When did lamp makers change from the "E" to "ED or ET " designation for lamp shape ?  My GE and Sylvania catalogs, both from 1980 list their lamps as "E" (i.e. E-17,E-18,E-23 1/2,E-28 )ETC.
even though both used different arctube support and bulb-crown design (E-18 mainly) . Now Sylvania says "ET-18" and GE says "ED-18"

I have had people correct me when I say "E-28 " or whatever regarding to vintage lamps.

THANKS !

Just seen this. I think 'ED' indicates that the lamp has a dimple in the top end for the support frame.

Not sure when they changed the designations but I will tell you what I think each stands for (in US terms):

- E = Elliptical, no dimple
- ED = Elliptical Dimple
- ET = Elliptical Tubular (the ET23.5 looks like a BT shape lamp though)

My reasoning for the question was that in my older lamp catalogs I can find a lamp listed as "E-28" but in a new catalog , the exact same lamp is listed as "ED-28" even though both are absolutely identical in shape and form . WHY ??

Not sure why they did that. Maybe it's because they realized they were using the wrong names for the shape of the lamps and wanted to put the correct names for the shapes?

Thanks ! I was curious since when I first joined a was corrected by a few members CONSTANTLY about "ED" instead of "E" . 

The shape that puzzled me the most was the ET23.5 shape as it really looks more like a BT shaped lamp than anything.

Same here. Wonder why it was called the ET23.5 instead of BT23.5?

The first letter E means Ellipsoidal, whereas B means Bulged.  Therefore if the bulb has a true mathematical ellipse shape it can be called E or ED or ET.  If non-pure ellipsoidal it should be called B or BT or BT. 

The second letter D indeed means a dimpled end, and T a tubular section at the end.

However Sylvania screwed this system up with the start of its more conically-shaped HPS lamps in the 1960s, which they called ET18 to differentiate from GE's ED18 bulb, and got it wrong.  They should have called it DC-18.  Since then, there have been other confusions introduced and the IEC nomenclature system for bulb shapes no longer applies in all cases.  For full details, see here

All the USA manufacturers used to use tubular shape envelopes for their main HID ranges.  It was not until 1952 that the first BT shape was introduced, by Westinghouse for an improved version of its 400W/C mercury lamp which originally had a large cylindrical bulb.  I think that may have been the first scientifically designed bulb envelope for an HID lamp - the much earlier fluorescent mercury lamps in Europe used A or PS shape bulbs.  The BT shape envelopes were copied by the other American and Asian producers, whereas the Europeans and British regions developed the ED shaped envelopes.  This global differentiation existed until 1966 when GE recognised the superiority of the ED shape lamps in terms of lamp life and light distribution, and began changing its USA types to the ED bulbs.  In 1988 Philips USA also made the same step so as to standardise its ex-Westinghouse USA production on the exact same lamp designs as its European operations - I believe they stopped making their own hardglass bulbs in the USA around that time.  Sylvania continued the latest with the BT shapes, because it also made its own glass.  Those types finally converted converted to the European ED style following the closure of the American Sylvania HID production, and switch to Chinese sourcing.  Osram's Chinese mercury lamp production was in fact relocated from the Osram English production, hence despite being in China where the BT bulb shapes were more popular, they were already producing only the ED shape.

A partial list of some of these bulb shape changes can be found here.

So why the US Philips, calling the envelope of their 1000W "E-25", despite they are in fact "DC-25"? See here .