Philips SL*18 Prismatic Compact Fluorescent Lamp

What you see here is a very, very important lamp.  This, in 1983, was the first ever single-piece compact fluorescent lamp that you could go out and buy.  The compact fluorescent lamps you see today quite probably owe a lot to the SL*18 as you see it here.  While this lamp itself did not prove to be a runaway success (for a number of reasons detailed below), it proved that the concept could work, "testing the water" so to speak, and opened the floodgates for many manufacturers to start producing CFLs of their own.  Also, it allowed Philips to refine the design itself, as with nothing like it having appeared on the market before, nobody knew how the public would react, so the feedback from the market would have been a valuable tool.

The SL*18 offered two main advantages over the incandescent lamp it was intended to replace, though in 1983, this was a tall order indeed!  The lifetime, at five thousand hours was nearly five times that of a conventional lamp; useful for those hard to reach fixtures, or places where a lamp blowing is hugely inconvenient.  In addition to this, the power usage was more than 75% lower than the 100W incandescent it was intended to replace.

An additional advantage for certain applications is that these lamps tend to dim over time, eventually becoming hard to start, before failing to start properly at all - just flashing on and off like older linear fluorescent lighting fixtures did (before electronic ballasts became commonplace).  They don't tend to just go "pop" like incandescents.  As such, even though they didn't attain huge popularity for everyday household lighting, the SL series of lamps saw unexpected success for lighting areas like loft spaces and store-rooms, where it was (and still is!) a real pain if all you're greeted with when flicking the switch is a night-vision destroying flash and a pop as the bulb blows.  It's for this reason that there are quite a number of these lamps still residing in peoples lofts or in industrial store-rooms which are only opened once in a blue moon.  This lamp apparently saw use in one such store-room until recently, now however in recognition of its importance in the evolution of modern lighting - not to mention the achievement of a piece of essentially disposable technology to have survived 23 years and still work perfectly -  it's going to live out a safe retirement in my virtual museum.

Unfortunately, the SL*18, revolutionary though it was, was not without its drawbacks.  Essentially, this lamp (aside from the phosphor used, which was developed for this, and its never-marketed predecessor the SL*1000) is all adapted technology from existing linear fluorescent technology, just stuffed into a small enclosure.  What is actually inside that glass jar (yes, it is actually glass) cylinder is the following: one conventional magnetic ballast (stick your ear next to one of these lamps when its running, and you'll hear that purr that used to always be present in libraries and classrooms from the fluorescent lights in the 80s!), a glow bottle starter (exactly what's inside the little white cylinders you have to replace now and then on the side of your linear fluorescent fixtures), and a 12mm diameter fluorescent tube folded into two U-shaped sections.  This adaptation of existing technology is where the problems arise.

The magnetic ballast is heavy - very heavy.  Heavy enough in fact to make things like table-lamps severely top heavy if this lamp was fitted.  I also imagine that many lamps got smashed when they fell out of lamp holders where one of the retaining lugs for the pins were broken.  The light weight of an incandescent lamp means that generally they'll stay quite happily in a damaged bayonet fitting, this however definitely would not, and would tend to end up smashing into a million pieces upon contact with the ground when dropped thanks to its weight.

Modern CFLs use electronic ballasts, which generally run the lamp on very high frequency AC, usually a few kilohertz at least.  The first advantage of this is that it totally eliminates visual flicker.  A magnetically ballasted lamp like the SL*18 will always flicker at twice the mains frequency, in the UK this means that the lamp will flicker at 100Hz, which some people can detect, while others cannot.  I can see it if I'm actually looking for it, but don't really find it objectionable - the SL*18 doesn't flicker anywhere near as badly as some linear fluorescent lamps I've encountered before.  Also, a high operating frequency will reduce the tendency for rapidly moving objects (such as wheels or pulleys on machinery) to appear to be standing still due to a stop-motion effect.  This is a major drawback of many discharge lamps using magnetic control gear, including the SL*18 - this is why many machine shops and such to this day still use incandescent lighting - it is actually a safety issue.

