Imagine a completely white room with totally even light on all surfaces. It would, in fact, be impossible to perceive anything. But nothing is ‘invisible’ as the eye will still see the light – we just cannot distinguish one thing from another. In other words, contrast is essential for perception.
The eye is capable of registering a contrast range of approximately 1,000:1. This means that light sources with a luminance value below 1/30th of the adaption level will be perceived as black, and light sources with a luminance value exceeding 30 times the adaption level will be perceived as white.
This limitation is the reason why it’s difficult to see details in the dark shadows on a bright and sunny day or to see the details in a bright cloudy sky.