The candela (cd), formerly called the candle or candlepower, is the unit of luminous intensity. It is defined in terms of the light emitted by a perfect radiator (see BLACKBODY RADIATION) at the temperature of the solidification of platinum (approximately 2,046 K). The candela serves as a standard in astronomical PHOTOMETRY.
Candlepower is an old term that has commonly been used to indicate the ability of a source to give off light, or as a measure of the "power" of a source as an emitter of light. It was based originally on the light emitted by a standardized candle. The more modern term for candlepower is luminous intensity, which measures the concentration of emitted light in a certain direction away from the source. Luminous intensity is the density of light, or the flux per unit solid angle. The present unit of luminous intensity is the CANDELA.
In photometry, the lumen is the SI unit of luminous flux, the rate at which radiant energy reaches a surface. It is defined as the flux emitted by a uniform point source of one CANDELA intensity in a cone having a solid angle of one steradian.
A solid angle is generated by an infinite set of line segments having a common vertex and passing through a simple closed curve. The solid angle subtended at P by the surface S is measured by the area U, which is the portion of the spherical surface with center P, and radius 1, lying within the solid angle. The unit solid angle is the steradian, the largest solid angle being 4 (pi) steradians. If the steradian is defined by the solid angle at the vertex of a right circular cone, opposite generating lines of the cone will make an angle of approximately 65 deg 32'.
Steradian = a solid angle of a sphere whose subtending surface areas = r2
In the above diagram, the red lines are radii leaving the center P and describe a cone. If the lamp is a point source of 1.0 cd intnesity at the center P of a 1.0 m. radius sphere, and the area S is 1.0 m2 (r2), and the angle subtending the area S is one steradian, then the flux through the diagramed cone is 1.0 lumen.
Below is a light bulb box that indicates it's luminance on the box in the lower right corner (1710 lm)
