Back in the old days, before the LED technology emerged, most of the people relied on the power wattage (W) of the bulbs to have an idea on how much light they were emitting.
With the appearance of the fluorescent and quickly transition to the LED technology, power consumption of the bulbs changed so much in relation to its light output that now it is hard to estimate it. Even more complicated to estimate it is if we consider most of the lamps are now including integrated LED with surface-mount devices (SMD) or Chip on Board (COB) light engines.
With the quickly introduction of the LED technology, wattage was not any more a valid measurement of light output but just of consumption. For that reason, most of the people started to use Lumens (lm) as a measurement of light output and brightness. Even Lumens is a valid reference and one of the most useful, it is important to be clear what it describes and the limitations of it, specially to avoid confusion between the common public. Therefore, we will try to simply describe for the public (without entering into much technical information) the different types of basic light measurements and in which situations is more convenient each of them.
Luminous flux or luminous power measures the total amount of light emitted by a light source and its measurement of unit is the Lumen (lm). It is important to remark that one lumen equals the amount of light emitted by a light source radiating equal amount of light in all directions.
Because of measuring the light in all of the directions this measurement is especially useful for traditional and spherical light bulbs or those luminaires which include shades as they create ambient light around it.
However, if what we need to measure is the light that can focus the maximum amount of brightness in a smaller beam such as a flash light or directional lights, then we should use the Candela value. The Candela (cd) is the luminous intensity or the quantity of visible light emitted by a light source in a given direction per unit solid angle. It is a useful measurement when comparing devices that produce focused beam like spot lights or reading lamps as they have a limited angle beam.
Finally, we have a third measuring unit of illuminance that it is the Lux (lx). Lux measures the luminous flux per unit area or how much light is falling on a given surface. So, the Lux determines how bright a surface is going to be. One lux equals one lumen per square meter of surface area, so basically we transform the lumens measurement to actual surface brightness. This measurement is most used on technical projects where the light has a very specific purpose like to illuminate objects or areas (exhibitors, banners, tables…etc).
After describing the three unit types of measuring the light emitted by a source, it is important to understand when to use each of them and the limitations of each measurement. Basically, to sum up we can say that when the light source is emitting to all directions (non-directional lighting) is better to use the Lumens measurements while when we are talking about lights beams (non-directional lighting) is better to use Candela.
If we use Lumens to measure directional light what will happen is that the Luminous flux will be reduced proportional to the viewing angle. So, comparisons of lumens between different viewing angle luminaires will give not give faithful results of luminous power.
Finally, if what we want is to measure the intensity of the light as perceived by the human eye in a surface we need to use the Lux, as the other values are not giving exact information of visual brightness perception on surfaces.