Delivered light defined
Instead of lumen output, the best and most relevant measurement for evaluating LED lighting fixtures is delivered light. The formal term for measurements of delivered light is illuminance. Generally defined, illuminance is the amount (or quantity) of light falling on a surface area. If the area is measured in square feet, the unit of illuminance is footcandles (fc). If measured in square meters, the unit of illuminance is lux (lx). While delivered light describes how much useful light a lighting fixture can deliver to a task area, useful light is the portion of a luminaire’s light output that is effectively directed to a task area, discounting any wasted light.
One way to think of illuminance is that it’s the density of light upon a surface. Generally, the denser the light on an object, the brighter that object will appear. Lumens can be envisioned as the total quantity of light, without describing directionality and the resulting density. Density – as measured by illuminance – is the most important factor when specifying light. If you are putting light in a defined location (e.g., a library reference room), enabling a specific task (e.g., reading), you need to know if there will be the right amount of light to perform that task – now and in the future.
This definition of illuminance covers any space or surface that needs to be illuminated, from an entrance hallway to a retail environment to an iconic building or national monument, such as the Eiffel Tower or the Empire State Building.