Assuming 25% fixture losses overall it’s 45,000 lumens, or about 55 lm/ft². That is kind of dim for a showroom. A good four tube 4 foot T8 fixture will give about 10,000 lumens after fixture losses while consuming approximately 110 watts.
Therefore, one of the challenges faced by the lighting industry as a whole is how to consistently measure (and describe) brightness across different technologies, especially when they can be drastically different in the magnitude of intensity.
Currently, one of the common ways to describe brightness that can be applied to both traditional and new technologies is to use the term “Lumen.” Wikipedia defines a Lumen as “the International System of Units derived unit of luminous flux, a measure of the total ‘amount’ of visible light emitted by a source.” (Luminous Flux is further defined here…)
What will 10,000 Lumens light up ?
Full sunlight can be over 10000 lumens per square foot so the answer is no. Even just skylight is something like 1000 lumens/ft². That being said, I find 100 to 200 lumens/ft² to be a good lighting level for indoors as it is pretty close to the light levels most rooms receive during the day from sunlight and skylight.
LEDs do offer interesting possibilities to eventually do what you suggested without inordinate power consumption. For example, should we eventually get to 250 lm/W efficiency levels then you can light a 100 square foot room to daylight levels (i.e. 1000 lm/ft²) using only about 400 watts. This is less power than many incandescent chandeliers now use to give a fraction of the light.
At this point in time the most cost efficient way to brightly light a room is to use 4 foot T8 tubes in as many fixtures as needed.
Before fixture losses that’s roughly 60,000 lumens. Assuming 25% fixture losses overall it’s 45,000 lumens, or about 55 lm/ft². That is kind of dim for a showroom. A good four tube 4 foot T8 fixture will give about 10,000 lumens after fixture losses while consuming approximately 110 watts. For the same power budget you can put in about ten such fixtures and more than double your light output.
This certainly seems like a reasonable unit to use, since it measures the total “amount” of visible light, and in many ways it is very useful. However, a challenge is created because Lumens actually measure the total amount of visible light emitted by a source without regard to the direction this light is emitted. Because of this, it does not accurately reflect the amount of light that actually reaches the eye of an observer.
Some reasons for this are uniform across technologies, such as the amount of dust and other particles in the air, so these don’t impact a meaningful discussion of how “bright” a light is, because they tend to have a relatively consistent impact on different lighting technologies.
However, there are some reasons that the total brightness of a light source doesn’t reach an observer’s eye that are different because of the technologies involved. The ones I’d like to focus on today are directional light and reflected light intensity.
Traditional (incandescent, high intensity discharge, etc.) lights emit light in all directions. In contrast, LED lighting is directional, meaning that all of the light energy is emitted in the direction the light is pointed (in whatever beam pattern it is designed to produce). For example, most Leading Edge Design lights are designed to emit a 120º beam pattern, so all light is directed within that 120º pattern. Traditional lights emit light in all 360º, meaning that essentially 50% or more of the light emitted is directed away from the intended illumination. Often times, light that is initially emitted in the wrong direction is reflected back generally toward the intended illumination point, however, this light looses brightness along the way, especially since some of the reflected light is reflected back into the original light source or is refracted along the way.
So, what does all of this mean? A rough rule of thumb to use is that, in order to compare an LED light source to a traditional light source, one must reduce the lumen results of the traditional light source by up to 50% or greater to account for brightness lost getting to the observer’s eye. I call this “Usable Lumens.” Who knows, maybe the industry will pick this term up to help better explain how two lights compare to each other. Either way, it is in part because of this difference in “Usable Lumens,” that a traditional light rated at 20,000 Lumens would be observed to be less bright that a 10,000 Lumen LED light under the same conditions.
Another common way to measure light intensity that is often used in regulations such as OSHA (Occupational Safety and Health Administration) standards is the unit of measure foot-candle. I’ll discuss the difference between this and a Lumen in a future post. In the meantime, you can learn more about the latest in LED light fixtures at our website, www.LED-ltd.us. Please post a comment or question on the blog, and we’ll get back to you as soon as possible.