by Bob Hawkins

My name is Bob Hawkins, and I am the Manager of Applications Engineering at The Lighting Agency. I have done lumen counting for TLA for over 20 years, and this week I’d like to touch on the topic of Light Trespass or Spill Control.

Input for site lighting design can come from many sources; ownership with experience, the Illuminating Engineering Society of North America, and local municipal codes being foremost among them.

Many municipalities have instituted, with forethought or not, rules and regulations that concern the concept of light “trespass,” whether or not they address the values within the lot itself. Many of the items written into Laws of Man may or may not even be possible with regards to the Laws of Physics when you try to meet all the requirements that can be written into such documents.

For example, area lighting (of pavement, for car movement) with a required uniformity or minimum foot-candle level occurring at, or very near to, a property line. These two requirements CAN end up conflicting with each other.

To reduce the conflict, we do have at our disposal some VERY good tools. By that, we mean the use of optical distributions that can radically reduce light thrown, say, behind a fixture.

The DSX series of fixtures from Lithonia Lighting ™ have, within their many optical offerings, a couple of different distributions to aid in fighting the dreaded spill light. The first and foremost is the BLC. Stands for, you guessed it, BackLightControl.

D-Series Area Size 1 – DSX1 LED Area Luminaire

D-Series Area Size 1 – DSX1 LED Area Luminaire

These optics use active lensing on the LED’s themselves and then a series of baffles to allow usable forward light, but radically reduce the light “spilling” behind the pole. The concept of a house side shield is in play here, but you can’t add anymore shielding to the already designed baffles. I’ve seen folks try to specify both, not allowed.

This is the BLC. The iso-contour lines represent .5fc, 0.25fc, and 0.1 fc. The 0.1fc value is called out in many codes as the number to be at or below at a certain point. In many cases that value is to be hit by 10’ beyond a property line. SO, you can see that 10’ back from the 20’ tall fixture (points 5’ o.c., lines 10’ o.c.) and 71w of power, we can hit that restrictive value while still providing really usable light in front of the fixture.

Two other distributions that come in very handy when approaching a corner or kink in the line that shall not be passed, are the RCCO and LCCO. Those can sound confusing, but they are Right Corner Cutoff Optics and Left Corner Cutoff Optics. The name describes the direction of BLOCKED light, not the throw of the light itself. Hence, the LCCO will cut off light to the left of the fixture as seen in the plan view below:

Same 20’ tall unit, same great limiting behind, but then we’ve also managed to CUTOFF the LEFT (LCCO) side in this case of the throw, also only 10’ away from essentially no light to the left of the fixture.

RCCO, same thing opposite direction:

Spill light can also reference how much light is emitted upward from a fixture. Any restriction here can be addressed by utilizing a fixture with a “U” rating of 0, meaning no uplight. The “U” comes from the current system of B-U-G, with nothing to do with insects. Backlight, Uplight, Glare. A 0 rating means no light is emitted upward. A full discussion of B-U-G rating is available in the IES TM-15 document.

If you would like more information about any of the products, manufacturers, or documents mentioned in this post, please email Bob Hawkins or your TLA Sales Rep.