Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Illumination
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Illumination
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Subject: Electrical Installation And Maintenance Work
Class: Senior Secondary 2
Term: 2nd Term
Week: 3
Theme: Workshop Practices
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Define terms used in illumination e.g. luminous flux, illumination etc. Solve problems on illumination Identify types of lamps State regulations guiding discharge lamps.
This section provides the core content necessary for the teacher to deliver the lesson comprehensively. 2.
1. Basic Terms Used in Illumination Light: Radiant energy that is capable of producing the sensation of vision. Measured in lumens. Luminous Flux (Φ or F): This is the total quantity of light energy emitted per second from a luminous body. It represents the rate at which light energy is radiated.
Unit: Lumen (lm).
Analogy: Imagine a water hose. The total amount of water flowing out per second is analogous to luminous flux.
Luminous Intensity (I): This is the luminous flux emitted per unit solid angle in a specific direction from a point source. It measures how "strong" the light source is in a particular direction.
Unit: Candela (cd).
Analogy: If the water hose has a nozzle that can direct the water, the intensity of the water stream in a particular direction is analogous to luminous intensity.
Illumination (E): This is the luminous flux received per unit area of a surface. It describes how brightly a surface is lit.
Unit: Lux (lx) or Lumen per square meter (lm/m2).
Analogy: If the water from the hose hits a specific area on the ground, the amount of water spread over that area is analogous to illumination. A high illumination means the surface is very bright.
Luminance (L): This is the luminous intensity per unit projected area of a surface, as viewed from a particular direction. It represents the "brightness" of a surface, indicating how much light it emits or reflects towards the eye.
Unit: Candela per square meter (cd/m2).
Coefficient of Utilization (CU): This is the ratio of the luminous flux reaching the working plane to the total luminous flux emitted by the lamps. It accounts for light losses due to absorption by walls, ceiling, and fittings, and the geometry of the room.
Factors affecting CU: Room dimensions, colour of walls/ceiling/floor, type of luminaire, mounting height.
Maintenance Factor (MF): This is the ratio of the illumination on the working plane after a certain period of use to the initial illumination when the lamps and fittings were clean. It accounts for depreciation of lamp output over time and accumulation of dirt on lamps and fittings. Typically ranges from 0.7 to 0.
9. Space to Height Ratio: The ratio of the distance between luminaires to their mounting height above the working plane. It helps ensure uniform illumination. 2.
2. Solving Problems on Illumination The primary formula for calculating illumination (E) relies on the inverse square law and Lambert's cosine law.
Inverse Square Law: For a point source, the illumination (E) on a surface is directly proportional to the luminous intensity (I) of the source and inversely proportional to the square of the distance (d) from the source to the surface.
Formula: E = I / d2 Where: E = Illumination in Lux (lx) I = Luminous Intensity in Candela (cd) d = Distance from the light source to the surface in meters (m)
Lambert's Cosine Law: When the light rays strike the surface at an angle (θ) other than perpendicular (normal), the illumination is further reduced by the cosine of that angle.
Formula: E = (I cosθ) / d2 Where: θ = Angle between the normal to the surface and the direction of the incident light. General Illumination Calculation (for room lighting design): The total luminous flux (Φ_total) required to achieve a desired illumination (E) over a given area (A) can be calculated using the following formula, incorporating the Coefficient of Utilization (CU) and Maintenance Factor (MF): Formula: Φ_total = (E A) / (CU MF)
Where: Φ_total = Total luminous flux required in Lumens (lm) E = Desired average illumination in Lux (lx) A = Area of the room/surface in square meters (m2) CU = Coefficient of Utilization (dimensionless) MF = Maintenance Factor (dimensionless) Once Φ_total is known, the number of lamps required can be determined by dividing Φ_total by the luminous flux per lamp efficiency.
Mercury Vapour Lamps (MVL): Principle: Arc discharge in mercury vapour under high pressure. Produces a bluish-white light.
Characteristics: Good efficacy, long life, often used for street lighting, industrial high bays in Nigeria. Sodium Vapour Lamps (Low Pressure - LPSV, High Pressure - HPSV): Principle: Electric discharge through sodium vapour. LPSV produces a monochromatic yellow light; HPSV produces a broader spectrum, yellowish-orange light.
Characteristics: Very high luminous efficacy (especially LPSV), extremely long life. LPSV used for road lighting where colour rendering is not critical; HPSV common for street lighting and security lighting in Nigeria.
Light-Emitting Diode (LED)
Lamps: Principle: Semiconductor device that emits light when an electric current passes through it (electroluminescence).
Characteristics: Extremely high luminous efficacy, very long lifespan, compact size, instant on/off, various colour temperatures, dimmable, low heat emission. Increasingly popular in Nigeria for all applications due to significant energy savings and durability, despite higher initial cost. 2.
4. Regulations Guiding Discharge Lamps The installation and use of discharge lamps are governed by various standards and regulations to ensure safety, efficiency, and longevity. In Nigeria, these often align with international standards such as those from the International Electrotechnical Commission (IEC), British Standards (BS), and local standards set by bodies like the Standards Organisation of Nigeria (SON).
