1979 Sylvania H39KC-175/N

The H39KC-175/N shown here is Sylvania’s first generation of “Warmtone” mercury lamp, first introduced in 1975. The production of warm-white light is realized thanks to a unique blend of yttrium vanadate and yttrium aluminum garnet (YAG) phosphors, complemented by an extra layer of blue-absorbing material (possibly cadmium silicate) so as to lower the color temperature to the 2900–3000 K level of warm-white fluorescent tubes. This H39KC-175/N was manufactured in Canada using an old coating technique that left the bulb crown free of phosphor. This measure was employed until the late 1970s so a to prevent the scraping and release of phosphor when the frame is inserted into the lamp during assembly. Any deposition of this material onto the burner surface results in premature blackening as a result of thermal decomposition. Refinements in the frame construction and in the coating technique introduced in the late 1970s made this clear-crown design unnecessary, and fully coated BT-shaped lamps then became the norm. Another detail of interest is the E40 screw cap, which clearly indicates that the lamp shown here was made for the export market.

The success of this particular lamp type was hampered by a critical lack of output stability. Before 1975–77, Sylvania’s first-generation yttrium vanadate phosphor was much less stable at high temperature than the superior yttrium phosphovanadate material introduced by Philips in 1969. As a result, early Deluxe White fluorescent mercury lamps (i.e., /DX type in the American nomenclature) made by Sylvania suffered from rapid color and output depreciations. This problem was exacerbated in the company’s /N lamps because of the strong blue absorption caused by a high YAG content (over 10 % of the phosphor) and the additional yellow filter, which resulted in a significant green shift of the emitted light color over time. This degradation proved particularly problematic in commercial and retail lighting applications due to the contrast with the far more stable light color of fluorescent tubes. As a result, Sylvania’s Warmtone mercury lamps of the first generation had to be replaced well before the end of burner life.

Following the improvements made on the stability of its ytttrium vanadate material, Sylvania began upgrading its /N lamps in 1977 with a new phosphor blend of 2nd-gen yttrium vanadate and magnesium fluorogermanate, a design similar to that introduced by Westinghouse in 1972 (except for the use of an yttrium phosphovanadate material and a silica pre-coat). Sylvania's new Warmtone mercury lamps delivered a more stable light color which was specified for a use in combination with incandescent lamps this time. Interestingly, some of the first-generation lamps were still manufactured until 1979, probably to finish stocks of the original blend of phosphor plus filter material. In 1980, Sylvania replaced both generations of its /N lamps with a new 3rd-gen design featuring an improved blend of yttrium vanadate plus YAG without the color filter, which boosted the initial light output from 7.0 to 9.8 klm and raised the average output from 5.60 to 7.56 klm. This lamp type was eventually discontinued during the 1990s as a result of the increasing popularity of greatly superior compact metal halide lamps.