RLOD#31 (2020.06.10) 1968 Philips HP175W RQ1034/32

The 175 W high-pressure mercury lamp was first introduced in the USA during the second half of the 1950s as a source of intermediate output between those of the 100 and 250 W mercury lamps. In Europe, Philips began developing this lamp type a decade later and released it as the HPL-N 175W in 1969, followed by a clear version, the HP 175W in 1970. The lamp shown here is an early development model featuring a 250 W burner with a mercury dosage adjusted for the lower power dissipation. Its operating vapor pressure becomes unsaturated at 4.37 atm when the dissipated power reaches 175 W exactly. The lamp’s electrical characteristics stabilize at this power level when driven from 220-V 50-Hz mains with a 82-Ω series inductor ballast, i.e., the impedance of Philips’s #58247 AH/00 choke designed specifically for the HP(L-N) 175W.

While this approach to lamp design is certainly an economical one as it minimizes the tooling costs in production (all that is needed is a 10.8 % reduction in mercury dosage), the reduced arc power load of 27.6 W/cm (it is 39.4 W/cm at 250 W) critically decreases the initial lumen efficacy to 44 lm/W. For this reason Philips eventually designed an optimized 175 W arc tube for its commercial lamps. To that end the electrode gap length was shortened from 58 to 48 mm, the burner inner diameter was reduced from 14.5 to 13.0 mm, and the operating pressure was set to 4.75 atm. While the 175 W HP/HPL-N was available in Europe for twenty years, its usage remained extremely limited. Philips eventually delisted this wattage from their European catalogs in 1990 due to a lack of demand.