The Point-by-point Method
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6 The Point-by-point Method

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: