In today's financial climate, we start to look at ways to save resources.  Although we've known for many years that outdoor lighting wastes billions of dollars by sending unnecessary light to the stars and planets of our Solar System, but too few have done enough about it. 

Even if every lighting installer knew the rules to avoid light trespass and would take the steps to conserve, they are still limited by technology.  For more than 70-years lighting experts have known the response curve of Human vision, and for nearly 40-years, the leading lighting suppliers have furnished lighting that emits a color misaligned with Human vision.

Why?    High Pressure Sodium (HPS) lighting gives great light-meter readings for the electrical energy consumed.  Can anyone offer another reason?

The following graph of Figure 1  plots our day and night visual response, and below that, Figure 2 aligns the spectral output graph of a typical High Pressure Sodium lamp. 

We can put aside any argument for Scotopic color because HPS is nowhere close to that curve.  While 50-nanometers may seem a small amount, it places the peak of HPS at only 40% on the response curve.

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. 

Comparing two colors in the Photopic range, 560nm and 625nm for example, 625nm will be only 30% as effective as 560nm. 

Comparing 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.  Projecting a line from the HPS peak up through the Photopic graph, we see that HPS is only about 40% as effective as a light color of 555 nanometers.

Using HPS is a shotgun approach; 60% of the power put into it is just to get the 40% effect.


Most Lighting Engineers Understand -
At luminance levels of less than 3 CD/m² (Mesopic on the way to Scotopic), 80% of HPS lighting is wasted.  HPS lighting is used predominantly to meet footcandle standards, it is not used for quality illumination. 

Producing light outside our visual range wastes energy and pollutes the otherwise dark night sky.

That's no bargain.