----- Original Message -----

HL, are you saying that in terms of high intensity fluorescent tubes, a 10,000K tube and a 20,000K tube are the same brightness but just a different colour? If so I didn't realise this…

Short answer - Yes

The 20 000K is more "blue" than the 10 000K, but for the same wattage it should be the same intensity.... Thinking about it, there is one difference, though (maybe one of our bright young academics can confirm this, my general physics is quite hazy after 25 years of non-use). As a higher frequency wave is more energetic than a lower frequency one, the 20 000K bulbs would theoretically have more energy - but nor twice that of the 10 000K bulbs. I'm not sure (gut feeling again) but I don't think the difference in wavelength between 10 000K and 20 000K is all that much (maybe 10 or 20 nm ??)