I think there is a bit of misinformation about this but here is a site that has an intereactive tool which makes te relationship between this stuff pretty clear:
Basically this is a simple scoop. The balck-body approach: This is based on a theoritical object which produces no inherent radiation of its own (some people erroneously assume this is black metal - though not exactly correct it is an easy way of looking at it. So you take this object and start heating it up. As it heats it starts to radiate. At lower temperatures it radiates low frequency (large Wavelength) radiation and as you heat it it continues through the radiation spectrum something like this:
Eimts from coolest to hotest, temps are approximates:
Radio Waves 1000K
Infared Waves (i.e. below red or below visable spectrum) 2000K
Visable Light (4500k-7200K)
Ultraviolet (7500K - 20,000K)
Gamma rays (30,000K +)
The temperatures provide for lightbulbs and the wavelength definitions are specifically peak outputs - i.e. where in the light spectrum the bulb will have its greatest output. This can be provided as a temperature (relating to a heated black body which would have the same peak - though not neccessarily the same color mix) or the specific wavelenght. The spectrum a bulb emits is usually referred to as the CRI (color rendering index) but let's leave that out for a moment.
The reason that actinics are not given in kelvins is that they typically are very narrow bandwidth i.e. they filter much of the output arround the peak wavelength - however, that does not mean that they don't have a peak wavelength or that the peak wavelength can not be directly related to a black-body object. The difference being that all the other wavelengths in the spectrum of an actinic are missing were as they are there in a black body object.
np on hijacking the thread - your input was valuable I just did not want folks to get a misunderstanding that the formula doesn't work - it does but the actinic bulbls are a bit of a special case and one has to be aware of how to apply the formula.
In my post by saying the bulb has a temperature of 6900K - I meant it has a peak output which would match a full spectrum bulb at the same temperature. Usually the peak is what we are interested in for organism health not so much for appearance - just so happens that many like the blue/violet tint of some of
these bulbs as well.
Well - hope I helped and didn't add anymore confusion and I hope you enjoy the site I've posted.