1986 Tesla SHRP 210W

Before the advent of dedicated sodium lamps, horticultural lighting was catered for with high-pressure mercury lamps, with some companies designing compact reflector lamps specifically for this application. One popular format was the elliptical lamp half-coated on its neck-side with layers of reflective a fluorescent materials so as to direct light downward and increase the red content of the emitted light. The small physical size of elliptical jackets makes the lamp unobtrusive, which is important in greenhouse so as to ensure that as much daylight as possible can reach the plants.

Although mercury lamps proved more effective than either incandescent or fluorescent lamps in this application, they have a too hight optical output in the UV and IR, which can damage plants at high flux densities. It is therefore only logical that the more efficient high-pressure sodium lamps became an interesting alternative soon after their introduction in the market. During the 1970s most new greenhouses were fitted with HPS lighting systems while the existing ones kept on using mercury lamps. At that time the newer sodium lamps and gears were significantly more expensive, which prevented the widespread adoption of this technology, while there was no sodium retrofit solutions dedicated specifically to plant lighting applications.

This latter issue was addressed in the early 1980s by Tesla of Czechoslovakia. To this end the stated-owned company developed and released a compact reflector sodium lamp based on its neon-filled SHP retrofit lamp platform. The resulting SHRP 210W shown here is thus the first horticultural sodium lamp and because it could operate on mercury lamp inductor ballasts, it enabled the upgrade of existing mercury lighting installations at a limited cost.
This lamp is built around the Tesla’s five-piece sodium burner, provided with white ceramic end-plugs designed to reduce the seal temperature via radiative cooling (this large-grained material has a higher optical emissivity than densely sintered PCA). A capacitive antenna consisting of a molybdenum wire coiled around the burner ensures the reliable ignition of the lamp at 220 V mains voltage. Since there is no need to convert UV radiation into visible light, the lamp is coated only with a simple layer of light-reflecting titanium dioxide.

Although the SHRP was a very useful lamp in its application, the use of a neon fill gas in place of the usual xenon fill has several unfortunate consequences. Not only does this causes a lower lumen efficacy, it also reduces the lamp’s optical output in the blue-green end of the spectrum. The latter can affect plant growth negatively, resulting in leaf and stem atrophies. The full benefits of the sodium lamp technology in plant lighting applications were eventually realized with the development in the late 1970s of efficient HPS sources with an increased xenon fill pressure. This improvement in the technology raised the efficiency and lumen maintenance of sodium lamps while the increased density of xenon atoms in the sodium discharge enhances the emission of blue-green light. Philips went one step further in the late 1980s by decreasing the sodium-to-mercury weight ratio of the amalgam in the burner so as to boost the optical output in the short-wave end of the spectrum, resulting in the introduction of the SON-T Agro in 1990. From this point on these highly efficient sodium lamps became popular in general and plant lighting applications and mercury lamp installations in greenhouses were quickly replaced with new HPS lighting systems.