What's the really colour temp (k e.g. 4000k) of Mercury vapour bulbs? Because usually they are green and I don't really see green on the colour temperature colour board. They definitely use over 4000k.

So, what is the really colour temperature of Mercury Vapour Bulbs (MV, MBF)

Well color temperature is mostly some kind of abstraction dealing with blue-to yellow color balance. It has a physical meaning for incandescent sources as a temperature of a hot black body having the same radiation spectrum. For non-white light, CCT quickly loses its original meaning but there is an agreement within the people working with light to ignore pink-green balance and interpolate resulting yellow-blue color balance of a light as being emitted by a thermal (incandescent) source too. They say "Isotherms perpendicular to Planckian locus" Weird, yeah?

That said, mercury lamp CCT is from about 5500-6000K (bare mercury arc, weak cheap phosphors) to about 4500K (regular MBF) to ~3700K (deluxe) to ~3200K super deluxe, golden lacquer filtered.






 

They usually say in the datasheet or catalog. Check out www.lamptech.co.uk for those books, or click on a few lamps that James has measured the color temp, CRI and so forth from.

Uncoated HPMV lamps get close to northern daylight. A brand new Philips HPL-N 50w sits somewhere around 4000-4500k with a better color neutrality (by eye closer to the black body locus - 'white' though the color temperature is allowed to differ) than the Narva 33-640 T10 lamp that's currently in my kitchen.



You'll sometimes encounter the chart shown above. In the middle there is the line that represents white light of different color temperatures. If you shift away from that line, your light gets a color hue. In the attachment a chart from James' website, which plots 3 different coated MV lamps in a cropped out section of the chart. HPL-N is very close to neutral white, but has a mediocre CRI (which cannot be plotted on these graphs). The HPL Deluxe has a better CRI but is shifted towards pink light. The final version, the HPL Comfort, is spot on the black body locus so it does not have a tint to the light.
Greened out coated MV lamps will of course shift towards the green part of the chart.

In fact, for me that pinkish color of a fresh deluxe mercury lamp is OK, and I kinda like it! Unlike super deluxe lamps which look cool with their golden coat but have some weird color that can be described as 'dirty' or 'muddy'...