Optical- Fluorescent/Discharge Lamps

Question 1

Basic arrangement of a fluorescent lamp

Answer 1

Evacuated transparent glass tube. 2 electrodes, one at each end. Small amount of Hg gas at low pressure. Electrical components are starter and ballast. Hot W filament emits electrons.

Question 2

How does fluorescent lamp work?

Answer 2

Electrons accelerated by electric field. Hit Hg atoms and excite their outer electrons into empty orbital. Excited atoms (Hg*) emit their excess energy as UV photons. These photons excite dopant ions in a phosphor coating inside of fluorescent tube. The excited dopant ions re-emit radiation at longer wavelengths (visible) to give various colours that sum to white.

Question 3

Energy conversions for fluorescent lamps

Answer 3

Electrical energy to kinetic energy of electrons. KE of electrons to excitation energy. Emission of UV photon from Hg*. Absorption of UV photon by phosphor. Emission of visible photon by phosphor.

Question 4

Efficiency of steps in fluorescent lamps

Answer 4

Only inefficient step is UV photon to visible photon. UV photons have wavelength near 200nm but visible between 400 and 700 so much lower energy. Difference between in and out energy is heat of phosphor. E=hν=hc/λ. Δhc/λ over initial hc/λ gives fractional energy loss. Efficiency at red end is lower than blue end but much more than incandescent tungsten so heat loss lower extending device life.

Question 5

Graph of cost vs operating time for W filament, fluorescent and LED

Answer 5

W filament steepest straight line through 0. Fluorescent much shallower starting a little above 0. Little kink upwards when have to replace it. LED a little shallower than fluorescent starting further up. Cross when fluorescent kinks.

Question 6

What does the colour of fluorescent lamps depend on?

Answer 6

The characteristics/chemistry of phosphor.

Question 7

Electrical characteristics of fluorescent lamps

Answer 7

Need pre heated W filament to emit e-. Initial gas resistance high so need 500-600V to start discharge. Once current has started, positive ions impacting on cathode help maintain its T and electron emission. Resistance decreases so current limiting device needed. Starter/ballast used to fulfil these functions but increase cost. They also mean lamp not pure resistive load causing extra power loss. Electrode at either end can act as cathode emitting e- so AC current can be used.

Question 8

Safety issues with fluorescent lamps

Answer 8

Hg can make health and disposal issue. Some phosphors may be poisonous. Also broken glass, sharp wires.

Question 9

Describe discharge lamps

Answer 9

Similar to fluorescent lamp but no phosphor. Na-gas discharge lamp uses excited Na ions to give light at wavelength near 589nm. Two operational modes are low and high pressure. Has poor colour rendering of illuminated objects (only suitable outdoors). Cannot use glass envelope as is attacked by Na vapour so need alumina tube (crystalline so hard to seal). Na pressure 10^-5 atm. Good light/£.

Question 10

3 components of phosphors

Answer 10

Active ion
Matrix in which ion is dissolved
Sensitiser ion designed to increase efficiency

Question 11

Emission process in phosphor

Answer 11

Excited atom can transfer some energy to lattice as heat to move to ground state. Or emission of photon with appropriate energy to move to ground state. Which process is dominant depends on host composition. Light emission favoured by lattices with low natural vibrational frequencies (small thermal energy quanta)

Question 12

Considerations for choosing phosphors

Answer 12

Minimise conversion of active ion excitations directly into heat. Phosphors absorb IR (heat) energy only at long wavelengths. If conversion to heat slow, excited ions have longer to emit excess energy as photons

Question 13

Examples of phosphors

Answer 13

ZnS, CaWO4, ZnSiO4, phosphates, halides

Question 14

Examples of active ions

Answer 14

Ag+, Sb3+, Eu2+, Mn2+, Sn2+, Pb2+, Cu+

Question 15

What do active ions need to have?

Answer 15

Excited state electronic energy levels separated from ground state by energies corresponding to visible photons.

Question 16

Why do active ions in phosphors give broad range of wavelengths?

Answer 16

They are in a solid so there is interaction between neighbouring ions. The electric field experienced by active ion changes with time as surrounding atoms vibrate causing the energy to be constantly changing.

Question 17

Why do ions in a solid absorb at one wavelength and emit at a longer one?

Answer 17

Absorption and emission involve atomic vibrations. Coordinating atoms move slowly compared with time for absorption/emission

Question 18

Absorption process for active ion

Answer 18

Before absorption, it is in ground state and surrounding atoms at equilibrium distances. When excited, surrounding atoms no longer at equilibrium so must move to new equilibrium position and effective size of active ion is increased.

Question 19

What is Stokes shift?

Answer 19

Absorption is at smaller wavelengths (higher energies) than fluorescence

Question 20

Energy states for absorption and emission in active ions

Answer 20

Absorption moves it to higher energy level than is ion was in free space. Energy difference between these levels (free or solid) lost as heat due to motion of ion to return to equilibrium position. Emits photon to get nearly to ground state. Difference between this and ground state lost as heat due to movement from non-equilibrium positions. Wavelength emitted of lower energy than absorbed.

Question 21

Roles of sensitiser ions

Answer 21

Allow wavelength shift between absorption and emission.
Role is to absorb, pass on energy to active ion, exciting it to emit. Reduces loss of efficiency due to active ion partially absorbing at the wavelength where it is emitting.