An ideal, point light source radiates its luminous flux evenly in all directions in the room, with its light intensity being equal in all directions. In practice, however, there is always an uneven spatial distribution of luminous flux, partly due to the lamp design and partly due to the manner in which the luminaire is formed. The Candela, as the unit of light intensity, is the basic unit of lighting engineering from which all other lighting engineering values are derived.
The spatial distribution of the light intensity of a light source results in a three-dimensional body of light intensity distribution. A section through this light intensity body will give the light intensity distribution curve, which describes the light intensity distribution in one plane. The light intensity is, usually displayed in a polar co-ordinate system as a function of the emission angle. To enable direct comparison of the light intensity distribution of different light sources, the values are expressed in relation to 1000 lm luminous flux. With rotationally symmetrical luminaires, a single light intensity distribution curve is sufficient to describe the luminaire. Axially symmetrical luminaires need two curves, although, these can usually be represented on one diagram.
The illuminance is a measure for the luminous flux density on a surface. It is defined as the ratio of the luminous flux incident on a surface to the size of that surface. The illuminance is not tied to a real surface, it can be determined anywhere in the room. The illuminance can be derived from the light intensity. Whereby, the illuminance reduces by the square of the distance from the light source (inverse square law).
The illuminance is a measure for the luminous flux density on a surface. It is defined as the ratio of the luminous flux incident on a surface to the size of that surface. The illuminance is not tied to a real surface, it can be determined anywhere in the room. The illuminance can be derived from the light intensity. Whereby, the illuminance reduces by the square of the distance from the light source (inverse square law).
