Lux is a way to quantify the intensity of light. For example, if you stare right at the sun, that quantifies getting over 1000 lux ... If you're in an office, it might be, say, 500 lux. If you're watching TV, it's in the 400 range, when it really should be in the 30 range in the 3-hour window before bed. If you're staring at your phone, it's about 60 lux, which is double what the expert consensus panel recommends.