Question

Three 100 cd point sources are located in a line 4.0 m apart, and each is suspended 3.0 m above a work bench. Neglecting reflections and assuming cosine law,
(a) calculate the total illuminance of the work bench at a point directly below the center source
(b) calculate the luminous intensity of one single point source in candela to replace the three point sources used in the above situation on the plane 3 m below at a point directly below the center source
(c) consider this single point source is provided by an incandescent lamp with luminous efficacy of 12 lumen/watt, calculate the wattage of the lamp
(d) this single light source is now replaced by a LED lamp with luminous efficacy of 175 lumen/watt and would last for 20,000 hours of use, whereas the incandescent lamp from (c) can last for 1,000 hours. The cost of incandescent and LED lamps cost $20 and $200 respectively and MARR is 5% p.a., calculate the total costs of lighting using these 2 kinds of lamps in a year for use of 12 hours a day if the cost of electricity is $ 1.21/kWh.

Answer 1

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