UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Question
The polar curve of a symmetrical downlight showing its intensity distribution per klm
is depicted here below. A lamp of 2700 lumen output is installed in this downlight.
Calculate the luminance of a horizontal surface, 2m underneath and 2m away
horizontally, reflecting lighting received from this downlight. The reflectance of the
surface is 0.4 and the surface is perfectly diffusing. It can be assumed that there is no
inter-reflection and atmospheric attenuation.
Answer
 2
The surface to be illuminated is tan 1   = 45o away from the downward vertical.
 2
From the polar curve of the downlight, the intensity at 45o is 300cd per klm.
Illuminance received by the surface is
300  2.7
cos 3 45 o
2
2
 
= 71.6 lux
Therefore luminance of the surface becomes
0.4  71.6

= 9.1 cd/m2
K.F. Chan (Mr.)
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Aug 2010