Mazda SLI/H Linear Low Pressure Sodium
First marketed in 1959 with ratings of 150, 200 and 250 watts, the Linear Sodium lamp was the first light source to break the 100 lumens-per-watt barrier.  It was widely known that it was possible to increase the luminous efficacy of a sodium lamp by adopting non-circular sections for the discharge tube, but A.E.I. Lamp & Lighting Co. of Leicester (formerly BTH) , was the first to commercialise this.

The range was characterised by a novel grooved discharge tube having an alternating crescent shaped cross-section, based on GE's American PowerGroove Fluorescent products.  Because hot sodium vapour absorbs its own radiation, tubes of narrow diameter are desirable to allow efficient transmission of light created at the centre of the tube.  However, efficacy also rises as the current density in the discharge falls, so to minimise this value a large diameter tube is called for.  In traditional SOX lamps, a compromise must be found to determine the optimum tube diameter.

Linear lamps benefited from a large diameter tube having a large cross-sectional area for low current density, and the crescent shape section minimised light absorption losses.  Efficacy was boosted considerably, and as a result the long U-shaped discharge tube was no longer required and compact Linear lamps could be made.  So confident was AEI in the Linear concept, that it abandoned traditional sodium lamp manufacture at this time and for the next decade, sourced in those lamps from Philips and Osram.  For thermal insulation, this lamp employs two additional glass sleeves.  The less complicated design of this lamp suffered fewer mechanical failures in service, and the exceptional reliability of Linear Sodium soon became very well respected.
Manufacturer: Associated Electrical Industries Ltd.
Lamp Power: 60 Watts
Lamp Current: 0.83 Amps
Lamp Voltage: 80 Volts
Cap Type: 2x G12 Aluminium
Bulb Finish: Clear Soda-lime glass
Bulb Type: Inner: TD-20 Outer: T-38
Overall Length: 427 mm 16¾ inches
Light Centre Length: N/A
Arc Length:
Electrodes: CCC-6 Black Tungsten BCT emitter
Atmosphere: Na|99%Ne,0.8%Ar,0.2%Xe Outer: Hard Vacuum
Luminous Flux: 5,700 lm (@ 100 hrs)
Luminous Efficacy: 95.0 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra-44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Burning Position: Horizontal ± 5°
Rated Lifetime: 4,000 hrs to 70% survival
Warm-up & Re-strike Time: 12 minutes Instantaneous
Factory: Leicester England
Date of Manufacture: July 1965 Date Code 2591
Original & Present Value: £3 15s 0d (1963) £52.57 (2005)
 
References: 1) AEI Mazda Lamp Catalogue, 1963.