Light Output - Defined

Luminous flux is the term to describe the total output of light from a light source. The unit we use for luminous flux is lumen. The lumen rating for a lamp is the total output of light in all directions from a lamp. It does not take the directionality of light into consideration. Furthermore lumen is not the total energy radiated from a light source but is the energy that falls within the range of human vision. Ultraviolet and infrared energy are also produced by most modern light sources and the human eye can not see that energy. The Illuminating Engineering Society defines light as "visually evaluated radiant energy." The graph below shows the human visual response curve.

Human Visual Response Curve

Our vision is not equally sensitive to all the wavelengths that comprise vision. Therefore lumen rating is based on our visual response. Our visual system responds more strongly to energy toward the center of the spectrum, the green-yellow area of the spectrum. We respond less strongly to the blue and red area of the spectrum. Consequently lumen output is affected more by central wavelengths than by wavelengths from either end of the spectrum. The end wavelengths are important for color but are not so important for lumen output.

In a directional lamp it is important to know how much light is going in one direction, and lumen output does not define that for us. We need to know the density of lumens contained within the cone of light. This density is know as luminous intensity, and the unit we use to describe this is candela or candlepower. More about directional light in a future post.

It is important to know about lumen depreciation. Manufacturers publish data on initial lumen output of their lamps. Usually this data reflects lumen output after 100 hours of operation. Lumen output gradually decreases as lamps operate beyond the initial 100 hours, and this decrease is know as lumen depreciation. The reason for the decrease in incandescent light bulbs is because the tungsten filament evaporates and/or the capsule darkens, and in fluorescent lamps it is due to phosphor depreciation and changes in the lamp cathodes and electrical properties.

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