TE Enriched Sample (E)

28 Lighting 1 Exercise (1.3) (c) The electricity cost can be reduced by 1. using a lamp of a lower rated power but higher luminous efficacy, or 2. using reflectors to reflect light onto the table. Checkpoint 5 1. True or false: (a) The illuminance on a surface decreases linearly with the separation between the light source and the surface. (b) Two surfaces equidistant from a light source must have the same illuminance. 2. A small surface and a small light source are 10 m apart. The illuminance on the surface is 1000 lx. If the surface is now 30 m away from the source with the same orientation, what is the illuminance on the surface? [Hint: The incident angle is fixed.] 3. A small surface is facing a small light source and they are 10 m apart. The normal illuminance on the surface is Y . If the surface is now rotated by 30° clockwise, what is the illuminance on the surface in terms of Y ? How about rotated by 30° anticlockwise? 4. A small lamp O is 2 m above a floor. A is a surface on the floor and OA = 2.5 m. If the lamp produces a luminous flux of 2000 lm, find the illuminance on A . cos + POA = . . 2 5 2 0 8 = cos E r POA 4 2 + r U = = ( . ) . 4 2 5 2000 0 8 2 # r = 20.4 lx 5. Now, the lamp in Q4 is replaced by another small lamp. The illuminance on A becomes 500 lx. Find the illuminance on P . 2 m A 2.5 m O P 1. Susan is designing the lighting for her office. Which of the following quantities best represents the amount of illumination on her desk? A. Luminous flux of all the light sources B. Illuminance of all the light sources C. Illuminance on her desk D. Electrical power consumption of all the light sources 2. Ignoring the response of the human eye, what does luminous flux measure? A. Total light energy given out by a light source B. Light energy given out per unit time by a light source C. Light energy given out per unit time per unit area of a light source D. Light energy given out per unit time per unit wavelength by a light source 3. Anotice board hanging on a vertical wall is illuminated by a ceiling lamp. Which of the following does not affect the illuminance on the notice board? A. Changing the lamp to one with a different efficacy B. Installing the lamp at a position farther from the notice board C. Changing the colour of the notice board D. Lowering the notice board 4. A small light source is illuminating two small surfaces A and B on the same plane as shown. Surface A is vertically under the light source. What is the ratio of the illuminance on A to that on B ? Y cos 30°; Y cos 30° 111 lx F F 977 lx B C C Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.