Trespass Issues

Energy Usage


Doesn't reducing unwanted light save energy?

The electrical energy consumed by a typical outdoor luminaire represents its single largest expense-- more than the initial price and maintenance costs combined. Selecting luminaires that direct more light in the intended direction, and less into the sky or toward neighboring property often can reduce lighting energy.  Reducing wasted light saves energy.

Point of Diminishing Returns

Eliminating all spill light, is not always the most energy effective choice. Limitations imposed on luminaires to meet this goal may produce the following results:

  Decreased efficiency (how much light actually gets out)
  Reduced pole spacing, increasing liminaire quantities
  More light directed to the ground and less on vertical surfaces (most of what we see at night is vertical surfaces)

The table below, illustrates this point.  It compares two lighting designs for a typical parking lot.  One design uses full cutoff luminaires, while the second employs fixtures that meet cutoff criteria.  The difference in performance of the two is that the cutoff luminaire emits a small amount of light at and above horizontal (less than 0.1%).  The resulting difference in performance is significant.  In this example, eliminating the last portion of uplight increases energy consumption by 24%.

Full Cutoff


Qty. of Luminaires 89 72
Total Power (KW) 41 33
Illuminance (fc) 4.2 horizontal

1.4 vertical

3.5 horizontal

1.7 vertical

Uniformity (max/min) 4.6 3.0
Sky Glow (fc) 

@ 50 ft. including 10% surface reflectance

0.3 0.3

400 watt metal halide luminaires mounted at 30ft.

  The above comparison highlights the need for a comprehensive lighting design to be multidimensional, to make intelligent choices addressing objectionable light, visibility, initial costs and operating expenses.