Willis Lamm's
Traffic Signal Collection

  Traffic Signal Bulbs
The effectiveness of a traffic signal relies a great deal on proper placement and visibility. Before the advancement of LEDs, all signals relied on incandescent bulbs. The clarity and effectiveness of the signal display was dependent upon the proper bulb wattage and bulb design in order to gain maximum optical output from the reflector and lens system. Wattage was somewhat limited to the amount of heat generated in a confined space, the need to prevent the signal from being overly glaring at night, and concerns over overall power consumption. In addition, in the days before use of multiple signal heads for each movement, a burned out bulb could create a significant safety hazard, so bulbs designed for long life was critical. Therefore a great deal of engineering was dedicated to getting the best light from traffic signal devices.

One critical factor was known as light center length or LCL. LCL is the distance from the bottom of the bulb's base to the filament. The filament had to be in the correct location for a particular reflector design to project the bulb's light correctly onto the lens. For 8-3/8 inch reflectors the correct LCL is 2-7/16 inches. For 12 inch reflectors the correct LCL is 3 inches.

Additionally, in an effort to reduce sun phantoms (lenses appearing to be lit due to early morning or late afternoon un) most lenses had beads on the inside designed to primarily pick up light from below the bead. Older bulbs had horseshoe shaped filaments. It was necessary to adjust the bulbs so that the horseshoe was oriented like the letter "U" (closed at the bottom) to properly light the lens and eliminate a dark shadow on the bottom. Many newer bulbs have "M" shaped filaments (or "W" shaped filaments, depending on how you look at the bulb) filaments that reduce shadowing if the bulb isn't correctly oriented. In addition more modern bulbs also had small reflector discs below the filaments to eliminate a center "dark spot."

L. Horseshoe filament, R. "M" filament.
Reflector Disc.
Relative shapes and sizes of common signal bulbs.
Bulb Identifications:
150 Watt
SP (Note 1)
40 Watt
51 Watt
69 Watt
69 Watt
Duro Test
60 Watt
Sylvania (Note 2)
67 Watt
Duro Test
67 Watt
68 Watt
Hytron (Note 3)
69 Watt
116 Watt
Duro Test
135 Watt
165 Watt


1. Used in 1, 2 and 3 bulb Darley signals where a single bulb lit four lenses.

2. Energy saver version of 69 watt bulb - using a different filament composite to provide same light with less wattage and still maintain long bulb life.

3. Extra long life bulb, guaranteed for two years.

Common Signal Bulb Chart

40 Watts - 410 Lumens 100 Watts - 1080 Lumens
60 Watts - 550 Lumens 116 Watts - 1260 Lumens
67 Watts - 610 Lumens 135 Watts - 1570 Lumens
69 Watts - 630 Lumens 150 Watts - 1900 Lumens
90 Watts - 1040 Lumens 165 Watts - 1950 Lumens

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