Four quantities, four questions
Luminous flux describes total visually weighted output. Illuminance describes light arriving per area. Luminous intensity describes output into a direction. Luminance describes the directional light associated with a surface. A product lumen total cannot answer all four questions. [1] [2] [3] [4]
Watts describe the input
Electrical power and luminous output describe different sides of a system. Luminous efficacy is output divided by input, in lumens per watt. When comparing reports, ask whether the boundary is the LED package, lamp, or complete luminaire. Optical and driver losses make those boundaries meaningful. LM-79 establishes methods for measuring SSL product optical and electrical performance. [6]
An original worked example
Suppose a complete luminaire produces 3,000 lumens while drawing 30 watts. Its measured efficacy is 100 lm/W. That ratio does not establish the illuminance on a desk: distribution, placement, room surfaces and geometry still matter. The numbers here are teaching assumptions, not a product claim.
Lux and footcandles
One lux is one lumen per square metre. One footcandle is one lumen per square foot; using the exact international foot gives 1 fc = 10.7639104167 lx. Converting units changes the number, not the lighting condition. Our calculator shows both directions. [5]
Choose the measurement for the decision
For a work surface, investigate illuminance and its distribution. For a bright surface in the field of view, investigate luminance and glare. For a beam at a distance, inspect directional intensity. For efficiency, use measured output and measured input under the same test boundary. Record the plane, location and operating state with every result.