For photoperiod (daylength) manipulation it has to be red light which is "measured" by a pigment in the leaves called phytochrome. Incandescents produce a lot of red light which is why they're commonly used for photoperiod control. (Actually plants really measure the length of the night but we still call it daylength). As little as 10 foot candles to the plants is enough to affect photoperiod, a 60 watt incandescent can do that, although you may need to space them out if covering a large area where one bulb isn't sufficient.
