TE Enriched Sample (E)

22 Measuring illumination A Illuminance The measure of how much light is incident on a surface is called illuminance E . It is defined as the luminous flux falling on a surface per unit area. The luminous flux represents the corrected total output power of a source, taking into account the sensitivity of the human eye to different wavelengths. (See p. 9.) Illuminance, therefore, depends on the sensitivity of eye as well. E A U = Its unit is the lux (symbol lx) and 1 lx = l lm m −2 . It can be measured with a lux meter (Fig. 1.22). Fig. 1.22 A lux meter with a measuring range of 200 to 50 000 lx In general, the illuminance on a surface due to sunlight is much larger than that due to artificial lighting (Table 1.2). condition illuminance/lx outdoors, plenty of sunlight 100 000 outdoors, very cloudy 1000 office 500 under street lamps 10 deep twilight （入黑時分） 1 under full moon 0.1 Table 1.2 Typical values of illuminance illuminance ๫׼ ܓ lux ਔд౶ 1.3 Teaching notes Remind Ss that sunshine delivers much more energy than artificial lighting even on a cloudy day. Teaching notes Remind Ss that 1. unlike the usual concept of flux, luminance flux is NOT per unit area. (C.f. magnetic flux) 2. the luminous flux of a source cannot be increased by adding reflectors around the source, but illuminance on the target surface can be. (See DSE 2016 Q3.1.) 3. both have taken into account the spectral sensitivity of the eye. Teaching notes Ss may confuse luminous flux with illuminance. There are three points that are worth noting: 1. Luminous flux usually describes the brightness of a point light source, while illuminance describes the brightness of an illuminated surface. 2. Luminous flux corresponds to light power (adjusted), while illuminance corresponds to light intensity (adjusted power per unit area ). (See DSE 2016 Q3.6.) 3. Their units, lumens and lux, are also confusing. Although the word lux spells like flux , the lux is a unit of illuminance. On the contrary, luminous flux is measured in lumens, which sounds like illuminance. A mnemonic for the correct association is lux is related to cross(x) -sectional area. 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.