True or false: Iwasaki SDX white HPS lamps are the world's only sodium based discharge lamps that produces the exact light of incandescent lamp at the same color temperature (2700K) and CRI (>90%), and so are considered the highest quality of any sodium based disharge lamp, without any relationship of the type of the lamp (HPS or LPS). Simply the other white HPS lamps don't attain the true color of the incandescent lamps (Usually 2500K and CRI of 85%).

Color of the sodium light is question of it's pressure.
You might get monochromatic glow with CRI=0 (=> LPS)
Or golden-yellow light with CCT=2000, CRI=~20 (=> "regular" HPS)
Or golden-white CCT=2200, CRI=~70 ("High CRI" HPS)
Or low temp white CCT=2500, CRI = 85
Or incandescent white CCT=2700, CRI >90
All this with one and the same lamp, depend how do you drive it. If you don't mind using special ballast and short lamp lifetime, you might easily get 2700K/CRI90 from regular HPS...

The issue with higher pressures is, then the system is becoming thermally unstable with regular ballasts (limit for HX or serial reactor gear is the 2200K/CRI70), the arctube has to withstand higher thermal stresses (otherwise it will fail soon) and the lamp become less efficient.
"White SON" need special controller, what stabilize the arctube temperature. And it is more question of manufacturing tolerances, how far it is possible to go with the CRI. Other reason for not going to the really high CRI regime is the efficacy - it drop significantly, as more and more radiation power shift towards IR, what is in lot of applications considered as too high cost for the a bit whiter light. Moreover as lot of people used to use underdriven incandescents (130V operated at 120V, 240V operated at 220V), the 2500K is assumed as "better, warmer light".
The difference between CRI 85 and 90 is nearly not noticeable, unless you really do fine color matching. But then the low CCT would be more a problem (lack of blue), so you would need at least 4000K source anyway...

So why in several lighting sites (Includes www.lamptech.co.uk) are written that Iwasaki SDX white HPS lamp is apparently the hiqhest quality or the best color of any type of sodium based light source available in the market (LPS, HPS, HPS deluxe and white HPS)?
Iwasaki SDX lamps can only compared in quality to another lamps of its category (Ie. the white HPS lamps). In this case, i bet they are the only white HPS that have the exact color characteristic of the incandescent lamp (2700K-3000K with >90% CRI) and it is unchanges all over their rated life and also the life is higher then another white HPS lamps and the build quality is excellent. The other white HPS lamps probably simply appears goldenish white, orangish white, pinkish white or simply the color of the carbon incandescent lamps, and the color is not stable. Iwasaki SDX is also the world only white HPS lamp that operated on an electronic ballast.
By the way, in Israel only the Philips SDW and SDW mini lamps are the white HPS lamps imported and several of them gives out a neutral white light that looks like 4000K or at least 3500K. Iwasaki is totally unknown in Israel.

I dont know about Iwasaki white hps lamps but my father has been using Philips MASTER SDW-T 100W lamps above his garage table. The color temp of the light is about 2500K (and thats what philips data sheet says too) and I have not noticed any color shifts during lifetime. Only negative thing about those lamps is their high price.

Roi_hartmann: White HPS lamps is always expensive. In fact the SDX lamps were much more expensive then the SDW-T lamps.

White HPS and electronic ballast:
IWASAKI operate ONLY at electronic ballast, as the required "intelligence" to stabilize the arctube temperature is only feasible to implement in an electronic system. Oerating the lamp with standard ballast would yield or to lamp destruction, or only "regular" CRI22 orange HPS mode or both...
Philips lamps do require controller, but it is way simpler and cheaper then the one required by Iwasaki lamps, as the CRI85 does allow wider temperature tolerance then the CRI90 mode.

The special ballast "intelligence" and accuracy is the main reason for the stable temperature (so light color), while actively stabilizing the arctube temperature is essential to be able to operate the lamp in the high CRI mode. Other part of the story is the lamp manufacturing quality and tight tolerances, that allow the lamp reasonable lifetime in such high temperature mode.

White SON (SDW-T and SDW-TG) lamps indeed do suffer from color runaway when driven on an electromagnetic ballast + controller; this sytem has limited color stabilizing capabilities. When the lamp ages, the color shifts from 2500K to 3500 ..  4000K.

A full ellectronic ballast will prevent this from happening.(e.g. HID Primavision ballast for SDW-T and SDW-TG lamps) because it controls the lamp much better.

Hmmh, The SDW-T operated with CSLS and choke ballast doesn't runaway. Instead they become yellowed when "white life" is spent. I newer had seen bluish SDW-T. The controller has maybe failed if this happend. The triac in controller can cut the power quite  low. The SDW-TG lamps with full electronic gear can be some degree bluish colour with the some of the lamps. With SDW-T driven by CSLS system the lamp may be bluish when all of sodium is lost, but arc-tube normally crack before that happend.

It is absolutely not true to say that the Iwasaki SDX or indeed any other sodium lamp produces the exact same colour as incandescent lamps of same correlated colour temperature.  The spectral power distribution is totally different.  However, when looking at the colour rendering indices and metamerism index values for each individual test colour, then the SDX (by Iwasaki’s own claims and data) does more closely approximate the colour rendering appearance of 2500K incandescent lamps than other discharge lamps.  I have not verified this claim myself, because of the impracticality of ageing multiple lamps of the different manufacturers for 100 hours to stabilise their spectra, and then average the results of dozens of identical lamps from different production batches to identify their true average performance.  However, I spent several years working for Iwasaki’s HID research labs and they did make such studies, and I trust their claims.  It also has to be borne in mind that Iwasaki had the full range of sodium wattages with white light, and all of them were highly optimised.  With the competitor brands, some of their lamp powers were better performing than others - and as such their average performance across the whole range may have been slightly inferior vs Iwasaki.

I must also note that for the vast majority of customers around the world, none would be aware of these differences or even be able to see them when pointed out.  Few countries are as obsessive about perfection in lighting colour rendering as the Japanese, which is partly due to the quite different colour sensitivity of the Asian dark eye vs the caucasian eye.  The correct rendition of certain colours is much more important to them.  So the fact that the Iwasaki lamps are technically the closest to incandescent should not be interpreted that the others are notably inferior.  And by the way, the Iwasaki lamps also shift considerably in spectrum during life, just like the Philips and other brands and even when operated on perfectly stable electronic ballasts.  This is an unavoidable consequence of sodium loss reactions, which make the spectrum proportionally richer in red radiation over time.  The main exception to that rule is the Osram DSX lamp, which contains no mercury, and therefore cannot cause the sodium spectrum to become redder as the mercury:sodium ratio increases.

That is the reason why Philips specify the white life for both SDW types T and TG. The white life difference between these is not big. It's more meaningfull that electrical life, what can be a much longer with wrong colour output.