Key aspects include:
1. Ballast Requirement: All discharge lamps (except some self-ballasted CFLs and LEDs) require a ballast (choke) to limit the current and provide the necessary starting voltage. The ballast must be correctly rated for the lamp and circuit.
2. Starter Requirement: Most fluorescent lamps (especially older T8/T12 types) also require a starter to initiate the arc discharge. This must be matched to the lamp. Modern electronic ballasts often integrate the starting function.
3. Correct Wiring: Wiring must be done according to IEE Wiring Regulations (BS 7671 or equivalent, adopted by Nigeria). Proper cable sizing to handle the starting current and operating current of the lamps and ballasts. Secure connections to prevent loose contacts and overheating.
4. Earthing (Grounding): Metal casings of luminaires (fittings) for discharge lamps must be properly earthed to prevent electric shock in case of insulation failure.
5. Protection Devices: Circuits supplying discharge lamps must be protected by appropriate circuit breakers (MCBs/MCCBs) or fuses against overcurrent and short-circuits. Residual Current Devices (RCDs) may be required for additional shock protection in certain areas.
6. Flicker Control: While not strictly a safety regulation, excessive flicker from discharge lamps can cause eye strain and headaches. Electronic ballasts help mitigate this.
7. Harmonic Distortion: Discharge lamps, especially those with magnetic ballasts, can introduce harmonic distortions into the electrical supply system. Large installations may require harmonic filters. Electronic ballasts often have lower harmonic distortion.
8. Disposal: Lamps containing mercury (e.g., fluorescent tubes, mercury vapour lamps) must be disposed of responsibly to prevent environmental contamination. While formal recycling infrastructure is developing in Nigeria, awareness of careful handling is important.
9. Mounting and Location: Luminaires must be mounted securely and at appropriate heights, considering heat dissipation and protection against mechanical damage or ingress of dust/moisture (IP rating).
1
0. Ventilation: Discharge lamps, especially high-intensity types, produce heat. Adequate ventilation around luminaires is necessary to prevent overheating and premature lamp failure. using the following formula, incorporating the Coefficient of Utilization (CU) and Maintenance Factor (MF): Formula: Φ_total = (E A) / (CU MF)
Where: Φ_total = Total luminous flux required in Lumens (lm) E = Desired average illumination in Lux (lx) A = Area of the room/surface in square meters (m2) CU = Coefficient of Utilization (dimensionless) MF = Maintenance Factor (dimensionless) Once Φ_total is known, the number of lamps required can be determined by dividing Φ_total by the luminous flux per lamp (Φ_lamp): Number of lamps = Φ_total / Φ_lamp Worked Example 1 (Inverse Square Law): A light source has a luminous intensity of 100 candela (cd). Calculate the illumination falling on a surface placed 5 meters away from the source.
Given: I = 100 cd d = 5 m Formula: E = I / d2 Calculation: E = 100 / (5)2 E = 100 / 25 E = 4 lux Answer: The illumination falling on the surface is 4 lux. Worked Example 2 (General Illumination for a Room): A classroom in a Nigerian secondary school is 10m long and 8m wide. The required average illumination is 250 lux. If the Coefficient of Utilization is 0.7 and the Maintenance Factor is 0.8, calculate the total luminous flux required. If each lamp provides 2000 lumens, how many lamps are needed?
Given: Length (L) = 10 m Width (W) = 8 m E = 250 lx CU = 0.7 MF = 0.8 Φ_lamp = 2000 lm/lamp Step 1: Calculate the Area (A) A = L W = 10 m 8 m = 80 m2 Step 2: Calculate the Total Luminous Flux (Φ_total) Φ_total = (E A) / (CU MF) Φ_total = (250 lx 80 m2) / (0.7 0.8) Φ_total = 20000 / 0.56 Φ_total ≈ 35714.29 lumens Step 3: Calculate the Number of Lamps Required Number of lamps = Φ_total / Φ_lamp Number of lamps = 35714.29 / 2000 Number of lamps ≈ 17.86 Since lamps cannot be fractional, round up to the nearest whole number.
Answer: The total luminous flux required is approximately 35714 lumens.
Therefore, 18 lamps are needed for the classroom. 2.
3. Types of Lamps Lamps are broadly categorised by their operating principle.
A. Incandescent Lamps: Principle: Light is produced by heating a filament (usually tungsten) to incandescence using electric current.
Examples: General service lamps (GLS), Halogen lamps.
Characteristics: Produce warm, continuous spectrum light. Relatively low luminous efficacy (lumens per watt), meaning they are not very energy efficient. Short lifespan compared to other lamp types. Widely used in homes in Nigeria for basic lighting where initial cost is a factor, though being phased out due to inefficiency.
B. Discharge Lamps: Principle: Light is produced by passing an electric current through a gas, exciting the gas atoms, which then emit light (or UV radiation, which is converted to visible light by a phosphor coating).
