A typical zonal flux diagram and associated
isoflux diagram is shown in Figure 2. This is divided through the plane of symmetry for
the luminaire, with **isocandela lines** plotted on the angular grid on the left
hand side, and figures indicating the **total flux** in each angular zone on the
right. Flux totals 344 lm (bottom right corner of the diagram) refer only to half diagram
so values should be multiplied by 2, i.e. the flux total should be 688 lm. The diagram is
used to give:

(a) the total flux intercepted by the area
to be lit (see Example 1), and

(b) point values of illuminance (see
Example 2).

Isoflux Diagram Zonal Flux Diagram

Figure 2 Typical Zonal Flux Diagram

Figure 3 Floodlighting

Example 1

An area measuring 20 m x 20 m is to
be lit using a single floodlight mounted on a 8 m high column, as shown in Figurer 3. The
floodlight is a 150 W SON-T lamp of 16000 lumens. The zonal flux diagram of the floolight
is given in Figure 2. Peak intensity for the floodlight is aimed at a distance 11.9 m
alonr line EF, i.e. at point P **(Normal floodlight aiming is about 2/3 of distance
along the centre line)**. Determine the total flux falling on the area, the
utilisation factor and the average horizontal illuminance.

Determination of Total Flux

The floodlight geometry for points
A, B, C and D is shown in Figure 3. The area to be lit (ABCD) is plotted to the zonal flux
diagram, as shown in Figure 4, by the angular data tabulated in Table 1.

(a) Point A (b) Point B

(a) Point C (b) Point D

Figure 3 Floodlight Geometry