But the only reason the Sapphires and Trafficvisions was to DOUBLE the light levels on the slip roads for no reason. Double the head on one column is only a good idea if the lanterns shine i.e. the left lantern to the left and the right to the right. That way less columns are used and less connections etc.

The reason why twin lamp lanterns haven't caught on is cost and practically. I'm pretty sure I have said all of this before.

The Mk1 Alpha 8 was a massive lantern - around twice the size of an Alpha 3. MBF lamps back then produced only around half the lumens of the same wattage SON lamp of today, so just to get the lumens, twin lamp lanterns are obsolete. Then add the cost of a larger lantern, twin sets of control gear and more complicated switching - doesn't make economic sense.

In Europe twin lamp lanterns were also used but they tended to be the same vintage of the original Alpha 8 with all the associated drawbacks. These lanterns also tended to be (but there are always exceptions) mercury based designs for elliptical lamps. Philips do produce twin PL (and SOX)  versions of the MA SOX lanterns  - there is room inside the enclosure, but you still need two sets of gear.

Twin lamp SON/MH lanterns are impractical as due to the point light source, the optic performance would be poor for a twin lamp fitting as the lamp is not in the optimum position – either side of the ideal position. So you need twin optics.

You could make twin PL Deltas/Libras etc but it is more practical and cheaper just to fit a dimming ballast. This is the key point, fitting a dimming ballast gives the same power savings at a fraction of the cost of a twin lamp lantern design.

For the twin and quad optic ZX4s, these are only 250W per optic so use less power than a 600W Sapphire (per lit carriageway side), I suspect the Rocades are also 2x 250W and not 2x400W. It is not practical to switch one lamp out as they tend to be group switched - they would all need to be converted to twin photocell - one of which would need to be part night.

The twin head Sapphires are used because the carriageway is so wide the conventional optics can't throw the light across to the outside lane - assuming verge mounted. So 2 approaches - use floodlights which have a wide "across road" beam but poor "along road" or shove up 2 Sapphires and use the brute force approach of so many lumens that some will spill over into the outside lane. What they should have done is just used some central reservation columns at these locations and with an opposite position, 2x400W would have probably sufficed rather than 2x600W.

With Cosmo reflectors, which are often quite small nowadays, it should be possible to design a lantern with two optics that is not any larger than a SON lantern with a single optic. However, you would still need two sets of control gear, unless Philips developed a smart ballast which could run two lamps, one part night. But why not use dimming?

I had a bit of a thought today. There are two terms, "energy efficiency" and "economy". Councils often talk about energy efficiency. That means that the product spoken about is efficient as a product in what it does. A CosmoPolis lamp and gear solution is relatively energy efficient, as it produces a lot of lumens for a relatively low power consumption. Whether it is economical or not is another matter. 140W CosmoPolis may be energy efficient, but not if it is used on a residential street. Economy not only deals with energy efficiency, but also application, and that is where councils have been going wrong. They have been using energy efficient products, but applying them in a manner that results in poor economy. We've all heard it, councils saying CosmoPolis is more efficient than SOX, but then installing 60W Cosmo in place of 35W SOX rather than doing the sensible thing of using 45W Cosmo instead.

I would like more councils to consider economy as well as energy efficiency.

E-On could do with a lesson from you Sotonsteve. They are responsible for Blackpool's PFI and in some areas have replaced 70w SON at 5m with 90w CPO at 8m. That would be all well and good if the lighting needed that kind of upgrade but in many cases this 90w CPO is used in quite, narrow streets, even in cul-de-sacs! I can see Blackpool's electric bill rising somewhat in the next few years, when the idea of these PFIs is partly supposed to be about making lighting more energy-efficient!  

Well it happened in Leeds, after installing "more energy efficient street lights"... because 250w SON on 10m columns is so much more efficient than 90w SOX!  

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I know exactly. I wonder if there are any PFIs that are making significant energy savings? Do our Southern correspondents think that the large South Coast PFI is making a big difference in that respect? I would guess it is, as they use CFL on the side roads.

Another PFI that could be making big savings could be Birmingham, due to their large stock of mercury lighting and it's replacement with LED.

Well with Birmingham, think about the company that is doing the work... which just so happens to be the same company that does the streetlight work in York!

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During the final stage of the planning for the south coast PFI, Hampshire County Council realised that for their element, if SON was the light source, the expected energy bill would be unaffordable. The bidders were told to go back and rebid using "white light". We thought (correctly) this would be CFL for residential and possibly Cosmo for main road.

In the end we did get Libras for side roads, but also Cosmo Arcs - which we weren't expecting - they didn't turn up until 3 months into the PFI. Main roads have generally been SON Iridiums but also Cosmo Arcs/Cosmo Iridiums in some built up areas.

There has been some good practice - 2 new columns replacing 3 old ones, dimming and the use of 26W Libras on some footpaths, but they could have gone much further.

Wider use of Cosmo on main roads would have saved more power compared to SON.

And to be honest, where (60W) Cosmo Arcs have been used at 5 and 6m, they could have just used 55W Libras instead..

True, Amey definitely seem to have their eye on the ball!  



At least they realised, before it was too late that SON would have crippled their budget (unlike Leeds).

CFL is a very efficient and useful light source but it seems to be very much out of favour in my local area, as it was never widely used here and no new CFL has been installed since about 2006. The Blackpool PFI does use LED, but, bizarrely, only in back alleys and often these alleys were either previously unlit or had only one or two lanterns but now have five or six at normal side road spacings so no energy saving there! One correspondent to the Blackpool Gazette website even complained that the new lighting in their back alley (which was previously unlit) had prompted metal thieves to steal the manhole covers, their argument was that had the alley remained unlit the thieves wouldn't have been able to see to steal! There is possibly some truth in that  

There is very possibly some truth in that. There are a few cycle routes round my area... the lit ones can have issues with youths loitering and causing a nuisance (with me once actually being threatened and pushed just for walking along... the youth in question was obviously schitzo, as he was hearing voices calling him names), but an unlit one going to Clifton Moor has nobody loitering around (only at the council estate end where the street lights overspill to the route entrance) simply because it's totally dark there and they can't see!

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