Here is an app note from Maxim describing how to implement light intensity sensing in the visible spectrum. This is used to adjust the brightness of displays in portable devices.
Human eyes only perceive a limited light spectrum. Sensors used to detect light intensity require complex conversions of their spectrum to emulate human eyes.
Another interesting fact is that human eyes perceive brightness logarithmically. What human eyes perceive as twice as bright, is in fact 10 times as bright. For example, bright sunlight provides around 100,000 lux of lighting, while a brightly lit office lighting provides only 300 lux.