The red line indicates the direction of the C0 plane. The C0 planes of the luminaires always show towards the x-axis, if it was not rotated. The luminous intensity distribution pattern for a light source shows in which direction and with what intensity the light is emitted. The individual values can be presented in tabular or graphic form, and a number of diagrams are normally employed for this purpose. Luminous intensity is usually based on a 1000 lm light source.
C plane system:
The interior around the light source is treated as comprising a large number of planes, which can be divided up into the A, B and C plane systems. The most frequent procedure is to use the C plane system, with the common axis represented by a vertical line passing through the centre of the luminaire. Every plane is then defined in terms of the angle formed with a defined line of reference, e.g. the axis of the luminaire. Luminous intensity is usually measured in the C-plane at 15° intervals and for emission angles in 5° increments.