Illumination system is one of the most important topics in electrical engineering and is widely asked in SSC JE, RRB JE, GATE, AE/JE, and diploma engineering exams. It deals with the production, measurement, control, and utilization of light for residential, commercial, and industrial applications.
In this article, we will discuss important terms related to illumination, laws of illumination, lamp efficiency, glare, utilization factor, modern lighting systems like LED and CFL, and lighting design concepts in detail.
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What is Illumination?
Illumination is defined as the effect produced when light falls on a surface. It is measured as the luminous flux falling per unit area.
Mathematically,
E = F / A
- E = Illumination (lux)
- F = Luminous Flux (lumens)
- A = Area in square meters
The SI unit of illumination is lux.
Important Terms in Illumination Engineering
1. Light
Light is the radiation energy from a hot body that produces visual sensation in the human eye.
2. Luminous Flux
Luminous flux is the total quantity of light energy emitted per second from a luminous body.
Unit: Lumen
3. Luminous Intensity
Luminous intensity is the luminous flux emitted per unit solid angle in a particular direction.
I = F / ω
- I = Luminous intensity
- F = Luminous flux
- ω = Solid angle
Unit: Candela (cd)
4. Lumen
Lumen is the unit of luminous flux. One lumen is the flux emitted in a unit solid angle by a source having one candela intensity.
Lumens = Candle Power × Solid Angle
5. Candle Power
Candle power is the light radiating capacity of a source in a given direction.
C.P = Lumens / ω
6. Lux
Lux is the unit of illumination and is equal to one lumen per square meter.
7. Foot Candle
Foot candle is another unit of illumination.
1 Foot Candle = 10.76 Lux
8. Brightness
Brightness is defined as luminous intensity per unit projected area of a surface.
Unit: Candela/m²
9. Glare
Glare is the excessive brightness within the field of vision that causes discomfort and reduces visibility.
10. Lamp Efficiency
Lamp efficiency is the ratio of luminous flux to power input.
Lamp Efficiency = Lumens / Watt
Unit: Lumens/Watt
11. Specific Consumption
Specific consumption is the ratio of power input to average candle power.
Unit: Watt/Candela
Space to Height Ratio
Space to height ratio is the ratio of distance between adjacent lamps to the mounting height of lamps above the working plane.
It helps in maintaining uniform illumination.
Utilization Factor
Utilization factor is the ratio of total lumens reaching the working plane to the total lumens emitted by the lamp.
Utilization Factor = Lumens reaching working plane / Total lamp lumens
Maintenance Factor
Due to dust and dirt accumulation, illumination decreases over time. Maintenance factor is the ratio of illumination under normal conditions to illumination under clean conditions.
Depreciation Factor
Depreciation factor is the reciprocal of maintenance factor.
Its value is always greater than unity.
Waste Light Factor
Some amount of light is wasted due to overlapping and edge losses. This is considered using waste light factor.
- For rectangular areas = 1.2
- For irregular areas = 1.5
Absorption Factor
In smoky or dusty environments, some light gets absorbed by the atmosphere. The ratio of available lumens after absorption to emitted lumens is called absorption factor.
Beam Factor
Beam factor is the ratio of lumens in the beam of a projector to the total lumens emitted by the lamp.
Reflection Factor
Reflection factor is the ratio of reflected light to incident light.
Its value is always less than 1.
Plane Angle
Plane angle is the angle subtended at a point in a plane by two converging lines.
θ = Arc / Radius
Solid Angle
Solid angle is the angle subtended at a point in space by an area.
ω = Area / Radius²
The unit of solid angle is Steradian.
Laws of Illumination
There are two important laws of illumination:
- Inverse Square Law
- Lambert’s Cosine Law
1. Inverse Square Law of Illumination
This law states that illumination is inversely proportional to the square of the distance between the source and the surface.
E ∝ 1/d²
Or, E = I / d²
- E = Illumination
- I = Luminous intensity
- d = Distance from source
Applications of Inverse Square Law
- Street lighting
- Flood lighting
- Interior lighting calculations
- Industrial lighting design
2. Lambert’s Cosine Law
Lambert’s cosine law states that illumination on a surface is directly proportional to the cosine of the angle between the normal and the direction of light.
E = (I cosθ) / d²
This law is applicable when the surface is inclined to the source of light.
Types of Lamps Used in the Illumination System
1. Incandescent Lamp
Incandescent lamp works on the heating effect of electric current. A tungsten filament produces light when heated.
Advantages
- Simple construction
- Low initial cost
- Easy operation
Disadvantages
- Low efficiency
- High power consumption
- Short life
2. CFL Lamp
CFL stands for Compact Fluorescent Lamp. It consumes less power compared to incandescent lamps.
Advantages
- Better efficiency
- Longer life
- Lower heat generation
Disadvantages
- Contains mercury
- Slow starting
3. LED Lamp
LED stands for Light Emitting Diode. It is the most energy-efficient modern lighting source.
Advantages
- Very high efficiency
- Long lifespan
- Low maintenance
- Eco-friendly
- Low power consumption
Disadvantages
- Higher initial cost
Energy Saving in Illumination Systems
Energy can be saved in lighting systems by using efficient lamps and proper lighting design.
Methods of Energy Saving
- Use LED lamps instead of incandescent lamps
- Use automatic lighting control systems
- Reduce glare and wastage
- Use proper spacing of lamps
- Clean lamps and reflectors regularly
- Use high utilization factor fixtures
Design of Lighting Scheme
The design of a lighting scheme depends on:
- Area of room
- Required illumination level
- Type of work
- Mounting height
- Utilization factor
- Maintenance factor
Flood Lighting
Flood lighting is used for illuminating large outdoor areas such as stadiums, monuments, buildings, airports, and playgrounds.
Advantages of Flood Lighting
- Uniform illumination
- Improved visibility
- Better security
- Enhanced appearance
Numerical on Lamp Efficiency
A 200 V lamp takes 1.2 A current and produces 2860 lumens. Find the lamp efficiency.
Given:
- Voltage = 200 V
- Current = 1.2 A
- Luminous Flux = 2860 lumens
Power Input = V × I
= 200 × 1.2 = 240 W
Lamp Efficiency = Lumens / Watt
= 2860 / 240
= 11.9 lumens/watt
Applications of Illumination Engineering
- Residential lighting
- Commercial buildings
- Industrial plants
- Street lighting
- Decorative lighting
- Stadium lighting
- Hospital lighting
- Educational institutions
Conclusion
Illumination engineering plays a vital role in modern electrical systems. Understanding concepts like luminous flux, candle power, illumination laws, glare, utilization factor, and lighting design helps engineers create efficient and energy-saving lighting systems.
Modern lighting technologies like LED lamps have significantly improved energy efficiency and reduced electricity consumption. Proper illumination design ensures comfort, safety, productivity, and better visibility in residential, commercial, and industrial applications.
FAQs on Illumination System
What is illumination?
Illumination is the luminous flux falling on a surface per unit area.
What is the SI unit of illumination?
The SI unit of illumination is lux.
What is luminous flux?
Luminous flux is the total quantity of light emitted per second from a source.
What is lamp efficiency?
Lamp efficiency is the ratio of luminous flux to power input in lumens per watt.
What is glare?
Glare is excessive brightness causing discomfort and reduced visibility.
State inverse square law of illumination.
It states that illumination is inversely proportional to the square of the distance from the source.
What is Lambert’s cosine law?
It states that illumination is directly proportional to the cosine of the angle between the normal and direction of light.
Which lamp is most energy efficient?
LED lamp is the most energy-efficient lighting source.