What is Illumination?

Generally, the illumination is the professional standard of the average brightness surroundings.
The minimun illumination means that the darkest enviroment which camera could take normal picture.
Illumination in normal situation as listed below:
Under Sunshine: 50,000 Lux ~ 1,000,000 Lux
Cloudy: 100 Lux ~ 10,000 Lux
Shop: 75 Lux ~ 300 Lux
Office: 300 Lux ~ 500 Lux
Street lamp at night: 10 Lux