Luminosity = lumens ÷ watts. Due to the widely varying efficiencies of illuminants, it is impossible to compare them only on the basis of output (watts). Thus, it is easier to compare them on their irradiated quantity of light. These value are expressed in lumens. Furthermore, the efficiency of illuminants can be easily compared based on their specific ratio of lumens to watts.
As a reference to luminosity, various candles were used in the past until the blackbody radiator made out of platinum was put in to use. The unit which measures luminosity is candela (cd).
Illuminance (unit: lux) refers to the distance of the illuminated object to the light source in question. It is hereby the light intensity which illuminates the object that is measured.
Conclusion: In order to assess types of lighting, it is most effective to compare their lumen values.