In one line
White LEDs, usually a blue LED chip coated with yellow phosphor, are used in bulbs, tubes, street lights, vehicles and display backlights because they give much more light per watt, last far longer and contain no mercury.
Explained simply
An old filament bulb is basically a heater that happens to glow: most of its electricity becomes heat. An LED makes light directly from electrons recombining, so the same light comes from a fraction of the power. That is why a small LED bulb of about 9 W is commonly sold as a replacement for a 60 W filament bulb.
Technically, lighting efficiency is measured as luminous efficacy (lumens per watt). LED lamps reach many times the efficacy of incandescent lamps and higher than CFLs, with lifetimes of tens of thousands of hours.
How it works
- White light: a blue InGaN chip excites a yellow phosphor coating; blue plus yellow looks white. Alternatively, red, green and blue chips are mixed (RGB), which also allows colour changing.
- Driver: mains AC is converted by a driver circuit to a regulated DC current suitable for the LEDs.
- Heat sink: although efficient, LED chips must be kept cool for long life, so lamps have aluminium bodies or heat sinks.
- Applications: household bulbs and tube lights, street lights, vehicle headlamps and tail-lamps, traffic signals, torches, backlighting for LCD TVs and phones, large outdoor video screens, decorative lighting.
| Feature | LED | Incandescent | CFL |
|---|---|---|---|
| Efficiency | highest | lowest | medium |
| Life | longest | shortest | medium |
| Mercury | none | none | contains mercury |
| Switch-on | instant | instant | slight delay |
| Robustness | solid-state, shock resistant | fragile filament | fragile glass tube |
Key formula
\( \text{Luminous efficacy} = \dfrac{\text{light output (lm)}}{\text{electrical input (W)}} \)
Light output in lumens (lm), input power in watts (W), efficacy in lm/W.
Quick example
Traffic signals in many cities now use LED clusters: they are brighter, use less power, and if one LED fails the rest still glow, unlike a single filament lamp that leaves the signal dark.
Common mistakes
- Saying white LEDs are made of a 'white semiconductor'; white comes from phosphor conversion or RGB mixing.
- Saying LEDs produce no heat at all; they produce less, but the chip still needs cooling.
- Saying LED lamps run directly on AC with no driver circuit.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain applications of LEDs in lighting" or "State advantages and disadvantages of LEDs" (3–5 marks). For full marks explain how white light is produced, list five or six lighting applications, give a comparison with incandescent and CFL lamps, and mention limitations (need for driver and heat sink, higher initial cost).