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notes on focusing

From the introduction of the GLS lamp, through to the 1950s, GLS lamps were made in various sizes and shapes. And there different sizes for different wattages. And there was also single-coil and coiled-coil. And to add to the number of combinations, some mercury lamps also had different dimensions depending on the manufacturer.

This was a problem for the lantern manufacturers. They were designing precision optical instruments, and variations, even a few millimeters, could detrimentally affect the spread of flux.

The solution was to allow a street lighting engineer to alter the bulbs position in the lantern, ensuring that the filament (or arc tube) was correctly focused within the optical system of the lantern. This act, known as focusing, was an error prone and laborious business. Early GLS lanterns featured a continuous focusing systems which catered for all the various sizes of bulbs.

By the 1950s, bulb sizes had been standardized, and so manufacturers could produce fixed focus lamps. However, for lanterns which supported several wattages of lamps, or different types, the manufacturers produced fixed focusing systems, which allowed the bulb holder to be positioned to several different positions depending on the bulb size.

Guide to focusing for continuous focusing systems:

  • Wing reflectors: Position the filament of the GLS bulb on the same horizontal plane as the lowest part of the lowest wing.
  • Dome refractors: Either position the filament of the GLS bulb at the base of the refractor or position the filament at the mark 'LC' if etched on the refractor ('LC' is 'Lamp Centre'). For discharge lamps, position the arc tube midway at the base of the refractor.

badly focused modern GLS bulb

This GEC Z5580 B lantern has been fitted with a modern GLS bulb, but not correctly focused. The problem can plainly be seen, as the flux emitted from the lantern appears to be from the upper parts of the refractor; not all of the refractor is emitting light e.g. it isn't evenly flashed.

correctly focused antique GLS bulb

Fitting the lantern with an antique single coil bulb clearly shows the difference. The bright area of the refractor clearly shows the direction of the main part of the flux whilst the whole of the refractor's surface is bright. The lantern is now correctly focused.

correctly focused MBTF bulb

Interestingly a modern MBTF elliptical bulb had the same length as the antique GLS. And when installed, it was again correctly focused, lighting up the whole of the refractor.