Lighting and the Human Eye

Part 3 of 3
The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute (RPI) in Troy, NY is using a Unified System of Photometry to design more energy efficient lamps, including LEDs, without sacrificing perceptions of visibility and safety.
Unified System of Photometry       

At LRC, Researchers measure how the eye SEES light, and NOT how Light Meters measure light. Using this new system, it is estimated that about half of U.S. streetlights could cut energy consumption by about 50 percent – annually saving one billion kilowatt hours.

With the Unified System of Photometry , future US street lights should be selected on the basis of how the eye sees light with Visually Effective Lumens (VEL’s) , and not on the basis of conventional ‘ Light Meter’ lumens.

Question: Why are there two types of lumens - aren’t all Lumens equal?

Answer: If one considers how the human eye perceives and responds to light, and how a Light Meter in a Test Lab perceives and measures light, the answer is “No.”
Image: The Midnight Sun, Anda Berczky, 2005
As shown in the Midnight Sun images above, the human eye sees differently throughout the day as the Kelvin color of the daylight changes with the time of day.
A Light Meter can measure 1 lumen of High Pressure Sodium (HPS) light at 2000 Kelvin.

The Same Light Meter can also measure 1 lumen of light at 5000 Kelvin.
" The Light Meter reports that both lumens are the same. But the human eye works differently from a Light Meter, and “sees” in the lower Kelvin light very differently than in the higher Kelvin light. To the eye, objects seen in the 5000 Kelvin light seem far brighter. "
With the Unified System of Photometry , Researchers measure how the eye Sees light (VEL’s), and NOT how Light Meters Measure light.

With the Scotopic/Photopic (S/P) Ratio Conversion Chart, lumens measured with a light meter, can be converted into VEL lumens so that the data provides a measurement of how well the eye can actually see.

The Unified System of Photometry unites these two different lumen systems into one lighting measurement standard.
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
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TJ-52 11.30.18