Old Bulbs = Algae

[MrGoodbytes]

Hmm interesting. I looked into it further with respect to fluorescent lamps in particular and it turns out to be quite a bit more complicated to determine actual correlated colour temperature (CCT). They use CCT to distinguish between actual CT since our lamps aren't actually radiating at that temperature.

You can use Wien's Law to find the peak of the blackbody spectrum which that of the lamp resembles but the lamp's spectral distribution (intensity per unit wavelength) will be different, due to the phosphors. Nevertheless, it still produces light that looks similar to our eyes, which "add" colours. That's why a low CCT lamp may appear white-"ish" but have a peak in the red - the other colours are playing a part when we observe them altogether.

Bah, sorry if this is way off topic or anything. Hope this clears things up somewhat.

Graeme