Total lumens has been a long-used term in lighting to describe the total light output of a given light source. Now it is common to see delivered lumens in it’s place, particularly with LED products. Understanding the difference between these two terms will help explain one of the reasons for the ascension of LED technology over the last several years.
Total lumens is a measure of the total quantity of light given out by a light source. Omni-directional light sources require up to 78% of the light output to be redirected. A majority of these reflections from the light source are directed back into the fixture. When looking at the light output that does reach the application area after being reflected, consider that energy is lost and compounded with every reflection. Most light traces require multiple reflections before hitting the application surface. Ultimately, less than 14% of the source lumens trace directly to the work surface.
Delivered lumens is used primarily with directed light sources. With these light sources, 80% of total lumen output is delivered directly to the work surface. The remaining 20% are reflected only once to the target. Overall, more than 94% of mono directional lumens are delivered. None of the lumens are reflected back into the fixture.
The formal term for measurements of delivered light is illuminance. In general, illuminance can be described as the intensity of light falling on a surface area. If the area is measured in square feet, the unit of illuminance is Foot-Candles. If measured in square meters, the unit of illuminance is lux.
Delivered light aka delivered lumens describes how much useful light a light fixture can deliver to a given area, discounting any wasted light. As detailed above, light can be wasted in any number of ways. It can be emitted in a direction away from the application area, it can be partially blocked or dispersed within the fixture housing, or it can be lost through filtering, lensing, fixture position, or other factors related to the specific installation.
Figures associated with total lumen output do not account for this wasted light. With LED lighting fixtures being fundamentally directional, LED fixtures typically waste much less light compared to traditional counterparts. This allows them to deliver more of their total light output to the target area. Therefore, an LED light fixture with a lower rated lumens may deliver the same or more useful light in a specific application than a comparable traditional lighting fixture that has a higher rated lumen output.
All of this signals that instead of lumen output, the best and most relevant measurement for evaluating LED light fixtures and for making accurate comparisons with traditional light fixtures is delivered light.