The term “cut-off” has a precise dictionary definition i.e. “A designated limit or point of termination.” However, in street lighting, whilst the spirit of the phrase is met, the actual parameters of the “cut-off” require extra qualification.

Therefore a “cut-off” street lighting lantern will be designed to limit the amount of light emitted beyond a certain angle. The percentage of light emitted above this angle, and the angle itself, is variable and depends on the specification to which the lantern was designed. However, the terms “cut-off”, “semi-cut-off” and “non-cut-off” can be applied to a lantern in a general sense and give a good indication of its lighting distribution.

So a “cut-off” lantern will cast light to a certain angle and then only small percentage will be allowed beyond; a “semi-cut-off” still has a cut-off point but it’s not as rigorously enforced and a larger percentage of light is allowed beyond the specified angle; whilst a “non-cut-off” does not have a cut-off angle at all.

That’s not to say that a “non-cut-off” lantern has no form of optical control. Many street lighting lanterns have optical system to produce the main beams which illuminate the road surface. These have their own angles and characteristics. So a “non-cut-off” lantern may exhibit a high degree of optical control, but doesn’t have a cut-off angle where lighting levels are limited.

Waldram, writing in 1952, stated that a “Cut-Off” distribution reduces light as completely and rapidly as possible above an angle which varies in different lanterns between 65 and 75 degrees with the vertical. This would’ve been the foundation for the specification of “Cut-Off” in the MOT Report (of 1937) and the resulting Code Of Practice CP:1004 (1952). Further standards have been published and to determine the parameters of a particular lantern, one would have to consult the specification under which it was designed.

So “no light cast above the horizontal” enters into the spirit of a cut-off system, but is only one specific case; but “flat glass lanterns” simply notes a feature of a lantern designed to produce a particular distribution. (And it falls down when one considers that some cut-off lanterns have curved bowls and some older ones don’t have bowls at all).

The Philips MA lanterns were designed against BS1788 (a British Standard which describes how lanterns should be constructed and has led to dull uniformity of design) and BS 4533:Part 2:Section 2.7:1976 (which will give you all the details of what denotes a “Cut-Off” system). Looking at the technical data for a “Cut-Off” for these lanterns, the upper limit of the beam is set at 70 degrees with the intensity ratio dropping to 1.2 by 74.5 degrees. (No stipulation is given above 74.5 degrees, but the Light Output Ratio in the upper hemisphere is given as 0.2, so a tiny percentage of light above the “Cut-Off” is permitted in this specification).

All the best,
Simon

Philips installation and servicing instructions. Note text at bottom of image. Also there is a "continental" semi cut off which is more cut off than for the UK semi cut off!





The reason why the MA series can indeed perform as a cut off lantern is shown by the design of the canopy and bowl below. This picture shows my MA conversion using twin PL -L lamps. The lamps are mounted "high" in the canopy - equivalent to the cut off position for SOX lamps.



The canopy forms the primary optical control. The lamp position is such that the canopy shields most light above the horizontal. When I powered my lamp up at night I was surprised how narrow in azimuth the beam pattern was on the ground. There was a distinct light/dark edge to the beam.

The MA bowl has two longitudinal lenses immediately below the canopy - marked red. These further control any light emitted at the horizontal by directing it downwards. However primary control is still by the canopy. When I closed the bowl on my conversion, the beam pattern on the ground wasn't significantly broader - confirming that the canopy provides the majority of the optical control.

The GEC bricks work on the same principle, the canopy provides the majority of the optical control - shielding light from being emitted above the horizontal. The reason why the bowl is shallow rather than flat is probably more to do with strength rather than optics. For the 135 and 180W lanterns an acrylic bowl if flat, would need to be made out of considerably thicker material for rigidity than for the curved shape. I found this out on my GEC Z9555, the acrylic sheet I used for the replacement bowl is about twice the thickness of the standard bowl, and even then I had to build a frame for it to give it sufficient rigidity.



