Nighttime Visibility vs Light Pollution

Definition
Visual
Response
(Color)
Luminance
cd/m2
(Brightness)
Interior
Lighting
Photopic
(grn - red)
> 3
(Daylight)
Outdoor
Lighting
Mesopic
(blu - grn)
0.001 > < 3
Starlight
Scotopic
(blue)
< 0.001
Color range we see best


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.
Human Visual Response
Figure 1

Spectral Output of HPS
Figure 2

The HPS light spectrum does not align with the Scotopic or Photopic response curves. 

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
Mesopic Color Spectrum
Figure 3

Also see:  New Look and Lumen Effectiveness Multiplier