The illuminance values for a uniformity
check are calculated using the point-by-point method. The inverse square law and cosine
law are used to calculate illuminance at a point from intensity data.
A floodlight of mounting height h lights up
the point p, as shown in Figure 5. From Eqn. (6) of lecture note 'Photometry and Light
Sources', the horizontal illumanance (EH) is given as follows:
(2)
where I is the illuminous intensity in the
direction of point p.
Since
therefore,
(3)
Taking maintenance factor (MF) and
atmospheric loss (AL) into account, Eqn. (3) is rewritten as:
(4)
Figure 5 The Point-by-point Method Figure
6 Illuminance on Pont B
Example 2
If both the maintenance factor and
atmospheric loss are 0.9, calculate the horizontal illuminance of point B (Figure 6) as
the floodlighting scheme proposed in Example 1.
From the isoflux diagram (Figure 3), the
luminous intensity of the floodlight in the direction of point B is I = 20 Candelas (based
on 1000 lamp lumens).
The horizontal illuminance of point B is
calculated as:
