In order to limit light spill into the environment, more streetlights are available with downward light distribution (most light aimed below horizontal), limited glowing surfaces and tightly controlled optics. For LED fixtures, the directional nature of the light source facilitates downward light distribution. Some fixture styles such as post-top fixtures accomplish the same goal by offering more “hat” or top cover options to minimize uplight into the night sky.
Optical System Design
The technical design of the optical control system determines the distribution of light emitted from the fixture and influences the fixture performance. The optical system is often hidden inside the fixture housing and sometimes supplemented by an outer lens. The optical controls -- reflectors, lenses -- in a light fixture shape the photometric light distribution that emanates from the fixture. Reflectors are commonly used to bounce light inside the fixture towards the aperture. The geometry of a reflector is designed to create a fairly specific light spread. Lenses, either within the fixture at the light source or outside over the aperture, are used to diffuse, refract, or otherwise shape the light.
Standard streetlight fixture classifications have been created by the Illuminating Engineering Society of North America (IESNA) to help identify fixture performance. The classifications are based on the shape of light delivered to the roadway surface. There are five standard shapes and their classifications are designated in Roman numeral form as Type I, II, III, IV and V.
IESNA Cutoff Classification System
The IESNA also has created two standards for describing the distribution of light exiting a streetlight. One of the two standards, the most well-known, is the IESNA Cutoff classification system, which has been in use since 1963 (updated in the 1990’s). It describes four types of fixtures – Full-Cutoff, Cutoff, Semi-Cutoff, Non-Cutoff. The IESNA Cutoff classification system is based on the maximum intensity of light (in candelas) and the percent intensity at high angles.
In a Full Cutoff fixture, there are 0 candelas at 90 degrees above nadir (horizontal), and the candelas per 1,000 lumens is less than 10% at 80 degrees above nadir. Full Cutoff fixtures emit no light at or above 90 degrees from nadir. In a Cutoff fixture, the candela per 1,000 lumens does not exceed 2.5% at 90 degrees above nadir, and the candelas per 1,000 lumens is less than 10% at 80 degrees above nadir. A typical characteristic of these fixtures is that the glare from the lens or reflector essentially disappears when viewed from a distance because most of the light is directed downward. Another characteristic is that their beam spreads are narrower than Semi-Cutoff and Non-Cutoff fixtures that are typically found in existing streetlight applications -- Full Cutoff and Cutoff streetlights require closer spacing, therefore more fixtures, to provide adequate light coverage on the roadway.
IESNA “BUG” Classification System
In 2007, the IESNA introduced their second standard for describing the distribution of light exiting a streetlight. It was originally called the Luminaire Classification System (LCS), but subsequently updated to Backlight, Uplight, and Glare “BUG” Ratings. The BUG system is a way to evaluate optical performance related to light trespass, sky glow, and high angle brightness control in terms of the quantity of light (lumens), as opposed to the percent intensity described by the Cutoff classification. In the BUG system, the optical performance is accessed in terms of an imaginary hemisphere around the light fixture that is divided into zones.
The Backlight (light emitted behind the fixture) rating of the fixture describes the amount of light emitted in the zones behind the fixture. The Uplight (a source of artificial sky glow) rating describes the amount of light emitted in the zones of the upper hemisphere. The Glare (annoying or visually disabling brightness) rating describes the amount of light emitted in the zones at the front.