IESNA Definition: the Standard International unit of luminous intensity. One
candela is one lumen per steradian.
The intensity of a light source in a specific direction is expressed
in candelas (cd). Any given light source will have many different
intensities, depending upon the direction considered. Since intensity
is a property of the source itself, the candlepower
(luminous intensity expressed in candelas) for a specified direction
remains the same, regardless of distance from the source.
It is interesting to compare lamps of the same wattage with regard to
luminous intensity. In the following examples, imagine the
lamps aimed straight down, with 0° representing
a point directly beneath each one, and 20° representing a point 20° up
from 0°. Although not true for all sources, the highest candela values
occur at 0° in these examples: