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Part 3- Everything you Need to Know About Lumens

Last month, we were shocked when we saw that the light was being directed to the floor. Today, we will finally find out why. The LED chips producing the light inPart3.jpg the lamp radiate from a single side of a flat surface. The radiance of this flat surface is not uniform, but will vary with viewing angle. The brightest irradiance can be seen looking directly at the radiating surface. As the viewing angle changes by looking at the radiating LED surface from the side, the amount of radiated light seen will reduce, to 50% at a 60 degree viewing angle, and to zero when viewed edge on.

This happens on one side, while the back side radiates no light at all. No 1.jpgwonder the light does not look the same! There are two different factors controlling how the radiated light from the LED chip is being distributed. We remember that the variation in radiance with viewing angle is called the Lambert’s Cosine Law. The figure to the right summarizes Cosine Law Radiance.

So, with such a lopsided radiance pattern, how can the lumen output of the LED lamp be the same as the 60 watt incandescent bulb? Becuase the lumen output  is simply the total light produced or radiated. Period. There are no directions or viewing angles specified in the lumen rating.

The following two pictures show the illumination patterns of a single, open LED lamp and an equivalent output incandescent bulb. The LED illumination pattern is clearly more directional.

F2.jpgortunately, the manufacturers of LED lamps have realized the limitations of directional task lighting for general illumination, and have produced lamps with the LED chips facing in several directions. This arrangement provides illumination that more closely resembles that of the older incandescent bulbs, and provides a more pleasing light.

One of the more popular arrangements is a circular or ring configuration as shown in the following figure. Here the directional characteristics of the individual LEDS blend together to provide a more omni-directional illumination.

As for the 3.jpgbirthday celebration, the LED lamps with the omni-directional lighting characteristics provided a distributed and more pleasing room illumination. The party was a success and Dande enjoyed every moment, especially the large, glowing balloons…amazing.




Written by: Ron Hughes

Ron has been with the engineering department of Ericson for 14 years. He has previously worked at Hubbell and Square D Company. He has a degree in engineering from The Indiana Institute of Technology. Ron has been married to his wonderful wife, Sandy, for 43 years, and they have one son together. In his free time, Ron likes to spend time with his Golden Retriever and taking nature photography.

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