GEC 'Dioptrion' Z8431

Lantern acquired in January 2011.

This lantern was one of several that were used to illuminate the North Way car park in Cirencester, Gloucestershire until mid-2010. They were attached to sleeved 25 ft Stanton concrete columns; however, they pre-date the sleeving, and are likely to have been the original lanterns fitted when these columns were new; then being reused and rewired once the original concrete brackets were replaced. The lanterns ended their days running 250 Watt SON-T lamps, although they would originally have operated 250 Watt MA/H (or MA/V, if electromagnetic arc deflectors were fitted) mercury lamps. With thanks to various collectors for their assistance in providing me with this lantern. The Dioptrion is not, as far as I have been able to ascertain, a lantern that saw any use on public roads in Derby; however, examples were installed on the roadways around the Crompton Parkinson works on Alfreton Road - curiously, CP also made street lighting lanterns, making the use of a rival's equipment rather surprising. One Dioptrion is known to remain in service in Worcester (as of 2022); details of which are located here.

The lantern comprises two glass bowls that surround the lamp; hence, the lantern's name being a portmanteau of "di-" (Greek for "two"), and "opt-" ("opticus") (Latin for "of seeing", as in "optics"). This design would mean that a certain amount of the emitted light would be able to escape into skies, the prismatic refractors moulded into the glass ensure that the majority of the luminous flux is beamed downwards. Two toggles at either end of the lantern secure the lower bowl to the middle support ring. With these disengaged, the lower section hinges open; allowing maintenance to be undertaken on the lantern.

When removed, the lanterns were simply unscrewed from their top-entry elbows; the threaded pipe entering the lantern being left attached. As always, the GEC logo and "Made in England" are cast into the lantern's aluminium top and displayed in a prominent position.

The two bowl securing toggles bolt to the top section. Although this is a top-entry lantern, the position of where a side-entry spigot would enter the lantern can be seen on the right of this section. The design of the Dioptrion reminds me of a hovercraft crossed with a submarine (not that such a craft could ever work!).

Two grub screws are positioned in indentations on the other side of the casting. These would be used to secure a side-entry bracket, but in this lantern, they do not serve any purpose. It is likely that the castings would all have been produced in such a way that they could have either accepted a side or top-entry spigot, and were then adapted for whichever mounting style was required later.

The refractors are engineered to maximise the output from the wide arc tube of early mercury lamps; although, as the bowl is symmetrical, the lantern could be positioned either way around on the top-entry bracket, without compromising the light levels on the roadway below. The style of the refractors is reminiscent of those moulded into the bowls of Philips main road SOX lanterns, so there is certainly a possibility that the Dioptrion provided inspiration for the design of the Philips lanterns.

The GEC logo is also moulded into the bowl...

...as is "Made in England", once again. Not visible here, but the number '59' is also moulded into the bowl, just below this text. This suggests a manufacture date of 1959 for this lantern; making it 51 years old upon removal. This long life can be attributed to the robust build quality, simplicity and regular maintenance of these lanterns.

This view shows how the toggle secures the aluminium centre ring that supports the lower bowl section.

Inside the lantern, the method of holding the top bowl section in place is revealed. There may once have been an electromagnetic arc deflector installed between the lampholder and the top section (so as to allow MA/V lamps to be operated horizontally without incurring damage to the arc tube), but if such a device were installed, it is now long gone. Advances in mercury lamp technology after this lantern was produced brought about the production of the MB/U lamp; the 'U' representing the universal burning position of these lamps. As previously mentioned, the Cirencester lanterns were later converted to run 250 Watt SON-T lamps, which will also operate correctly without the need of arc deflectors. The rubber gasket (normally attached to the ring supporting the lower bowl section) has become loose over the years - only a small portion remains glued to the ring.

The gasket was temporarily repositioned on the flat edge of the ring for the above photograph. The differing spaces between the outer and inner refractor prisms are clearly shown here.