Important Terms in Illumination Engineering | Light, Lumen, Lux, Candle Power and Glare
Important Terms in Illumination Engineering Explained in Simple Words
Illumination engineering is an important topic in electrical engineering because it deals with the proper use of light for homes, offices, streets, industries, workshops, schools, hospitals and public places. A good lighting system is not only about brightness. It should also provide comfort, safety, energy saving, proper visibility and less glare.
Before designing any lighting scheme, beginners must understand some basic terms such as luminous flux, lumen, luminous intensity, candle power, lux, brightness, glare, utilization factor, maintenance factor and depreciation factor. These terms are used to calculate the number of lamps, light output, working plane illumination and overall performance of a lighting installation.
What is Illumination Engineering?
Illumination engineering is the study and design of lighting systems. Its main aim is to provide proper light at the required place with minimum wastage of electrical energy. It includes selection of lamps, reflectors, mounting height, spacing, lighting level, glare control and maintenance of the lighting system.
Why Are Illumination Terms Important?
These terms help us understand how much light is produced by a lamp, how much light reaches the working surface, how bright a surface appears to the human eye and how much light is lost due to dust, reflection, absorption or poor design.
- They help in designing indoor and outdoor lighting.
- They are useful for electrical engineering exams and interviews.
- They help in selecting efficient lamps and luminaires.
- They improve comfort and reduce eye strain.
- They support energy-efficient lighting design.
1. Light
Light is a form of radiant energy that produces the sensation of vision in the human eye. In simple words, light is the visible part of electromagnetic radiation that allows us to see objects.
Light may be produced by natural sources such as the sun or artificial sources such as LED lamps, fluorescent lamps and incandescent lamps.
2. Luminous Flux
Luminous flux is the total quantity of visible light emitted by a light source per second. It tells us how much useful light is produced by a lamp.
It is denoted by F or Φ and measured in lumens (lm).
3. Lumen
Lumen is the unit of luminous flux. It represents the amount of visible light emitted by a source. When buying modern LED bulbs, the lumen rating is more useful than watt rating because lumens directly tell us about light output.
For example, an LED lamp with higher lumen output gives more light than a lamp with lower lumen output.
4. Luminous Intensity
Luminous intensity is the luminous flux emitted by a source per unit solid angle in a particular direction. It tells how strong the light is in a given direction.
Where I is luminous intensity, F is luminous flux and ω is solid angle in steradian. The unit of luminous intensity is candela (cd).
5. Candle Power
Candle power is the light-radiating capacity of a source in a particular direction. It is an older term related to luminous intensity. It indicates how much light is emitted per unit solid angle.
6. Illumination
Illumination is the amount of luminous flux falling on a surface per unit area. In simple language, it tells us how much light is received by a table, floor, road or working plane.
Where E is illumination, F is luminous flux in lumens and A is area in square metres. The unit of illumination is lux.
7. Lux or Metre-Candle
Lux is the SI unit of illumination. One lux is equal to one lumen per square metre.
For example, a classroom, office, workshop and street all require different lux levels depending on the type of work performed there.
8. Foot-Candle
Foot-candle is another unit of illumination. It is equal to one lumen per square foot.
This unit is commonly found in older lighting design books and some practical lighting standards.
9. Candle
Candle is an older unit of luminous intensity. In modern engineering, the SI unit candela is used. It represents the intensity of light emitted in a particular direction.
10. Mean Horizontal Candle Power (MHCP)
Mean Horizontal Candle Power is the average candle power of a light source in all directions in the horizontal plane passing through the source.
It is useful for comparing lamps whose light distribution changes with direction.
11. Mean Spherical Candle Power (MSCP)
Mean Spherical Candle Power is the average candle power of a light source in all directions and in all planes around the source.
It gives a more complete idea of the overall light distribution from the source.
12. Mean Hemispherical Candle Power (MHSCP)
Mean Hemispherical Candle Power is the average candle power in all directions above or below the horizontal plane passing through the light source.
13. Reduction Factor
Reduction factor is the ratio of mean spherical candle power to mean horizontal candle power.
It helps in understanding how uniformly a light source distributes light in space.
14. Lamp Efficiency
Lamp efficiency is the ratio of luminous flux output to electrical power input. It is expressed in lumens per watt.
A higher value of lumens per watt means the lamp is more energy efficient. This is why LED lamps are preferred in modern lighting systems.
15. Specific Consumption
Specific consumption is the ratio of power input to average candle power. It is expressed in watt per candela.
Lower specific consumption indicates better lighting performance.
16. Brightness or Luminance
Brightness, also called luminance, is the luminous intensity per unit projected area of a source or reflecting surface. It tells how bright a surface appears to the eye.
The unit of luminance is candela per square metre (cd/m²), also called nit.
17. Glare
Glare is excessive brightness within the field of vision that causes discomfort, eye strain, annoyance or difficulty in seeing clearly. Glare may occur due to direct exposure to bright lamps, shiny surfaces or poor lighting arrangement.
Types of Glare
- Direct glare: Caused by looking directly at a bright light source.
- Reflected glare: Caused by reflection from polished surfaces, screens or glass.
- Discomfort glare: Causes irritation but may not fully block vision.
- Disability glare: Reduces visibility and makes work difficult.
18. Space Height Ratio
Space height ratio is the ratio of distance between adjacent lamps to the mounting height of the lamps above the working plane.
This factor helps in deciding the proper spacing of lamps to get uniform illumination.
19. Utilization Factor or Coefficient of Utilization
Utilization factor is the ratio of total lumens reaching the working plane to the total lumens emitted by the lamps.
It depends on room size, wall colour, ceiling reflection, fixture design and mounting height.
20. Maintenance Factor
Maintenance factor accounts for the reduction in illumination due to dust, dirt, ageing of lamps, dirty walls and poor maintenance. With time, lamps produce less light and surfaces reflect less light.
Its value is always less than or equal to one.
21. Depreciation Factor
Depreciation factor is the reverse of maintenance factor. It is the ratio of initial illumination to the maintained illumination on the working plane.
Its value is always greater than one.
22. Waste Light Factor
Waste light factor considers the loss of light due to overlapping, uneven distribution and light falling outside the required area. For rectangular areas, a factor of about 1.2 is commonly used, while for irregular areas or monuments, a factor of about 1.5 may be used.
23. Absorption Factor
Absorption factor is the ratio of lumens available after absorption to the total lumens emitted by the light source. Smoke, fumes and dust reduce the amount of light available in a room.
Its value may be close to one in clean rooms and much lower in foundries or dusty industrial areas.
24. Beam Factor
Beam factor is the ratio of lumens in the beam of a projector to the total lumens emitted by the lamp. It considers losses due to reflector and front glass of the projector.
Its value usually varies from 0.3 to 0.6 depending on the design of the projector.
25. Reflection Factor
Reflection factor is the ratio of reflected light to incident light on a surface. A white wall has a high reflection factor, while a dark wall has a low reflection factor.
This factor is important in indoor lighting because wall and ceiling colours strongly affect the brightness of a room.
26. Plane Angle
Plane angle is the angle subtended at a point in a plane by two converging lines. It is usually measured in degrees or radians and denoted by θ.
27. Solid Angle
Solid angle is the three-dimensional angle subtended by an area at a point. It is measured in steradian. The concept of solid angle is important in luminous intensity calculations.
Important Illumination Units at a Glance
| Term | Symbol | Unit | Meaning |
|---|---|---|---|
| Luminous Flux | F or Φ | Lumen | Total visible light output |
| Luminous Intensity | I | Candela | Light emitted in a particular direction |
| Illumination | E | Lux | Light falling per unit area |
| Luminance | L | cd/m² | Brightness of a surface |
| Lamp Efficiency | — | lm/W | Light output per watt |
| Solid Angle | ω | Steradian | Three-dimensional angle |
Difference Between Lumen, Lux and Candela
| Parameter | What It Tells | Example |
|---|---|---|
| Lumen | Total light produced by a lamp | A bulb gives 1000 lumens |
| Lux | Light received on a surface | A study table has 500 lux |
| Candela | Light intensity in one direction | A torch has high candela in forward direction |
Modern Importance of Illumination Engineering
In the modern era, illumination engineering is closely connected with energy saving, LED technology, smart lighting, automation and human comfort. Proper lighting design helps reduce electricity bills and improves productivity in offices, factories and educational spaces.
- LED lighting: Provides high efficiency and long life.
- Smart lighting: Uses sensors and automation to control light.
- Energy saving: Reduces unnecessary electricity consumption.
- Workplace safety: Prevents accidents in factories and workshops.
- Visual comfort: Reduces glare and eye strain.
Common Mistakes in Lighting Design
- Using only watt rating instead of lumen rating.
- Ignoring glare and shadow formation.
- Not considering wall and ceiling colour.
- Keeping lamps too far apart.
- Ignoring dust and maintenance factor.
- Using the same lighting level for every type of work.
Frequently Asked Questions
What is the difference between light and luminous flux?
Light is visible radiant energy, while luminous flux is the total visible light emitted by a source per second.
What is the unit of illumination?
The SI unit of illumination is lux, which is equal to one lumen per square metre.
What is the difference between lumen and lux?
Lumen measures total light output from a lamp, while lux measures how much light falls on a surface.
Why is glare harmful?
Glare causes discomfort, eye strain and difficulty in seeing objects clearly. It should be minimized in good lighting design.
What is maintenance factor?
Maintenance factor represents the reduction in illumination due to ageing of lamps, dust, dirt and poor maintenance.
Why are LEDs preferred in modern illumination?
LEDs provide high lumens per watt, long life, low heat generation and better energy efficiency compared with traditional lamps.
Key Takeaways
- Luminous flux is measured in lumens.
- Illumination is measured in lux.
- Luminous intensity is measured in candela.
- Brightness or luminance is measured in cd/m².
- Utilization factor shows how much lamp light reaches the working plane.
- Maintenance factor considers reduction in light due to dust and ageing.
- Good lighting design must reduce glare and provide uniform illumination.
Conclusion
Important terms in illumination engineering form the foundation of lighting design. Concepts like lumen, lux, candela, luminous flux, illumination, brightness, glare, utilization factor and maintenance factor help engineers and students understand how light behaves and how a lighting system should be designed. In the modern era, efficient illumination is not only required for visibility but also for energy saving, safety, comfort and productivity.
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