LAWS of ILLUMINATION Factors Affecting Illumination

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LAWS OF ILLUMINATION Factors Affecting Illumination Assumption that the source is a point source, or is otherwise sufficiently far away from the surface: 1) E is directly proportional to the luminous intensity of the source. 2) Inverse Square Law Laws for Point Sources of Light 3) Lambert’s Cosine Law Inverse Square Law • Established by J. H. Lambert as one of the earliest lighting laws to enable the calculation of illuminance • The illumination of a surface is inversely proportional to the square of the distance of the surface from the source. 퐼 퐸 = 푟2 Example The luminous intensity of a spotlight varies with angle. It may have a maximum value of 1000 candelas at the center of the beam. a)If this spotlight is aimed directly downwards onto the floor 2 meters below, what will be the illuminance? b) If the spotlight is angled so that the luminous intensity directed downwards is 100 candelas, what will be the illuminance? Lambert’s Cosine Law • Lamberts Second Law • According to this law, if the surface is turned so that the rays hit it at an angle, the illuminated area will increase in size and the illuminance will drop accordingly. Lambert’s Cosine Law • The ratio of the original illuminated area to the new area is equal to the cosine of the angle through which the surface has been moved. • Therefore the illuminance will fall by the factor of the cosine of angle. 퐼 퐸 = 푐표푠퐴 푟2 Example • If a spotlight is 3 meters above the floor, aiming at a point 3 meters away and its intensity in this direction is 1000 candelas, calculate the illuminance. Activity • A lamp giving out 1200 lm in all directions is suspended 8m above the working plane. Calculate the illumination at a point on the working plane 6m away from the foot of the lamp. Illumination At Multiple Points 3 3 3 • EB =EAcos θ1 EC =EAcos θ2 ED =EAcos θ3 Example • A 2400 lumen lamp is suspended 10m above point A. find the illumination at points A, B and C if B and C is 5 and 10m away from point A respectively. Activity • A corridor is lighted by 4 lamps spaced 10m apart and suspended at a height of 5m above the center line of the floor. If each lamp gives 200 candela power in all directions below the horizontal, find the illumination at the point on the floor on the mid-way between the second and third lamps. Coefficient of Utilization or Utilization Factor(η) • It is the ratio of the lumens that actually received by a particular surface to the total lumens emitted by a light source. η = Lumens actually received on the work plane Lumens emitted by a light source Coefficient of Utilization or Utilization Factor(η) The value of this factor varies widely and depends on the following factors: 1.Type of lighting system (direct or indirect) 2.Type and mounting height of the fittings 3. Color and surface of walls and ceilings 4. Shape and dimension of the room Example • Determine the average illumination of a room measuring 9.15 m by 12.2 m illuminated by a dozen 150W lamps. The luminous efficiency of the lamps are 14 lm/w and the coefficient of utilization is 0.35..

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