Examples: Fluorescent Lamps (CFLs, Linear Fluorescent Tubes): Principle: Electric current excites mercury vapour, producing UV radiation. This UV radiation then strikes a phosphor coating on the inside of the tube, causing it to glow (fluoresce) and emit visible light. Require a ballast (choke) and starter.
Characteristics: High luminous efficacy, longer lifespan than incandescent, various colour temperatures. Common in Nigerian offices, schools, and homes due to energy efficiency.
Mercury Vapour Lamps (MVL): Principle: Arc discharge in mercury vapour under high pressure. Produces a bluish-white light.
Characteristics: Good efficacy, long life, often used for street lighting, industrial high bays in Nigeria. Sodium Vapour Lamps (Low Pressure - LPSV, High Pressure - HPSV): Principle: Electric discharge through sodium vapour. LPSV produces a monochromatic yellow light; HPSV produces a broader spectrum, yellowish-orange light.
Characteristics: Very high luminous efficacy (especially LPSV), extremely long life. LPSV used for road lighting where colour rendering is not critical; HPSV common Teacher Activities: Introduction (10 minutes): Begin by asking students about their observations of lighting in different environments (e.g., home, school, market, street). Introduce the topic of illumination, emphasizing its practical importance in electrical installation. State the learning objectives for the lesson.
Key Concepts Explanation (25 minutes): Define and explain terms: Luminous flux, Luminous intensity, Illumination, Luminance, Coefficient of Utilization, Maintenance Factor, Space to Height Ratio. Use simple analogies relevant to daily life in Nigeria (e.g., brightness of a lantern vs. a floodlight). Clearly write down the definitions and units on the board. Encourage students to ask questions for clarification. Problem Solving – Theoretical Basis and Worked Examples (25 minutes): Explain the Inverse Square Law and Lambert's Cosine Law for illumination calculations. Write the formulas clearly on the board. Go through Worked Example 1 (Inverse Square Law) and Worked Example 2 (General Illumination for a Room) step-by-step, explaining each stage of the calculation. Use the worked examples provided in section 2.
2. Emphasize units and how to substitute values correctly.
Types of Lamps (20 minutes): Describe the working principle, characteristics, and common applications for Incandescent lamps (GLS, Halogen) and various Discharge lamps (Fluorescent, Mercury Vapour, Sodium Vapour, LED). Use visual aids if available (e.g., actual lamps, pictures of different lamp types and their uses in Nigeria). Discuss the pros and cons of each type, particularly their energy efficiency and lifespan in the context of electricity costs and availability in Nigeria. Regulations Guiding Discharge Lamps (15 minutes): Explain the key regulations and safety considerations for discharge lamps (ballasts, starters, earthing, wiring, protection, disposal). Relate these to safety practices in electrical installations in Nigeria (e.g., proper earthing of streetlights, safe disposal of broken fluorescent tubes). Discuss the role of regulatory bodies like SON and local electrical installation codes.
Recap and Q&A (5 minutes): Briefly recap the main points of the lesson. Address any remaining student questions.
Student Activities: Active Listening and Note-taking: Students actively listen to explanations and take comprehensive notes on definitions, formulas, and lamp characteristics.
Questioning: Students ask clarifying questions during explanations.
Observation: Students observe actual lamp types (if available) or pictures/diagrams, noting their features.
Participation in Discussions: Students engage in discussions about the characteristics and applications of different lamps, and the importance of regulations.
Problem Solving: Students follow along with the teacher's worked examples, attempting to solve them simultaneously or immediately afterwards. Group Work (Optional, if time permits): Students could be briefly put into small groups to discuss a real-life lighting scenario (e.g., "How would you choose lights for a small retail shop in Lagos?") before the teacher moves to guided practice.
Energy Efficiency and Cost Savings in Nigerian Homes and Businesses: Students can apply their knowledge to advise on switching from inefficient incandescent or older fluorescent lamps to modern LED lamps. Given the cost of electricity and unreliable power supply in Nigeria, understanding how to achieve optimal illumination with minimal energy consumption is highly valuable for reducing utility bills and dependence on generators. For instance, recommending LED retrofits for commercial spaces or public buildings can lead to significant operational cost reductions.
Safety and Productivity in Workplaces: Proper illumination is critical for safety in workshops, factories, and construction sites across Nigeria. Students can understand the need to calculate and ensure adequate lux levels to prevent accidents, reduce eye strain, and improve worker productivity. For example, ensuring proper task lighting for mechanics in a vehicle repair shop or adequate general lighting in a manufacturing plant. This knowledge supports local businesses in creating safer and more efficient working environments.
Public and Street Lighting Design: The principles of illumination are directly applied in designing street lighting systems for Nigerian communities. Understanding luminous intensity, illumination levels, and the selection of appropriate lamp types (like high-pressure sodium or LEDs for durability and efficiency) is essential for improving road safety, reducing crime, and enhancing the quality of life in urban and rural areas. This involves balancing cost, light distribution, and maintenance considerations specific to the Nigerian climate and infrastructure.