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 W SON-T lamps, although they would originally have operated 250 W MA/H (or MA/V, if electromagnetic arc deflectors were fitted) mercury lamps. Photographs of the lanterns whilst still extant can be seen here. 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.


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... 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 W 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.

The lantern was disassembled in preparation for its restoration, in December 2017. Amazingly, the components separated with relative ease; however, the lampholder bracket and earth terminal screws were beyond repair, and so the casting was sent away to a local engineering firm for these to be replaced. This returned in April 2018, with bolts replacing the seized screws. Given the relatively good condition of the aluminium, I decided not to paint it, but simply to rub down the worst of the corrosion on the inside. The (suspected) asbestos gasket was replaced with an adhesive foam strip at this time too.


The worst of the corrosion was found on the central aluminium support ring. Again, this was rubbed down using an abrasive soap pad, until the aluminium became smooth.


The two glass bowls were treated to a soak in the bath (yes, really!), in order to remove the worst of the ingrained dirt. After that, they received an extended cleaning courtesy of the dishwasher - well, it was known as the 'Double Dish' lantern, after all! As can be seen, two especially stubborn deposits remained in place on the upper bowl, but apart from that, the (by then) 59-year-old glassware cleaned up exceptionally well.


The lantern was attached to a specially-modified (in order to accommodate the top-entry lantern) AC Ford AC 872 Mk II wall bracket on Friday, 20th April 2018. Thanks to the original 1 inch BSP coupler still being attached to the lantern, fastening it to the new bracket was simply a case of screwing it into position. This will have been the third bracket that has supported this lantern, after the original Stanton concrete bracket, and later, the replacement sleeve, but on every occasion, the same coupler has performed amicably! I decided to reassemble the lantern with the casting secured to the bracket, rather than attempting everything at ground level, for ease.


With the casting in place, the top bowl and support brackets were attached.


Actually, forget that - the support frame has to go on first! The four bolts that hold the connecting pieces in place are 14 ″ BSW (British Standard Whitworth) headed - these have a slightly different head size to equivalent AF (Across Flats) imperial bolts, and required the purchase of some dedicated BSW sockets.


Right...let's try that top bowl again, shall we!


The second bowl was added next.


The porcelain lampholder and internal connector block had also been given a clean, and were duly fitted at this time, complete with new wiring to replace the rather burnt (non-original) wires.


Naturally, re-fitting a SON lamp in this lantern was never an option, but with the arc deflector for allowing MA/V lamps to be burnt horizontally having long since been removed (if one ever existed in this lantern at all), and the extreme rarity of MA/H lamps (medium pressure mercury vapour lamps that could be burnt horizontally, thanks to a thicker arc tube) in 2018, I decided on a compromise solution - fit the lantern with my 1960s' Atlas 250 W MB/U lamp! Although the arc tube in an MB mercury vapour lamp is physically smaller than the equivalent MA type, owing to it being quartz, rather than glass, the lamp can be operated at any burning position without causing premature failure, and the colour output is very similar.


With the hinged lower bowl back in position, the smaller arc tube was still visible through the refractors, but was reasonably well disguised.


Looking the other way, the GEC logo seemed somewhat more prominent than it did previously - that's what a good clean does for brand advertising!


The gear for this lantern came in the form of a Tridonic OMB 250 W 120 ballast and PED capacitor fitted into an Industrolite-branded metal enclosure. This is new, old stock, and dates from 1987, according to the obligatory manufacture date that is printed on the capacitor's casing.


With a bit of a grumble from the lamp's internal resistor-based ignitor, the lamp sprang into life, producing an attractive pattern on the underside of the bowl in the process.


From the side, the distribution of the main beam was very apparent.


The output whitened as the lamp warmed up, though of course, a strong blue-green tint remained. Had this been a coated mercury lamp, a broader colour spectrum would have been witnessed.


Despite the majority of the lantern being glass, the refractors are engineered in such a way that ensures that more of the flux is directed downwards - this photograph demonstrates this effect nicely.


One final side view of the lantern, with the lamp now operating at full power.


Lamp warm-up video:

Testing the lantern with my energy monitoring device revealed the following results:

Test Voltage (V) Current being drawn at full power (A) Measured wattage (W) Apparent Power (VA) Frequency (Hz) Power Factor True Power (W) Difference to rated wattage Percentage Difference
244.3 1.3 290 318 49.9 0.9 285.83 35.83 14.33 %

Thorn Gamma Basique | Thorn Beta 5




© 2002 - English Street Lights Online