Zonal Flux Diagram
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5 Zonal Flux Diagram

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


Table 1 Angles from Beam Axis





Angle from beam axis (horizontal),


Angle from beam axis (vertical),



Figure 3(a)


Figure 3(b)


Figure 3(c)


Figure 3(d)


If part of the area to be lit falls outside the lowest isocandela contour, it is recommended to:

(a) increase the number of floodlight,

(b) use a wide beam floodlight, or

(c) increase the mounting height of floodlight.

The lumen values inside each box are added up (If the line cuts only part a box, the zonal lumen value within the lines is estimated.) and as the areas to be lit is symmetrical, the total flux = 2 x 222 = 444 lm.

Figure 4 Annotated Zonal Flux Diagram

Determination of Utilisation Factor

The utilisation factor (UF) is given by

Determination of Average Horizontal Illuminance

The average horizontal illuminance (Eav) is calculated as