Could I seek some sort of explanation on this as I must admit it's a new one on me.
Are we saying that the spectrum of a lamps output will vary over a cycle? If so what dictates the speed of the cycle and is it the full visible spectrum or just part of it? I'm assuming that the slower the cycle the more likely we are to get a variance in colour temp if using faster shutter speeds, so does the same apply to luminosity?
Cheers.
The speed of the cycle is dependent upon the frequency of the AC current. In the US that frequency is 60 Hz; I think in many European countries the frequency is 50 Hz. This means that the instantaneous voltage will change from zero to a maximum, back to zero, to the maximum again in the opposite direction, then back to zero sixty times a second.
Incandescent bulbs, by their nature, don't react to the voltage change that quickly. They sort of "average" out the changes and are relatively constant, both as far as luminosity and color temp.
Other kinds of lights, particularly sodium and mercury based lights, do not "smooth" things over. The color temperature and light output actually change with the input voltage fairly substantially. Fluorescent lights also have the same problem, although not as strongly as those others.
If your shutter speed is slow enough, you end up "averaging" the light through at least one or more cycles. If, on the other hand, your shutter speed is fast you can end up with only part of the cycle, and your color temp and measured light can vary from frame to frame, depending upon when you hit your shutter.
Finally, those lights themselves, used in the US for gym, industrial, and street lighting, don't have anywhere close to a continuous spectrum at any time. Go grab a cheap diffraction grating or prism and you'll see really strange spikes in the spectrum. These lights are bright, fairly low in heat production, fairly efficient, long lived, and cheap for their light output compared to incandescent, which explains their use.
Tom Frerichs