Street lighting is changing.
Lighting technology is advancing with better efficiency, lower maintenance, and more environmental friendliness. Our understanding of nighttime lighting is better than ever in the past 50 years, and so we will save energy, and will improve nighttime visibility at the same time.
Experts have studied the effects of lighting on our night vision, and they direct us back onto the lighted path. The lights in your neighborhood will soon be whiter, or maybe more blue. Will they be Metal Halide, or Induction, or perhaps Light Emitting Diodes? Today's high output LEDs and other bluer lighting technologies offer greater savings and safety because they can provide better visual acuity than the popular HPS (High Pressure Sodium) with LESS lumens. Not only will this save energy, it means darker night skies. The down side... Lighting technology is emerging but standards setting bodies are lagging behind.
Background
In 1992 Sam Berman at Lawrence Berkeley Laboratory1 documented his research on the effects of color on our nighttime vision. Dr. Berman's work illustrated that a bluer light source was actually more effective per watt of power than the "efficient" orange-yellow sodium light sources. Further, he demonstrated that some light spectrums were actually worse than darkness, they upset our eyes' normal dark adaptation. His work is quite revealing.
In 1995 Mark Rea at Rensselaer Polytechnic Institute2 documented his studies similar to Berman, and he charted the efficacies of modern light sources. In this study he demonstrated peripheral vision deficiency as a result of luminance and color. Dr. Rea's conclusion is that nighttime light measurement is quite different than daytime, and he coined a phrase 'Nighttime Lumen'; however, the lumen already is a defined unit of measure. His direction is illuminating.
Perhaps the most enlightening (or disturbing) research was documented in 2000 by Ian Lewin at Lighting Sciences Inc.3 Dr. Lewin studied the effects of High Pressure Sodium lighting on Human peripheral vision. In his paper Lamp Color in Outdoor Lighting Design6, he compares nighttime vision under sodium lighting as driving a motor vehicle while looking through 50-inch long tubes; as having almost no peripheral vision. (This tracks Dr. Rea's findings of 5-years earlier.) Lewin demonstrated that at lower luminance levels, our visual acuity and reaction times are worse under Sodium lighting than under a bluer light. Lewin's work is a real eye opener.
Under HPS light, motorists are driving with blinders.
Remember that the next night you are standing in a pedestrian crosswalk beneath HPS lights. Drivers with perfect vision take their peripheral vision for granted.
The irony - nighttime luminance criteria has been based on the wrong color, and has given a false impression of safety. Can the protective shield against tort liability deteriorate in the absence of timely and proper standards?Getting back to basicsWhat natural characteristic makes something more or less visible to us? Answer: BRIGHTNESS Putting it together Lewin's proposal4,5 is that we use a multiplier to show the actual effectiveness of a light source based upon its color and application. |
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Task, Energy, Response & Color
Table 1 | ||||||||||||||||
Our sensitivity to color does change, moment by moment, with varying levels of illumination, which we perceive as brightness. Table 1 shows three distinct scales of this variance.
When luminance is greater than 3 cd/m2, our "daylight vision" peak sensitivity is 560nm, this is the Photopic scale. For less than 0.001 cd/m2, our color sensitivity peaks at 500nm, the Scotopic scale. Between these is the Mesopic scale, expanded in Figure 3 below, showing luminance and a color spectrum. Most outdoor lighting aligns with the center of this scale at 0.1 cd/m2.
Figure 1 | |
Mesopic Color Spectrum Figure 3 |
Figure 2
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Unfortunately the HPS light of Figure 2 does not align with either of the Human visual response curves shown in Figure 1.
All of the HPS light is higher than 575nm, which will appear as less than 1/5th as bright as a bluer light source.
What it means Relative brightness of an object may be plotted with the CIE graph in Figure 1. Given two objects illuminated in the Photopic range, 560nm and 625nm for example, 625nm will appear to be only 30% as bright as 560nm. Given two objects illuminated in the Scotopic range, 500nm and 575nm for example, 575nm will appear to be only 20% as bright as 500nm. |
Knowing the typical illuminance levels for a given task, and knowing that some lighting is more effective than others for such tasks, it seems reasonable to provide lighting that aligns with the task for the most efficient utilization of electrical energy and require the least amount of light.
This HPS 'shotgun approach' wastes billions of dollars and pollutes our night skies!
An object's brightness makes it more or less visible. At Mesopic luminance, our eye is less sensitive to the red spectrum and is about 6 times more sensitive to blue. Our visual response is logarithmic, so we can't easily place a number on these comparisons, but Human Factors response time tests have done just that.
Table 2 |
Table 2 compares several lights to the popular HPS. LED and MH are 7 and 8 times more effective than HPS. HPS produces almost 20% more lumens per watt than MH; but, it's 80% less effective. Mercury Vapor not shown due to environmental obsolescence. |
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Conclusion: At night, Color Temperature has a strong role in visual effectiveness. Now we know that bluer light sources can be much lower wattage to save more energy than HPS, and, bluer light supports peripheral vision, critical to night driving safety.
See more about Lumen Effectiveness Multiplier
Less lumens means darker skies. Get ready for a new look at night.
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