The SL*18 also takes a good three or four minutes to fully run up, giving out a really pitiful amount of light when first started - modern lamps usually are up to about 90% within a minute - which while not really a problem, is an inconvenience you don't face with an incandescent lamp.

Possibly its biggest failing though, is its size.  This is a problem which has plagued CFLs from the early days of this SL*18, right through to the modern day.  Due to the fact that you need a certain length of tubing (as there is a relationship between the length of the tube and the power dissipation of the lamp), its is very hard to make CFLs compact - hence the rather long shape of the SL*18.  Improved glass working techniques and the development of phosphors which can withstand higher operating temperatures has allowed more tightly curved tube shapes to be used of narrower diameters, but back in the days of this lamp - that was not possible.  To this day, the tendency for all but a few designs of CFL to stick out of the top of lamp shades, or refuse to fit where you want them remain.  Most notable for co-operating as far as fitting and being discreet are concerned, are the spiral style lamps.  As you can see though - they have shrunk a lot since the SL*18 was launched.

As with any new technology, cost was a factor too.  These lamps cost at least £5.00 in 1983, depending where you bought them, that could have been as high as £8 - a local store to us stocked them at £7.99 in 1987 (Based upon RPI increases, that equates to £16.20 in 2007).  Even today where CFLs are available for as little as £1.00 (some supermarkets even selling them for that price, on a buy-one-get-one-free basis), in terms of price, they cannot compete with the £0.10 each incandescent lamps they are trying to replace.  In 1983 though, that was very much more true.  £5 was relatively more then thanks to inflation, and energy prices were lower - this meant that the payback for the extra purchase cost in terms of energy saved would be a period approaching infinity.  Still, enough people bought it that the technology didn't die...it proved that the concept could work...hence the fact that we have CFLs today.

As a side note, it should be noted that the SL*18 (and its sister products such as the SL*25, SL*Comfort (a technically identical lamp with a light grey base and opal glass diffuser, the 13W version of which I used as a nightlight for a number of years a child)) has demonstrated a truly impressive ability to operate long, long beyond its estimated lifetime.  It's for this reason that working examples can still be found, just a tribute to how over-engineered and well put together these early lamps were.

The SL*18 wasn't a perfect design by any means, but it was proof of a concept, and it worked well enough that the idea stuck in the minds of the lighting industry, and has evolved into what we know as the compact fluorescent lamp today - in that way, it deserves a very big salute from anyone interested enough to have read this I think.

It's taken me a while to find one of these, but I am very, very glad to have it in the collection.

Philips SL*18 Prismatic Compact Fluorescent Lamp - General overview of lamp

Philips SL*18 Prismatic Compact Fluorescent Lamp - Detail of lamp cap

Philips SL*18 Prismatic Compact Fluorescent Lamp - Detail of diffraction pattern moulded into outer bulb

Philips SL*18 Prismatic Compact Fluorescent Lamp illuminating my workstation from a distance of approximately two metres

Philips SL*18 Prismatic Compact Fluorescent Lamp - Overview of lamp shown while lit

Philips SL*18 Prismatic Compact Fluorescent Lamp - Detail showing size of lamp

Philips SL*18 Prismatic Compact Fluorescent Lamp output spectra

Philips SL*18 Prismatic Compact Fluorescent Lamp - Detail of text printed on lamp base (1/2)

Philips SL*18 Prismatic Compact Fluorescent Lamp - Detail of text printed on lamp base (2/2)

This lamp added to the Virtual Display Shelf on the 12th December 2006 at 23:05.

References: Lamp markings and Lamptech, website of a fellow lamp collector.

Acknowledgements: Many, many thanks to the reader of this website who donated this lamp I had been seeking for so long for display!

This page was last updated on the 15th of June 2023: Page format changed to improve readability on mobile devices and some background code changes were made to improve search engine behaviour.