Programmable Visibility Signals of Traffic Lights
Signals such as the 3M High Visibility Signal and McCain Programmable Visibility signal, utilize light diffusing optics and a powerful fresnel lens to create the signal indication. Lit via a powerful 150W PAR46 sealed-beam lamp, the light from the lamp in these “programmable visibility” signals passes through a set of two glass lenses at the back of the signal. The first lens, a frosted glass diffusing lens, diffuses the light into a uniform ball of light around five inches in diameter. The light then passes through a nearly identical lens known as an optical limiter (3M’s definition of the lens itself), also known as a “programming lens”, also five inches in diameter.
Using a special aluminium foil-based adhesive tape, these signals are “masked” or programmed via the programming lens so that only certain lanes of traffic will view the indication. At the front of these programmable visibility signals is a 12″ fresnel lens, each lens tinted to meet United States Institute of Transportation Engineers (ITE) chromaticity and luminance standards. The fresnel lens (3M utilized a square lens masked to create a ball indication, McCain used a circular lens itself) amplifies the light output created by the lamp, and creates a uniform display of light for the lane in which it is intended. These signals were first developed by the 3M Company in the late 1960s, and were wildly popular in the late 1970s as traffic density increased.
Typical applications for these signals were skewed intersections, specific multi-lane control, left-turn pocket signals or other areas where complex traffic situations existed.