Definitions and Recommendations

Definitions

Adapted from IDA's Information sheet #9:

FULL-CUTOFF FIXTURE: a luminaire having a light distribution where zero candela intensity occurs at an angle of 90 degrees above nadir, and at all greater angles from nadir. Additionally, the candela per 1000 lamp lumens does not numerically exceed 100 (10 percent) at or above a vertical angle of 80 degrees above nadir. This applies to all lateral angles around the luminaire. This kind of luminaire emits no light above the horizontal.

DIRECT LIGHT: light that leaves a fixture and travels directly to a location, in contrast to light that is reflected from e.g. a building surface or ground. The full cutoff definition is (neglecting the 80 degree rule) roughly equivalent to saying that no direct light shines at any angle above 90 degrees above nadir.

RPI has a page with more definitions.

Recommendations

Only FULL-CUTOFF FIXTUREs shall be installed for exterior lighting. All fixtures shall be installed in the proper orientation so as to emit no light above horizontal when installed.

Any exterior lighting fixture with an INITIAL DESIGN LIGHT OUTPUT of more than [VALUE], in addition to being FULL CUTOFF, shall be shielded so as to shine no DIRECT LIGHT onto any STREET or any other LOT. Appropriate values might be 100 lumens for direct light trespass onto conservation land, 4000 lumens for direct light trespass onto residential and recreation conservation zones and perhaps 10000 for trespass onto commercial/industrial zones. This is a stricter requirement than full cutoff, which allows light to shine sideways onto other properties, and down onto properties at lower elevations. Note that we suggest the values be according to the LOT receiving light, rather than the LOT generating it.

No exterior lighting fixtures with an INITIAL DESIGN LIGHT OUTPUT of more than 4000 lumens may be installed. (This might be appropriate in residential zones, and allows fixtures with two 100-watt incandescent bulbs.)

Any exterior lighting fixture installed within a setback of 25 feet from lot lines must cast no DIRECT LIGHT onto other LOTS or any STREET, and must not cause an illuminance of more than 0.2 lux on any other LOT.

All exterior lighting on [the lot in question], taken together, must not caues an illuminance of more than 1 lux on any other LOT, or more than 0.1 lux at any point 40 feet onto any other LOT, or more than 0.01 lux on the surface of any dwelling.

Outdoor lighting should be limited to the places where it is necessary, the times when it is necessary, and to the levels required to meet articulated needs. (In the past, lighting has been installed that produces far more light on the ground than is necessary to see e.g. to walk to parked cars, as well as light shining into the sky, and such lighting is operated dusk to dawn.)