Luminous flux
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Definition: Brightness of a source, taking account the sensitivity of the human eye to
different wavelengths
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Unit: Lumen (lm)
1. Amount of light energy emitted per unit time (power)
2. Response/sensitivity of human eye to wavelengths/colour of light emitted
Efficacy
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Definition: how efficiently a lighting device converts electricity into light
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Unit: lumen/W (lm W-1)
How does matter emit light?
1. When an atom is excited by heat or radiation, it jumps from a lower to a higher energy
level
2. Unstable, electron at higher energy level will return to lower energy level by emitting
EM waves
3. The larger the difference in energy levels, the higher the frequency of EM waves
4. With a suitable wavelength, visible light is emitted
★ Jumping up and down energy levels produces EM waves = light
Incandescent lamp
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Components: glass globe with coiled filament
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Major source of energy loss: invisible infrared radiation, (lost to surroundings by
conduction)
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Why coil? Longer, higher resistance, prevent short circuit
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Filament: the thicker, the higher power; the hotter, the more visible light emitted, the
shorter the lifetime; tungsten as it has high m.p.
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Why does the filament get thinner? Why are there dark spots on the globe? Tungsten
evaporate (and condense on the glass)
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How to reduce evaporation rate? Use argon, an inert gas
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Working principle:
1. Filament is heated to around 2000°C
2. Electrons gain energy and jump to many different energy levels (tungsten has a
high atomic number)
3. When they return to lower energy levels, EM waves of various wavelengths are
emitted
4. Continuous light spectrum is produced
Gas discharge lamp (FTL & CFL)
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Components:
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Starter: to provide large enough voltage to start gas discharge/accelerate e-
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Ballast: stabilize current
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Filled with argon gas (inert) at low pressure: ionize gas, increase chance of
electrons interacting with mercury atoms
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Mercury vapour:
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Coating of phosphor (different phosphors for different colours of lights) on
internal side of tube: give off visible light
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Filament at two ends
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Major source of energy loss: heat dissipated during gas discharge
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Working principle:
1. Large pd is applied to set up an electric field and accelerate electrons from
filament
2. Argon gas is ionized, current is formed (electrons are knocked out, resistance
inside tube is reduced)
3. Mercury atoms are bombarded by electrons, gain energy, excite to higher
energy level
4. Mercury atoms emit UV radiation when returning to lower energy level
5. Phosphor atoms absorb UV, gain energy
6. Phosphor emits visible light when returning to lower energy levels →
fluorescence
LED lamp
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Components:
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p-type semiconductor with +ve charge carriers called holes (absence of electron
in an atom)
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n-type semiconductor with -ve charge carriers electrons
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Positive pole of cell to p-type and vice versa
Major source of energy loss: heat produced by semiconductors
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Working principle:
1. Electric field from p-type to n-type, holes move to n-type, electrons move to
p-type (opposite directions)
2. Electron falls into a hole, jumps down to lower energy level
3. Energy is released as visible light
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Important things to note:
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Step down the voltage (operates at low dc voltage)
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Convert ac to dc with a rectifier (otherwise electrons cannot fall into holes)
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Heating up → lifetime shortens, efficiency decreases
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Frequency of light: depends on difference in energy levels of p-type and n-type
semiconductors