Hi John,
I will confess I am a comparative amateur and hobbyist when it comes to this field and dabble in installs occasionally (although I may start to delve into it more professionally now). Having come from a telecommunications background with ILECs however I have a pretty good handle on doing installs. At the moment my toolkit lacks a light meter so I can't give you quantified data on light levels just yet, however that will hopefully be rectified shortly.
I will say however that I don't trust Hikvision's light level stats because at the moment I'm using two models - the DS-2DE4A204IW-DE PTZ which I did the mini-review on here, and the DS-2CD2135FWD-I which is a 3.1MP fixed dome, and Hikvision claims color images down to 0.002 and 0.008 lux respectively. In fact for the latter there is conflicting data because their literature claims 0.008 lux while the box label claims 0.005... Rather than relying on them for actual low light performance figures I think they serve more as an abstract rating that you can compare against their other cameras and know, "oh, this one is going to perform better in low light versus that one".
For the PTZ it was always billed as a Darkfighter unit and you can see in the link above what kind of images it is producing at night with ambient light. Again I'm afraid I don't have hard numbers. It does however seem to do extremely well and at less than $1000 CAD wholesale, seems to be a great unit for the money if you don't need long zoom or smart analytics.
The dome on the other hand isn't quite as good in low light despite being a step higher in their Value Plus line but we already knew it wouldn't be from the stats. It benefits more from ambient light (overcast sky reflecting streetlights for example) but can still deliver a good color picture at night if you turn up the WDR. I had WDR turned down to 20% and with overcast skies it was great, clear skies fairly dark, so I'm going to try it with slightly higher WDR.
That brings up a significant point about ultra low light, at least with these Hik units. There is a LOT of tweaking you can and need to do to tailor the image to the particular area so it performs well in all conditions. For a professional installer that means they need to do it at night in order to get the image they want. At my house toying with my own gear that's hardly a concern but for larger companies scheduling techs during the evening might incur overtime and could factor into the cost of an install. I've found that higher WDR helps at night but increases noise and blows out colors. Cranking DNR helps with noise but also kills the resolution and detail. There isn't a one size fits all configuration here and needs a fair amount of experimenting.
I should point out that the Value Plus line originally didn't bill these 0.008 lux cameras as Darkfighters but in the latest literature they do. You can also get the same sensor package in their bullets, mini-bullets, and turrets, and a 2.1MP version is also available. Note that only the 2.1MP and 3MP are billed as Darkfighters; the 5MP and 8MP are not. In the latest catalog Hikvision also introduced this sensor package in varifocals in all formats as well.
Having seen the performance difference between the PTZ sensor and the dome I'd bill the domes with the 0.008 lux rating as more of a quasi-Darkfighter. Since the PTZ is so much more complicated to put up I haven't been able to try it in a darker area to see how it compares to the dome, although I did buy some more of the domes to replace the 5MP turret I compared with in the link above, so that will give a bit more of a comparison.
Note that Hikvision does have more 0.002 and 0.0027-rated Darkfighter cameras in their Smart line but you'll be paying $1000 for a dome versus $160. For the money I think the new Value Plus "Darkfighter" line is a great bang for the buck. And that PTZ well, let's just say I wish I could use them everywhere.
I think the overall takeaway from this is Hikvision is aggressively expanding its line of ultra low light cameras so that they become a much more regular commodity versus being specialized units. I'd say within a few years most manufacturers will have similar lines because color at night is so much more superior for identification than IR. I really want to try the new DarkfighterX but truth be told, I don't have the cash flow to buy one as a toy just yet!