Illumination and Lamps
Overview
Illumination and Lamps is a core chapter in ITI Electrician Semester 4 (NCVT). It covers fundamental concepts of light, photometric quantities, laws of illumination, and types of electric lamps including incandescent, fluorescent, mercury vapour, sodium vapour, and LED lamps. Understanding luminous flux, illuminance, luminous intensity, and efficiency is essential. This chapter regularly appears in NCVT theory papers with numerical and conceptual questions.
Typed Notes
Fundamental Photometric Quantities
Luminous Flux (Φ): Total light energy emitted per second by a source, unit is Lumen (lm). Luminous Intensity (I): Light emitted per unit solid angle (steradian), unit is Candela (cd). Illuminance (E): Luminous flux falling per unit area of a surface, unit is Lux (lx); 1 Lux = 1 lm/m². Luminance: Brightness of a surface, unit is cd/m². Solid angle (ω) is measured in Steradians; a complete sphere subtends 4π steradians. Relation: Φ = I × ω. These quantities form the base of all illumination calculations in NCVT papers.
Laws of Illumination
Inverse Square Law states that Illuminance (E) = I/d², meaning if distance doubles, illuminance reduces to one-fourth. Lambert's Cosine Law states E = (I × cosθ)/d², where θ is the angle between the incident light ray and the normal to the surface. When light falls perpendicularly (θ = 0°), cosθ = 1, giving maximum illuminance. These two laws are frequently combined in numerical problems. Example: A lamp of 100 cd at 2 m distance gives E = 100/4 = 25 lux on a horizontal surface directly below.
Types of Electric Lamps — Construction and Working
Incandescent Lamp: Tungsten filament heated to ~2700K emits light; filled with argon/nitrogen gas; efficacy 10-15 lm/W; life ~1000 hours. Fluorescent Lamp (TFL): Mercury vapour produces UV rays; phosphor coating converts UV to visible light; efficacy 50-80 lm/W; needs choke (ballast) and starter. Mercury Vapour Lamp (HPMV): High-pressure mercury arc; bluish-white light; used in street/industrial lighting; warm-up time 4-5 minutes. Sodium Vapour Lamp (LPS/HPS): LPS gives monochromatic yellow light, highest efficacy (~180 lm/W); HPS gives golden-white light; used in highways and tunnels.
LED Lamps, CFL and Comparison
LED (Light Emitting Diode) lamp works on electroluminescence — electrons recombine with holes releasing photons. Efficacy: 100-200 lm/W; life: 25,000-50,000 hours; no warm-up time; available in all colour temperatures. CFL (Compact Fluorescent Lamp): Miniaturised fluorescent lamp with inbuilt ballast; efficacy 50-70 lm/W; life ~8,000 hours. Comparison tip for exam: LED > Sodium Vapour > Fluorescent > Mercury Vapour > Incandescent in terms of efficacy. Colour Temperature: Warm white ~2700K, Cool white ~4000K, Daylight ~6500K. CRI of LED is 80-90, incandescent is ~100.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Laws of Illumination
- Inverse square law E = I/d²
- Cosine law
- Units: lumen, lux
Same lamp 2x door — brightness 1/4 (inverse square).
02Incandescent and Halogen
- Tungsten filament 2800 K+
- Only ~12-17 lm/W
- Being phased out
100 W bulb vs 12 W LED — same brightness, 8 guna bachat.
03Fluorescent Tube and CFL
- Tube: choke + starter, 50-60 lm/W
- CFL: compact
- Mercury content — disposal care
Tube light blink kare to pehle starter badlo — 90% time wahi problem.
04LED Lighting
- Light emitting diode
- 80-120 lm/W, 25000+ hrs
- Driver important
LED street light 50 W = 150 W HPSV replacement — municipality bachat.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Luminous Flux (Φ) is measured in Lumens (lm); Luminous Intensity (I) in Candela (cd); Illuminance (E) in Lux (lx); Luminance in Candela/m².
- 02Inverse Square Law: E = I/d² — Illuminance is inversely proportional to the square of the distance from the source.
- 03Lambert's Cosine Law: E = (I × cosθ)/d² — Illuminance depends on the angle of incidence of light on the surface.
- 04Luminous Efficacy = Lumens output / Watts input; LED has highest efficacy (~100-200 lm/W); Incandescent lamp has lowest (~10-15 lm/W).
- 05Colour Rendering Index (CRI) measures how accurately a lamp renders colours compared to natural light; maximum CRI = 100 (sunlight).
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
State the Inverse Square Law of illumination and derive the formula. A lamp has luminous intensity of 200 cd. Find the illuminance on a surface 4 m directly below the lamp.
What is Lambert's Cosine Law? A lamp of 500 cd is mounted 5 m above a point P. Find the illuminance at point Q which is 5 m horizontally away from P (directly below the lamp).
Compare incandescent lamp, fluorescent lamp, and LED lamp on the basis of efficacy, life, and working principle.
Why is a choke (ballast) used in a fluorescent lamp circuit? What is the role of the starter?
What is luminous efficacy? Which lamp has the highest luminous efficacy and why is it not used for indoor general lighting?
Illumination chapter covers photometric units (lumen, candela, lux), Inverse Square Law and Lambert's Cosine Law for numerical problems, and detailed comparison of lamp types. LED lamps offer highest efficacy and longest life. Fluorescent lamps need choke and starter. Sodium vapour LPS has highest efficacy among discharge lamps. Mastering formulas E = I/d² and E = Icosθ/d² is critical for scoring in NCVT Semester 4 theory papers.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.The unit of Illuminance is:
Q2.If the distance between a light source and a surface is doubled, the illuminance becomes:
Q3.Which of the following lamps requires a 'starter' for its operation?
Q4.The working principle of an LED lamp is:
Mark karo — dashboard par progress dikhega.