1988 Norma H4 T.P. 75/70W

Incandescent lamps for automotive headlights come in different designs and specifications depending on the luminaire's optics and the intended usage. The H4 is a very common type of bi-filament tungsten halogen headlight lamp whose typical wattage for standard road usage is 55/60 W on 12 V circuits. The Norma H4 T.P. shown here is a special variant, made by Philips for the French market (Norma was a French automotive lamp manufacturer before it was assimilated by the Dutch) with 70 and 75 W coiled coil filaments designed for an operation on the 24-V circuit of diesel-powered vehicles. Because of the lower efficacy of the thinner filament, the light output is only 20/15 % higher than that of the standard 12-V 55/60 W model and is still street legal. T.P. in the reference stands for Travaux Publics, which in French means "public works". This mention refers to a lamp provided with vibration-resistant filaments that can withstand the rough operating conditions on construction work equipments.

Since this particular model was intended for the French market, it is also provided with a yellow filter in the form of a globe made of cadmium glass. Headlights in this country had to be colored yellow until 1995, when France began applying EU regulations in this matter and clear white headlights then became the norm. Typical of H4 lamps, the two filaments are offset along the lamp's axis and a reflector cup is placed next to the topmost filament so as to properly define the cut-off profile of the projected low-beam pattern. To reduce glare and stray light the tip extremity of the tubular halogen capsule is painted black.

Back then Philips had an interesting way to ensure that the filaments would be properly aligned in the bulb during the production process. First, the geometry of the filament assembly was fixed by two quartz rods fused together along their length, trapping the three lead wires. Next, the assembly, including the moly foil seals, was inserted in the quartz bulb and after the filaments were properly positioned using machine vision, the quartz rods were fused to the bulb using two narrow flames. Finally, the bulb was sealed and variations in the pressing of the quartz tube on the moly foils and lead wires had no incidence whatsoever on the location of the filaments. Interestingly, such construction contributed to the robustness of the lamp, especially its resistance towards shocks and vibrations.