Switching on a light isn’t something that you think about in great detail – you flick a switch and the light comes on. But some lights actually take up to a few minutes to operate at their full potential – you may have noticed this in particular with CFL lights. This is known as the light’s 'warm up time'.
CFL warm-up times
Incandescent bulbs turn on to full power within a fraction of a second. CFLs take a little bit longer, though – some take a few seconds, while others may take up to a few minutes to warm up to full brightness. While it's not necessarily a terrible thing in most situations, you may wish to avoid using CFLs in lights that are turned off and on frequently, and where the immediate presence of strong lighting is important.
If this can’t be avoided, however, there are some CFLs that are known as ‘instant on’ bulbs, which will don't have warm-up time problems. Also, a handful of manufacturers have developed hybrid CFLs that will also be free of warm-up time issues.