Lighting units

Luminous Flux or Light Output… lighting units

In modern lighting, luminous flux or light output is defined as the total amount of light emitted per second by a light source. The unit of luminous flux is lumen (Lm).
Lumens of the lamp = the amount of light emitted from the light source, whose unit is also lumens.

Luminous Efficacy:

Optical efficiency or optical efficiency of a light source is defined as the ratio of output light (lumens) to input energy (watts).
Light efficiency is measured in lumens per watt (lm/W). The efficiency of different light sources varies dramatically. From less than 10 lumens/watt, to more than 200 lumens/watt.
Light source efficiency = lamp lumens / lamp watts

Luminous Flux Density or Lighting Level

The light flux density at a point of the surface is defined as the light flux per unit of surface. Luminous flux density is also known as illuminance or the amount of light on a surface or illuminance surface.

The SI unit is lux illuminance (lx), lx = 1 lm /m2 1. When the measurement is in imperial units, the unit of illuminance is candela (fc), fc = 1 lm /ft2 1
The relationship between fc and lux is lux 1 = 10.76 fc. By the way, this is the same relationship between square meters and square feet: 1m2 = 10.76 ft2
In simple terms, we can say lux = unit of illumination at one point of the surface.
Lux = lumens/area.