This article describes measurements for outdoor lighting and proposes a standard system to improve safety of night lighting and its utilization of energy. 

Background
Visible light measurement revolves around a Photopic standard developed in 1924 by the CIE.  Photopic measurement is based upon Human retinal response to visible light centered about 560 nanometers and energy  >3 cd/m². 

Lumens, footcandles, candela, etc., are Photopic.

Lower than the Photopic level, retinal color sensitivity begins to shift until at very low light energy, our visible spectrum peaks at the 510nm (blue) region, defined as our Scotopic response. 

Application	Luminance cd/m2	Visual Response
Interior Lighting	> 3	Photopic
Outdoor Lighting	0.001 >><< 3	Mesopic
Starlight	< 0.001	Scotopic

The area between is known as Mesopic, in which nearly all outdoor lighting falls.  Mesopic luminance is defined by CIE, and in Figure 1 we have added the dimension of color.

The user community faces a lumen legacy; lumens rule over footcandles and seemingly less important is visual effectiveness.  Despite the lumen overdose, numerous studies on Human peripheral vision and motor response time show that night lighting is often spectrally inadequate.  Standards are needed to improve safety and get the most from energy dollars.

Proposed
The LEM (Lumen Effectiveness Multiplier) correlates Photopic data to the Mesopic scale. 

We can quantify the effectiveness of a light source as compared to an established reference light source with Mesopic Color.

Figure 2 illustrates the Mesopic measurement of two light sources: High Pressure Sodium spectrum against a Metal Halide spectrum. 



Table 2 shows a sum of six comparison samples from Fig. 2.  MH is 7.8x more effective than HPS

In the Figure 2 comparison using a Mesopic graph, a MH source has considerably greater effect on our vision.  Table 3 compares effectiveness of other lighting with HPS.

        For example, at the same luminance:
         1 = same effectiveness of HPS
         6 = 6 times more effective than HPS
      0.9 = 10% less effective than HPS

The LEM could apply to any Photopic measurement so that lightmeter readings and Isofootcandle or Isolux charts may be factored to predict the visual effectiveness for that given light source.

The matter of night lighting effectiveness has become very pronounced with new technology lighting sources.  Now that Light Emitting Diodes are bright enough for illumination and those systems can be tailored to produce light in the spectrum most necessary, measurement systems must catch up. 

HPS produces more lumens per watt than nearly any other outdoor lighting source.  But, if you can't use those lumens in the dark, where's the bargain?

This article is derived from many published papers on the subject and is a response to a need in the industry.  The approach shown allows a Standard to be drafted while leaving room for evolving lighting sources.  We encourage your input.

Also see:  New Look