What is luminous intensity?
How much light a source sends in each direction, which changes with angle but not with distance.
Luminous intensity is the quantity that says how much light a source sends in a given direction, measured in candela. It belongs to the source and its direction, not to any surface the light lands on.
Two points matter for AGL. First, intensity varies with direction: a fixture is brightest along its beam axis and weaker as the angle moves away, so one fixture has many intensity values, not one. Second, intensity does not change with distance. What changes with distance is the illuminance on a surface, which falls with the square of the distance. That is why a fixture’s performance is described by intensity in each direction, and why it is measured at many angles.
Frequently Asked Questions
Q1What is luminous intensity?
The amount of light a source sends in a particular direction, measured in candela. It describes the source and the direction, not the surface that receives the light.
Q2Why does a fixture have many intensity values?
Intensity depends on direction. It is usually highest on or near the beam axis and lower at other angles, so a fixture is described by intensity at many angles, not by a single number.
Q3Does intensity change with distance?
No. The intensity of a small source in a given direction stays the same however far away you are. The illuminance on a surface is what falls with distance, by the square of it.
Q4What is the difference between luminous intensity and candela?
Luminous intensity is the quantity. Candela is its unit, one lumen per steradian. In practice the two words are often used together.
Q5How does luminous intensity relate to flux?
Flux is the total light emitted. Intensity is the part of it going into one direction per unit solid angle. Adding up the intensity over all directions (integrating over the solid angle) gives back the flux.
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