The question is rather vague. Your apparently talking about artificial light, but it's characteristics can be quite different.
- Incandescent lamp light is almost unmodulated due to the large time constant of filaments
- mains frequency operated fluorescent lamps have strong 100/120 Hz modulation
- electronical ballasts of fluorescent energy saving lamps operate at high frequencies between 20 and 50 kHz, the light is also modulated with double drive frequency, but with lower modulation index caused by fluorescence time constant
From on optical communication viewpoint, you face an increasing enviromental pollution of modulated light sources.