And, more importantly, that's a deluxe HPS (2200 K, 70 Ra8)! It's really an oddball lamp there. It is clearly intended for export (i.e., E40 end cap) but it is not optically compatible with any other standard 400 W HID lamps (except the US cram MH ones, but those are not found/used outside North America). It looks like GE's US engineers didn't want to run the fatter arc tube in the usual ED18 bulb of their 400 W Lucalox (overheating issue with their softer leaded borosilicate glass?) and went for the ED28 jacket used for their 175 and 250 W HID lamps instead. That lamp's output is also too high (37.14 klm initial) to be similar to that of usual 250 W HID lamps, so that LU400/DX/40 must have been intended for dedicated lighting systems/installations only, which keeps it out to the retrofit market (in applications involving reflector luminaires at least). It's all very odd indeed.

Thanks for the information Max. I guess it wasn't available for very long. It does indeed have a notably better CRI than a standard 400W SON.

No problem. I dug a little bit in the literature and I think I have a better idea of what exactly this lamp is. It was first introduced in 1990 in the USA (with an E39 end cap) and this was the last Deluxe Lucalox released by GE. The E40-capped variant for the export market was introduced the following year. It appears that this was GE's solution to a 400 W cram HPS lamp. The cram lamp concept is typically American and consisted in greatly reducing the bulb size of standard MH lamps (e.g., 1000 W arc tube in a BT37 bulb) so as to fit compact luminaires. This concept arose most probably because of the large size of standard MH lamps there due to the use of soft leaded borosilicate glass, which required large luminaires and limited the system's optical efficiency. Cram lamps solve this problem with their smaller bulb but this came at the cost of a higher operating temperature which reduced the lamp's service life. Those lamps proved reasonably successful regardless.

However, that concept proved much more difficult to apply to HPS lamps given their much longer arc tubes. Philips addressed this issue by using two burners connected in series (see there) while GE opted for a shorter Deluxe arc tube which could fit an ED28 jacket. So, the present lamp was intended for a use in compact luminaires (those designed for cram MH lamps) and not to retrofit standard 400 W HPS lamps (although it could in luminaires without a specular reflector). This makes sense considering that the two lamps (standard and Deluxe Lucaloxes) targeted completely different applications, so it did not really matter that the Deluxe version had a different light center length. Moreover, the reduced lumen efficacy of the Deluxe Lucalox is compensated by the higher luminaire efficiency of the cram lamp system, resulting in similar projected beam intensities as for standard 400 W Lucalox systems, with the Deluxe variant offering an improved light color quality as a benefit (but with a mean service life reduced to 10 kh initially, consistent with cram lamps). GE's intention with the E40-capped variant of their LU400/DX released in 1991 -they had a clear ED28-jacketed LU250/DX available too (not sold in North America though)- was to sell those lamps abroad as a compact alternative to their already existing E40-capped tubular 250 and 400 W Deluxe Lucaloxes, based on Thorn's SONDL-T 250 and 400W and produced in Europe. The latter were optically compatible with standard HPS lamps.

Thanks again Max, that solves a mystery I haven't fully known since acquiring the lamp a good number of years ago. Might have to look after it a bit better now I know as it just sits on a windowsill.

Yes, take care of it, that's certainly not your usual HPS lamp (I have one from 1995 with the E39 end cap for the domestic market).