HQL/5 "Caution Yellow" Mercury Discharge
These so-called "Caution Yellow" mercury lamps were marketed during the 1950's and 60's primarily to provide an alternative colour light source for streetlighting service.  Owing to the greater number of road accidents occurring at intersections and sharp bends, it was decided to try changing the colour of illumination in these areas in an effort to improve driver alertness, and hopefully reduce the accident rate as a consequence.  Indeed a strong link was proven in due course, and a market was created for this colour-modified mercury lamp.

The concept was pioneered by Sylvania in the USA, and was later offered by competitors General Electric and Westinghouse.  In Europe the only known producers are Osram & Radium in Germany, and Tesla of Czechoslovakia, while various Japanese firms also made them available.  In these countries it provided an excellent visual contrast against the conventional blueish-green colour of standard mercury lamps which were universally employed in exterior lighting.

However in the UK and many other Western European countries, the low pressure sodium lamp was the preferred source for streetlighting, and where colour contrast was called for here, ordinary mercury lamps were of course more appropriate.  Thus the Yellow Mercury lamp was never offered in the UK and is totally unknown in several countries.

Its met its demise in the 1960's however when high pressure sodium lamps were installed first in these accident black spots, both to raise lighting levels and continue to provide the similar colour contrast.  After this time there was no longer a requirement for the yellow lamp and it promptly disappeared.
Manufacturer: Osram GmbH
Lamp Power: 250 Watts
Lamp Current: 2.5 Amps
Lamp Voltage: 125 Volts
Cap: E40s/45 (Ceramic)
Bulb Finish: Mg4(F)GeO6:Mn with external yellow glaze
Bulb Type: ED-90
Overall Length: mm
Light Centre Length: N/A
Electrodes: Backwound tungsten with triple carbonate emitter
Atmosphere: Inner : Argon Outer : Nitrogen
Luminous Flux: 8600 lumens
Luminous Efficacy: 34.4 lm/W
Colour Temperature & CRI: CCT: 3700K CRI: Ra 36.4
Chromaticity Co-ordinates: CCx: 0.444 CCy: 0.533
Burning Position Universal
Rated Life:
Factory: Berlin, Germany
Date of Manufacture: April 1960 (Date code: fmX)
Original / Present Value: