The phrase "illumination levels" refers to the amount of light in a particular area or space. It indicates how bright or dark a place is, often measured in units like lux or foot-candles. Full definition
You will also see all of the different illumination levels on the front - lit screen in complete darkness and witness how the reader performs with being submerged in water. (goodereader.com)
I was looking forward to a video comparing these three readers at a similar illumination level. (goodereader.com)
Astronomers may not be able to see the planet in question, but the resulting star dimming allows them to determine such key planetary life factors as illumination level and temperature. (science.howstuffworks.com)