Optical- Filament Lamps and Radiation

Question 1

What is emittance?

Answer 1

Symbol E. The total energy (summed over all wavelengths) emitted per second per square metre by a blackbody. Measured in joules.

Question 2

Stefan-Boltzmann equation

Answer 2

E=σT^4
Where σ is 5.67x10-8
T is temperature in K
E is emittance
This is for a blackbody

Question 3

What is spectral emittance?

Answer 3

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

Question 4

Describe emission curve

Answer 4

Intensity against wavelength. Curves up steep to a peak then goes back to 0 slower. Greater temperature means higher peak and shifted to left.

Question 5

Wien’s law

Answer 5

λmax=constant/T

Question 6

Spectral emissivity

Answer 6

Ratio of actual to theoretical emittance (for a real and ideal blackbody) at the same temperature and wavelength.

Question 7

Emissivity

Answer 7

Symbol e. An integrated value of the spectral emissivity. Used in Kirchoff’s equation E=eσT^4

Question 8

How does emission curve change for different emissivities and real materials?

Answer 8

Lower emissivity means similar shape to e=1 but lower peak. Real material has irregular shape but can be averaged with constant emissivity.

Question 9

Colours of hot bodies for different temperatures

Answer 9

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.

Question 10

Temperature of sun

Answer 10

Effective surface temperature of 6000C

Question 11

Visible light wavelength range

Answer 11

400nm-700nm

Question 12

How does shape of emission curve limit efficiency of incandescent source lamp?

Answer 12

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.

Question 13

How to produce a brighter source

Answer 13

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)

Question 14

Why are actual efficiencies less than theoretical ones?

Answer 14

Eyes do not respond equally through the visible spectrum

Question 15

At what temperature is maximum % visible light output?

Answer 15

11000C, hotter than melting point of any known solid

Question 16

Carbon as a filament

Answer 16

High melting point (3650C) easily oxidises so needs perfect vacuum. Evaporates too quickly

Question 17

Why is tungsten used as filament?

Answer 17

High melting point (3410C). Low vapour pressure at high temperatures (boil point 5800C, vapour pressure 10-4 torr at 2757C)

Question 18

What is vapour pressure at boiling point?

Answer 18

1atm. Which is 760torr

Question 19

How does evaporation work for tungsten?

Answer 19

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.

Question 20

How to reduce evaporation in bulbs

Answer 20

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.

Question 21

Disadvantages of putting inert gas in envelope

Answer 21

It increases convective heat loss, reducing filament temperatures and lowering efficiencies. Coiled-coil design used to minimise convection.

Question 22

Describe quartz-halogen lamp

Answer 22

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.

Question 23

UV emission in standard filament and quartz-halogen lamps

Answer 23

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.

Question 24

Reactions in quartz-halogen lamps

Answer 24

W+3Cl2 to WCl6

At thin, hot spot, WCl6 decomposes back to W and Cl2

Question 25

Mechanisms in quartz-halogen lamps

Answer 25

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.