1964 Philips LL93110E (91 W)

In the early 1950s Philips introduced a series of compact spectral lamps with a short discharge length and a high intrinsic brightness, useful for scientific and technical purposes. Because of its intense spectral lines located across the visible and the UV domains, mercury is one of the most commonly used elements in these applications, especially in spectroscopy. Spectral sources filled with this transition metal are usually made in a similar way as the standard discharge lamps intended for general lighting service, with some aspects of the source design changed in order to meet certain specific requirements related to e.g. the emitted spectrum, the lamp size or the output stability.

The LL93110E shown here is a first-generation high-pressure mercury spectral source which features an early quartz burner design of the sort that was used in Philips’s 80 W HP and HPL lamps produced in the 1940s. This arc tube is built with two vacuum-shrunk end seals, an obsolete design which survived the transition to mechanically pinched seals in the early 1950s because of the specific way spectral lamps were made in Eindhoven, relying entirely on skilled glassblowers for the production of the special burners. The other key difference compared to standard HP/HPL arc tubes lies in the presence of quartz diaphragms placed in front of the electrodes to ensure a stable, flicker free mercury arc, and to limit the blackening rate of the quartz vessel over time. This is critical to maintain a high source luminance and to ensure an optimal spectral output, especially in the shortwave end of the spectrum as electrode deposits are particularly absorbant towards blue and UV light. The electrode chambers are coated with platinum paint for the conservation of heat at the burner extremities, enabling a high operating pressure. In the intended application of the lamp, these coatings also serve the useful purpose of blocking off the parasitic light emitted by the incandescent electrodes.



The burner is mounted inside a nitrogen-filled outer jacket made of fused silica, rim-sealed to a quartz disk provided with two vacuum-shrunk quartz-molybdenum feedthroughs that connect the burner to the lamp’s E27s end cap. The low concentration of metallic impurities in the material bulk of the quartz jacket allows a light transmission down to about 180 nm, thus giving access to most of the discrete optical energy radiated by the hot mercury plasma. The short-wave output of the lamp is concentrated mainly in three spectral regions located around 185, 254, and 365 nm, whose spectral profiles are broadened and self-reversed. The lack of spectral finesse these regions originates from the elevated temperature of the plasma (~6000 K), the high pressure of its mercury vapor atmosphere (~14 bar), and the strong temperature gradient (i.e., contraction into an arc) that results from the elevated input power load of 27 W/cm. Although broadened by pressure and temperature, spectral lines in the visible and infrared domains are much narrower and are in general suitable for interferometric work, among other applications. In the visible there are five main lines located at 404.7, 435.8, 546.1, 577.0 and 579.1 nm, with the last two nearly merged into a cluster due to their overlapping broadened profiles. In the near infrared there are three lines of interest: 1014.0, 1128.7, and 1692.0 nm.

The arc power load mentioned earlier is the highest of all spectral lamps produced by the Dutch, including high-wattage sources such as the helium and mercury-cadmium-zinc lamps. This characteristic confers a particularly high luminance to the mercury light source, which makes it useful in applications other than spectroscopy, such as photochemistry, photoelectric emission characterization and demonstration, fluorescence analysis, and general light projection. The latter application, especially where color fidelity was not critical, was catered for with high-pressure mercury lamps since the early days of the technology (i.e., mid-1930s), precisely because the high intrinsic brightness of mercury arcs allows an highly effective optical control of the emitted light. In that case, the projector's efficiency benefits from a compact lamp design since it allows the placement of the bright arc close to the light-collection optics.

Typical of all lamps from the Philips ‘LL’ spectral family, the 93110E is designed for an operation on commercial 0.9 A leakage-flux auto-transformers of the type used for SO 140W low-pressure sodium lamps. The ballast's 470 V open-circuit voltage is high enough to ensure a reliable discharge ignition, aided by the auxiliary electrode located on the cap side of the lamp.