Standard International lighting is a measure of intensity. A candela is a lumen/steradian. It was previously referred to as a "candle".
The luminity intensity of a light source in a certain direction is expressed in candela (cd). Depending on the direction involved, any light source will have many different lighting intensities. Since the luliation intensity is a feature of the light source itself, the candela value in a certain direction remains the same regardless of the distance to the source.
It is interesting to compare lamps with the same wattage value according to lighting intensities. In the following examples, let's consider that the lamps are positioned at a position of 0° facing directly down and at an angle 20° up from this direction. Although it does not apply to all resources, the highest candela value among these examples occurs at 0°.
|lamp||Kandela at 0°||Kandela at 20°|
|150-W R40 Projection||1,100 cd||820 cd|
|150-W R40 Spot||5,800 cds||780 cd|
|150-W PAR38 Projection||4,000 cds||1,100 cd|
|150-W PAR38 Spot||11,800 cds||500 cd|
It is a curve that represents a change in the lighting performance of a lamp or light in a plane that cuts the center of light and is usually polar. The lighting intensity of light coming from a lamp or light at certain angles (candle value measured in candela) can be shown on a table or chart.
If the distribution of light is not symmetrical in all directions around a vertical line, such as in standard ceiling-mounted 2 ft x 4 ft fluorescent lamps, the lighting values (in candela) can be taken in several vertical planes that cut the light. The three most common planes used are: planes parallel to the lamp axis, planes perpendicular to lamp axes, and planes that angle lamp axes by 45o.
From here, light generation curves can be drawn according to the preferred plane. Each combination of lamp and lamp-light has its own separate light lighting distribution.
Adapted from Interior Lighting for Designers.
Vertical lighting distribution curve is obtained by making measurements at various elevation angles in a vertical plane that cuts the center of light. Unless the plane is specified, the vertical curve is considered to represent a curve, such as the curve obtained when rotated around the lamp or light vertical axis. The lighting distribution of a light is based on reflector design, the type of housing used in the lamp and the choice of lamp-ballast.