Review the effectiveness of Sodium lighting for an outdoor application. The far right column of the table shows a luminance range for each of 3 categories.
The interior lighting category includes daylight, stadium lighting, or other high brightness applications.
The peak of our color sensitivity shifts when luminance changes; with less light, we can actually see less of the red spectrum and we're more sensitive to the blue.
The following graph of Figure 1 plots our day and night visual response.
When luminance is greater than 3 cd/m2, our "daylight vision"
peak sensitivity is 560nm or green. For less than 0.001 cd/m2, our color sensitivity
peaks at 500nm, or blue. Figure 3 expands on that range in between. | |
Figure 1
Figure 2
The HPS light spectrum does not align with the Scotopic or Photopic response
curves.
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What it means
Relative effectiveness of a color may be plotted with the CIE graph in Figure 1. Given two colors in the Photopic range, 560nm and 625nm for example, 625nm will be only 30% as effective as 560nm. Given two colors in the Scotopic range, 500nm and 575nm for example, 575nm will be about 20% as effective as 500nm. The HPS plot in Figure 2 shows that all of its light is higher than 575nm, which at low luminance levels is less than 20% as effective as a bluer light source. Most Lighting Engineers Understand - At luminance levels of less than 3 CD/m2, 80% of HPS lighting is wasted. Producing light outside our visual range wastes energy and pollutes the otherwise dark night sky. That's no bargain. HPS lighting is used predominantly to meet footcandle standards, it is not used for quality illumination. 
Mesopic Color Spectrum Figure 3 |
To benefit from the efficacy of HPS lighting, luminance must be at least 3 CD/m2.
This often requires more lumens which requires more energy, which offsets the perceived
savings, and then ... |
Also see: New Look and Lumen Effectiveness Multiplier