As Simon C points out, it is the definition of cut off which is the key.

And these Philips and GEC lanterns show some characteristics for a cut-off design:

1. Bulb mounted high in the canopy of the lantern.
2. Reflectors in the canopy (or the canopy's shape) reflect the flux into two main beams (using the idea of a "cross-over" distrubution).
3. A bowl can further spread the light below the lantern and/or focus the main beams.

All the best,
Simon

That'll be specified by C.I.E. 12. (which is the European standard).

All the best,
Simon

Thank you everyone for your responses. It's certainly an interesting discussion.

I must admit that I had previously applied a more literal meaning to the phrase 'cut-off', and with the new breed of flat glass SON lanterns on the market, which do not leak any light at all above the horizon, I thought 'full cut-off' in fact had a different and stricter meaning to 'cut-off' (like a 'service history' on a second-hand car advert implies an incomplete service history, whereas 'full service history' denotes a complete service history).

Even many years ago, I was confused by how a bowled lantern could be marketed as having 'cut-off' light properties yet still waste light into the sky. Back in February 1999, I had an opportunity to see whether Philips' MA SOX lanterns really could be seen from the air whilst visiting Exeter for Tesco. The M5 motorway, which ends at Exeter, goes through a steep cutting with a high bridge over the top to close the gap. If I was able to locate that bridge, which crossed over the motorway in a lit section, I would be able to see once and for all whether MA SOX lanterns could be seen from the air.


The end of the M5 motorway at Exeter in February 1999, and lit at the time by MA60s. The shadow of the high bridge I was standing on can be seen at the back of the photograph.


Returning to the bridge later the same day answered my question!

You do have to wonder whether a flat-glass option of the MA SOX lantern, if such a design was possible, would have extended the life of SOX lanterns on the motorway in the light of today's stiff competition from flat-glass SON lanterns.

If these lanterns have been replaced with flat-glass SON lanterns since my visit 12 years ago, I would not expect to see any of these lanterns from above!

As Simon C points out it is a ratio of light reduction, so although you can see light above the horizontal the proportion won't be very great compared to that directed to the road surface. Don't forget how sensitive the human eye is, a humble 5mm led with less than 1 lumen can be seen from a considerable distance.

But were these lanterns configured for cut-off or semi-cut-off?



It isn't design nor current fashion which dictates what is used and how: it's the specifications. And when the street lighting specifications changed so that the light emitted had to have a colour rendering index greater than 40 (IIRC) then good ol' monochromatic low-pressure sodium was effectively killed off no matter how efficient it was.

All the best,
Simon

Firstly excellent photographs of the M5

I too have always associated anything 'cut-off' (or 'full cut-off') to be a flat glass lantern. Semi cut-off is a term I'd use to describe lanterns with shallower bowls and I'd also use this term to describe lanterns such as the Residium which have the bulb held in the canopy but still have a bowl. I'd use the term 'deep bowl' to describe lanterns which are far from being cut-off such as the ZX3 or Iridium in their deep bowl format.

However, I will reconsider my terminology after reading the contents of this thread.

I only considered lanterns with flatglass or bowls which sides have been covered (GR151 etc) to be cut-off but thanks for that information

_________________
"I used to rule the world... Now I sweep the streets I used to own."
Remembering GEC.

The GEC Z9545/55/65 "Bricks" wiki page makes reference to the low mounted installations on the A40 (now replaced by Sapphire 1s). I recently read in The Story of London's Underground by John Day and John Read the following:

"...work began on a new £10 million station for Hillingdon at the start of 1991. It was needed because the main A40 trunk road was being realigned and the old wooden station stood right in its path."

This therefore suggests that the line of remaining bricks (as seen here) are only 20 years old, hence the reason why the columns and lanterns look to be in fairly good condition, with few casual replacements. In a post I made a while back, I estimated that they were installed in the late '80s / early '90s and so this book has attested my assumption.