones emit effectively zero UV and any produced is absorbed by envelope. Quartz-halogen can give significant UV emission because of greater transparency of pure vitreous quartz and higher operating temperatures.
Reactions in quartz-halogen lamps
W+3Cl2 to WCl6
At thin, hot spot, WCl6 decomposes back to W and Cl2
Mechanisms in quartz-halogen lamps
WCl6 has too low boiling point (347C) to condense on envelope. W preferentially redeposits at hot spots so thin, hot spots are self-repairing. Allows higher operating T and less W deposited on envelope so blackening reduced. Leads to higher efficiency than old filament lamps.
