Light measurement

The luminaire’s light distribution is measured on several C-planes around the luminaire, at intervals of at least 15°. First measurement plane (C=0°) is across the lamps’ longitudinal axis. γ-angles – several angles are measured, at least every 5 degrees (see figure).

Light distribution curve

The light distribution curve drawn in a polar diagram denotes the luminaire’s luminous intensity in different directions as a function of viewing angle in one or more planes. It is usually represented by an unbroken line that indicates the light distribution perpendicular to the light source’s longitudinal axis and with a dashed line that indicates the light distribution in direction of the longitudinal axis.

The values of the light distribution curves are scaled to correspond to 1000 lm from the light source (cd/1000 lm, cd/ klm). This is why it is often possible to show luminaires with different outputs on a common polar diagram.

Asymmetrical light distribution
At a maximum luminous intensity angle of 25° a 28 W fluorescent lamp gives approximately 580 cd/klm.

Symmetrical light distribution
A 28 W fluorescent lamp (2600 lm) gives, directly beneath it, approximately 375 cd/klm.


Isolux diagram


The diagram shows, using curves (or scales) a predefined area, inside which the horizontal illuminance exceeds the curve’s lux value. The position of the luminaire is usually indicated on the diagram. Alternatively, the isolux diagram can be represented by a 3D diagram, which is best suited for showing the uniformity of the lighting installation.

The calculation points’ illuminance values can be introduced onto the premises’ layout drawing and the results given in table form. The results are available in all the above mentioned forms in DIALux.